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I have been observing my cat and found that when confronted with an unknown item, she will always use her front left paw to touch it.
This has me wondering if animals exhibit handedness like humans do? (and do I have a left handed cat?)
One note of importance is that with an unknown item, her approach is always identical, so possibly using the left paw means allowing a fast possible exit based on how she positions her body.
This question is related to Are there dextral/sinistral higher animals?. However, I question the "paw-ness" as a consequence of how the cat is approaching new items (to be ready to flee), whereas the other question remarks about the high number of "right-pawed" dogs and questions the influence of people for this preference.
Yes. handedness (or Behavioral Lateralization) has been documented in numerous vertebrates (mammals, reptiles and birds) as well as invertebrates.
- This includes domestic cats (see Wells & Millsopp 2009).
There have been numerous studies that have documented behavioral lateralization in many groups of animals including lower vertebrates (fish and amphibians), reptiles (even snakes!), birds and mammals. More recent work (e.g., Frasnelli 2013) has also shown that lateralization can also occur in invertebrates. In other words, "handedness" (or pawedness, footedness, eyedness, earedness, nostriledness, toothedness, breastedness, gonadedness, etc.) occurs rather extensively across the animal kingdom.
These studies suggest that the evolution of brain lateralization, often linked to lateralized behaviors, may have occurred early in evolutionary history and may not have been the result of multiple independent evolutionary events as once thought.
Although this view of brain lateralization as a highly conserved trait throughout evolutionary history has gained popularity, it's still contested (reviewed by Bisazza et al. 1998; Vallortigara et al. 1999).
Note: Laterality of function may manifest in terms of preference (frequency) or performance (proficiency), with the former being far more often investigated.
And no, right-handedness is not always dominant.
One hypothesis is that brain lateralization was the evolutionary result of the need to break up complex tasks and perform them with highly specialized neuronal units to avoid functional overlap (i.e., to account for "functional incompatibility").
In humans, many hypotheses have been developed including: division of labor, genetics, epigenetic factors, prenatal hormone exposure, prenatal vestibular asymmetry, and even ultrasound exposure in the womb.
Snake studies (see below) have suggested lateralization behavior can be dictated by environmental conditions (specifically, temperature).
Other work (Hoso et al. 2007) suggest that lateralization could be the result of convergent evolution. In this case, snakes developed feeding aparati that allow them to better consume more-common dextral species of snails.
- Note: dextral (meaning "clockwise") is a type of chirality -- another form of "handedness"
Lateralization in non-human primates: McGrew & Marchant 1997.
Lateralized behaviors in mammals and birds: Bradshaw & Rogers 1993; Rogers & Andrew 2002.
Lateralized behaviors in lower vertebrates: Bisazza et al. 1998; Vallortigara et al. 1999.
Some spiders appear to favor certain appendages for prey handling and protection (Ades & Novaes Ramires 2002).
Octopi (or octopodes) preferably use one eye over the other (Byrne et al. 2002; with seemingly no preference for right/left at the population level: Byrne et al. 2004) and also apparently have a preferred arm (Byrne et al. 2006).
Preferential ventral fin use in the gourami (Trichogaster trichopterus). [Bisazza et al. 2001].
Preferential eye use in a variety of fish species [Sovrano et al. 1999, 2001].
Lateralization of neural control for vocalization in frogs (Rana pipiens). [Bauer 1993].
Preferential use of hindlimbs (Robins et al. 1998), forelimbs (Bisazza et al. 1996) and eyes (Vallortigara et al. 1998) in adult anurans.
Preferential use of right hemipenis over left under warm conditions. [Shine et al. 2000].
Coiling asymmetries were found at both the individual and population levels. [Roth 2003].
- Tendency for parrots to use left-feet when feeding. [Friedmann & Davis 1938].
Pawdness in mice. [Collins 1975].
left forelimb bias in a species of bat when using hands for climbing/grasping. [Zucca et al. 2010]
Behavior experiments show domesticated cats show strong preference to consistently use either left or right paw and that the lateralized behavior was strongly sex related (in their population: ♂ = left / ♀ = right). [Wells & Millsopp 2009].
- Posture, reaching preference, tool use, gathering food, carrying, and many other tasks. See McGrew & Marchant (1997) for review.
Ades, C., and Novaes Ramires, E. (2002). Asymmetry of leg use during prey handling in the spider Scytodes globula (Scytodidae). Journal of Insect Behavior 15: 563-570.
Bauer, R. H. (1993). Lateralization of neural control for vocalization by the frog (Rana pipiens). Psychobiology, 21, 243-248.
Bisazza, A., Cantalupo, C., Robins, A., Rogers, L. J. & Vallortigara, G. (1996). Right-pawedness in toads. Nature, 379, 408.
Bisazza, A., Rogers, L. J. & Vallortigara, G. (1998). The origins of cerebral asymmetry: a review of evidence of behavioural and brain lateralization in fishes, reptiles and amphibians. Neuroscience and Biobehavioral Reviews, 22, 411-426.
Bisazza, A., Lippolis, G. & Vallortigara, G. (2001). Lateralization of ventral fins use during object exploration in the blue gourami (Trichogaster trichopterus). Physiology & Behavior, 72, 575-578.
Bradshaw, J. L. & Rogers, L. J. (1993). The Evolution of Lateral Asymmetries, Language, Tool Use and Intellect. San Diego: Academic Press.
Byrne, R.A., Kuba, M. and Griebel, U. (2002). Lateral asymmetry of eye use in Octopus vulgaris. Animal Behaviour, 64(3):461-468.
Byrne, R.A., Kuba, M.J. and Meisel, D.V. (2004). Lateralized eye use in Octopus vulgaris shows antisymmetrical distribution. Animal Behaviour, 68(5):1107-1114.
Byrne, R.A., Kuba, M.J., Meisel, D.V., Griebel, U. and Mather, J.A. (2006). Does Octopus vulgaris have preferred arms?. Journal of Comparative Psychology 120(3):198.
Collins RL (1975) When left-handed mice live in righthanded worlds. Science 187:181-184.
Friedmann, H., & Davis, M. (1938). " Left-Handedness" in Parrots. The Auk, 55(3), 478-480.
Hoso, M., Asami, T., & Hori, M. (2007). Right-handed snakes: convergent evolution of asymmetry for functional specialization. Biology Letters, 3(2), 169-173.
McGrew, W. C., & Marchant, L. F. (1997). On the other hand: current issues in and meta‐analysis of the behavioral laterality of hand function in nonhuman primates. American Journal of Physical Anthropology, 104(S25), 201-232.
Robins, A., Lippolis, G., Bisazza, A., Vallortigara, G. & Rogers, L. J. (1998). Lateralized agonistic responses and hindlimb use in toads. Animal Behaviour, 56, 875-881.
Rogers, L. J. & Andrew, R. J. (Eds) (2002). Comparative Vertebrate Lateralization. Cambridge: Cambridge University Press.
Roth, E. D. (2003). 'Handedness' in snakes? Lateralization of coiling behaviour in a cottonmouth, Agkistrodon piscivorus leucostoma, population. Animal behaviour, 66(2), 337-341.
Shine, R., Olsson, M. M., LeMaster, M. P., Moore, I. T., & Mason, R. T. (2000). Are snakes right-handed? Asymmetry in hemipenis size and usage in gartersnakes (Thamnophis sirtalis). Behavioral Ecology, 11(4), 411-415.
Sovrano, V. A., Rainoldi, C., Bisazza, A. & Vallortigara, G. (1999). Roots of brain specializations: preferential left-eye use during mirror-image inspection in six species of teleost fish. Behavioural Brain Research, 106, 175-180.
Sovrano, V. A., Bisazza, A. & Vallortigara, G. (2001). Lateralization of response to social stimuli in fishes: a comparison between different methods and species. Physiology & Behavior, 74, 237- 244.
Vallortigara, G., Rogers, L. J., Bisazza, A., Lippolis, G. & Robins, A. (1998). Complementary right and left hemifield use for predatory and agonistic behaviour in toads. NeuroReport, 9, 3341-3344.
Vallortigara, G., Rogers, L. J. & Bisazza, A. (1999). Possible evolutionary origins of cognitive brain lateralization. Brain Research Reviews, 30, 164-175.
Wells, D. L., & Millsopp, S. (2009). Lateralized behaviour in the domestic cat, Felis silvestris catus. Animal Behaviour, 78(2), 537-541.
Zucca, P., Palladini, A., Baciadonna, L. and Scaravelli, D. (2010). Handedness in the echolocating Schreiber's long-fingered bat (Miniopterus schreibersii). Behavioural processes, 84(3): 693-695.
Stanley Coren, The Lefthander Syndrome, says that humans are distinctive, not in having preferred handedness, but in having a high majority of individuals right-sided instead of odds approaching 50-50.
There is a lot of urban legend biology that might be answered. For instance, right-handedness is a human combat advantage, because a right-handed attack or defense puts the heart slightly further away from an opponent than an attack that's left-handed. And that one doesn't require a real knowledge of biology to answer: 48% of Olympic fencers are left-handed, and among people who've studied it left-handed people are far more likely to be excellent at combat sports than right-handed people.
(I'd love to see someone debunk the array of folk explanations for why a large majority of humans are right-handed.)
Visually Inexperienced Chicks Exhibit Spontaneous Preference for Biological Motion Patterns
When only a small number of points of light attached to the torso and limbs of a moving organism are visible, the animation correctly conveys the animal's activity. Here we report that newly hatched chicks, reared and hatched in darkness, at their first exposure to point-light animation sequences, exhibit a spontaneous preference to approach biological motion patterns. Intriguingly, this predisposition is not specific for the motion of a hen, but extends to the pattern of motion of other vertebrates, even to that of a potential predator such as a cat. The predisposition seems to reflect the existence of a mechanism in the brain aimed at orienting the young animal towards objects that move semi-rigidly (as vertebrate animals do), thus facilitating learning, i.e., through imprinting, about their more specific features of motion.
Citation: Vallortigara G, Regolin L, Marconato F (2005) Visually Inexperienced Chicks Exhibit Spontaneous Preference for Biological Motion Patterns. PLoS Biol 3(7): e208. https://doi.org/10.1371/journal.pbio.0030208
Academic Editor: David C. Burr, Istituto di Neurofisiologia, Italy
Received: January 31, 2005 Accepted: April 13, 2005 Published: June 7, 2005
Copyright: © 2005 Vallortigara et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Competing interests: The authors have declared that no competing interests exist.
Abbreviations: ANOVA, analysis of variance cd, candela
- Right-handedness is by far the most common type. Right-handed people are more skillful with their right hands. Studies suggest that approximately 90% of people are right-handed. 
- Left-handedness is far less common than right-handedness. Studies suggest that approximately 10% of people are left-handed. 
- Mixed-handedness or cross-dominance is the change of hand preference between different tasks. This is very uncommon in the population with about a 1% prevalence. 
- Ambidexterity refers to having equal ability in both hands. Those who learn it still tend to favor their originally dominant hand. This is very uncommon, with about a 1% prevalence. 
Handedness may be measured behaviourally (performance measures) or through questionnaires (preference measures). The Edinburgh Handedness Inventory has been used since 1971 but contains many dated questions and is hard to score. The longer Waterloo Handedness Questionnaire is not widely accessible. More recently, the Flinders Handedness Survey (FLANDERS) has been developed. 
There are several theories of how handedness develops. Occurrences during prenatal development may be important researchers studied fetuses in utero and determined that handedness in the womb was a very accurate predictor of handedness after birth.  In a 2013 study, 39% of infants (6 to 14 months) and 97% of toddlers (18 to 24 months) demonstrated a hand preference. 
Language dominance Edit
One common handedness theory is the brain hemisphere division of labor. In most people, the left side of the brain controls speaking. The theory suggests it is more efficient for the brain to divide major tasks between the hemispheres—thus most people may use the non-speaking (right) hemisphere for perception and gross motor skills. As speech is a very complex motor control task, the specialised fine motor areas controlling speech are most efficiently used to also control fine motor movement in the dominant hand. As the right hand is controlled by the left hemisphere (and the left hand is controlled by the right hemisphere) most people are, therefore right-handed. The theory implies that left-handed people have a reversed organisation. 
However, this theory does not address the fact that the majority of left-handers have left-hemisphere language dominance—just like right-handers.   Only around 30% of left-handers are not left-hemisphere dominant for language. Some of those have reversed brain organisation, where the verbal processing takes place in the right-hemisphere and visuospatial processing is dominant to the left hemisphere.  Others have more ambiguous bilateral organisation, where both hemispheres do parts of typically lateralised functions. When tasks investigating lateralisation are averaged across a group of left-handers, the overall effect is that left-handers show the same pattern of data as right-handers, but with a reduced asymmetry.  This finding is likely due to the small proportion of left-handers who have atypical brain organisation.
Genetic factors Edit
Handedness displays a complex inheritance pattern. For example, if both parents of a child are left-handed, there is a 26% chance of that child being left-handed.  A large study of twins from 25,732 families by Medland et al. (2006) indicates that the heritability of handedness is roughly 24%. 
Two theoretical single-gene models have been proposed to explain the patterns of inheritance of handedness, by Marian Annett  of the University of Leicester, and by Chris McManus  of UCL.
However, growing evidence from linkage and genome-wide association studies suggests that genetic variance in handedness cannot be explained by a single genetic locus.         From these studies, McManus et al. now conclude that handedness is polygenic and estimate that at least 40 loci contribute to the trait. 
Brandler et al. performed a genome-wide association study for a measure of relative hand skill and found that genes involved in the determination of left/right asymmetry in the body play a key role in handedness.  Brandler and Paracchini suggest the same mechanisms that determine left/right asymmetry in the body (e.g. nodal signaling and ciliogenesis) also play a role in the development of brain asymmetry (handedness being a reflection of brain asymmetry for motor function). 
In 2019, Wiberg et al. performed a genome-wide association study and found that handedness was significantly associated with four loci, three of them in genes encoding proteins involved in brain development. 
Epigenetic factors Edit
Twin studies indicate that genetic factors explain 25% of the variance in handedness, and environmental factors the remaining 75%.  While the molecular basis of handedness epigenetics is largely unclear, Ocklenburg et al. (2017) found that asymmetric methylation of CpG sites plays a key role for gene expression asymmetries related to handedness.  
Prenatal hormone exposure Edit
Four studies have indicated that individuals who have had in-utero exposure to diethylstilbestrol (a synthetic estrogen based medication used between 1940 and 1971) were more likely to be left-handed over the clinical control group. Diethylstilbestrol animal studies "suggest that estrogen affects the developing brain, including the part that governs sexual behavior and right and left dominance".    
Prenatal vestibular asymmetry Edit
Previc, after reviewing a large number of studies, found evidence that the position of the fetus in the final trimester and a baby's subsequent birth position can affect handedness. About two-thirds of fetuses present with their left occiput (back of the head) at birth. This partly explains why prematurity results in a decrease in right-handedness. Previc argues that asymmetric prenatal positioning creates asymmetric stimulation of the vestibular system, which is involved in the development of handedness. In fact, every major disorder in which patients show reduced right-handedness is associated with either vestibular abnormalities or delay,  and asymmetry of the vestibular cortex is strongly correlated with the direction of handedness. 
Another theory is that ultrasound may sometimes affect the brains of unborn children, causing higher rates of left-handedness in children whose mothers receive ultrasound during pregnancy. Research suggests there may be a weak association between ultrasound screening (sonography used to check the healthy development of the fetus and mother) and left-handedness. 
Infants have been observed to fluctuate heavily when choosing a hand to lead in grasping and object manipulation tasks, especially in one- versus two-handed grasping. Between 36 and 48 months, there is a significant decline in variability between handedness in one-handed grasping it can be seen earlier in two-handed manipulation. Children of 18–36 months showed more hand preference when performing bi-manipulation tasks than with simple grasping. 
The decrease in handedness variability in children of 36–48 months may be attributable to preschool or kindergarten attendance due to increased single-hand activities such as writing and coloring.  Scharoun and Bryden noted that right-handed preference increases with age up to the teenage years. 
In his book Right-Hand, Left-Hand, Chris McManus of University College London argues that the proportion of left-handers is increasing, and that an above-average quota of high achievers have been left-handed. He says that left-handers' brains are structured in a way that increases their range of abilities, and that the genes that determine left-handedness also govern development of the brain's language centers. 
Writing in Scientific American, he states:
Studies in the U.K., U.S. and Australia have revealed that left-handed people differ from right-handers by only one IQ point, which is not noteworthy . Left-handers' brains are structured differently from right-handers' in ways that can allow them to process language, spatial relations and emotions in more diverse and potentially creative ways. Also, a slightly larger number of left-handers than right-handers are especially gifted in music and math. A study of musicians in professional orchestras found a significantly greater proportion of talented left-handers, even among those who played instruments that seem designed for right-handers, such as violins. Similarly, studies of adolescents who took tests to assess mathematical giftedness found many more left-handers in the population. 
Conversely, Joshua Goodman found that evidence for left-handers was overrepresented amongst those with higher cognitive skills, such as Mensa members and higher-performing takers of SAT and MCAT tests, due to methodological and sampling issues in studies. He also found that left-handers were overrepresented among those with lower cognitive skills and mental impairments, with those with intellectual disability (ID) being roughly twice as likely to be left-handed, as well as generally lower cognitive and non-cognitive abilities amongst left-handed children.  In a systematic review and meta-analysis, Ntolka and Papadatou-Pastou found that right-handers had higher IQ scores, but that difference was negligible (about 1.5 points). 
Early childhood intelligence Edit
Nelson, Campbell, and Michel studied infants and whether developing handedness during infancy correlated with language abilities in toddlers. In the article they assessed 38 infants and followed them through to 12 months and then again once they became toddlers from 18 to 24 months. What they discovered was that when a child developed a consistent use of their right or left hand during infancy (such as using the right hand to put the pacifier back in, or grasping random objects with the left hand), they were more likely to have superior language skills as a toddler. Children who became lateral later than infancy (i.e., when they were toddlers) showed normal development of language and had typical language scores.  The researchers used Bayley scales of infant and toddler development to assess all the subjects.
In two studies, Diana Deutsch found that left-handers, particularly those with mixed hand preference, performed significantly better than right-handers in musical memory tasks.   There are also handedness differences in perception of musical patterns. Left-handers as a group differ from right-handers, and are more heterogeneous than right-handers, in perception of certain stereo illusions, such as the octave illusion, the scale illusion, and the glissando illusion. 
Left-handed people are much more likely to have several specific physical and mental disorders and health problems. For example:
Lower-birth-weight and complications at birth are positively correlated with left-handedness. 
A variety of neuropsychiatric and developmental disorders like autism spectrum disorders, depression, bipolar disorder, anxiety disorders, schizophrenia, and alcoholism has been associated with left- and mixed-handedness.  
A 2012 study showed that nearly 40% of children with cerebral palsy were left-handed,  while another study demonstrated that left-handedness was associated with a 62 percent increased risk of Parkinson's disease in women, but not in men.  Another study suggests that the risk of developing multiple sclerosis increases for left-handed women, but the effect is unknown for men at this point. 
Left-handed women have a higher risk of breast cancer than right-handed women and the effect is greater in post-menopausal women. 
At least one study maintains that left-handers are more likely to suffer from heart disease, and are more likely to have reduced longevity from cardiovascular causes. 
Left-handers are more likely to suffer bone fractures. 
One systematic review concluded: "Left-handers showed no systematic tendency to suffer from disorders of the immune system". 
As handedness is a highly heritable trait associated with various medical conditions, and because many of these conditions could have presented a Darwinian fitness challenge in ancestral populations, this indicates left-handedness may have previously been rarer than it currently is, due to natural selection. However, on average, left-handers have been found to have an advantage in fighting and competitive, interactive sports, which could have increased their reproductive success in ancestral populations. 
In a 2006 U.S. study, researchers from Lafayette College and Johns Hopkins University concluded that there was no statistically significant correlation between handedness and earnings for the general population, but among college-educated people, left-handers earned 10 to 15% more than their right-handed counterparts. 
However, more recently, in a 2014 study published by the National Bureau of Economic Research, Harvard economist Joshua Goodman finds that left-handed people earn 10 to 12 percent less over the course of their lives than right-handed people. Goodman attributes this disparity to higher rates of emotional and behavioral problems in left-handed people. 
Left-handedness and sports Edit
Interactive sports such as table tennis, badminton and cricket have an overrepresentation of left-handedness, while non-interactive sports such as swimming show no overrepresentation. Smaller physical distance between participants increases the overrepresentation. In fencing, about half the participants are left-handed.  The term southpaw is sometimes used to refer to a left-handed individual, especially in baseball and boxing. 
Other, sports-specific factors may increase or decrease the advantage left-handers usually hold in one-on-one situations:
- In cricket, the overall advantage of a bowler's left-handedness exceeds that resulting from experience alone: even disregarding the experience factor (i.e., even for a batsman whose experience against left-handed bowlers equals his experience against right-handed bowlers), a left-handed bowler challenges the average (i.e., right-handed) batsman more than a right-handed bowler does, because the angle of a bowler's delivery to an opposite-handed batsman is much more penetrating than that of a bowler to a same-handed batsman (see Wasim Akram). 
- In baseball, a right-handed pitcher's curve ball will break away from a right-handed batter and towards a left-handed batter. While studies of handedness show that only 10% of the general population is left-handed, the proportion of left-handed MLB players is closer to 39% of hitters and 28% of pitchers, according to 2012 data.  Historical batting averages show that left-handed batters have a slight advantage over right-handed batters when facing right-handed pitchers.  Because there are fewer left-handed pitchers than right-handed pitchers, left-handed batters have more opportunities to face right-handed pitchers than their right-handed counterparts have against left-handed pitchers.  Fourteen of the top twenty career batting averages in Major League Baseball history have been posted by left-handed batters.  Left-handed batters have a slightly shorter run from the batter's box to first base than right-handers. This gives left-handers a slight advantage in beating throws to first base on infield ground balls. 
- Because a left-handed pitcher faces first base when he is in position to throw to the batter, whereas a right-handed pitcher has his back to first base, a left-handed pitcher has an advantage when attempting to pick off baserunners at first base. 
- Defensively in baseball, left-handedness is considered an advantage for first basemen because they are better suited to fielding balls hit in the gap between first and second base, and because they do not have to pivot their body around before throwing the ball to another infielder.  For the same reason, the other infielder's positions are seen as being advantageous to right-handed throwers. Historically, there have been few left-handed catchers because of the perceived disadvantage a left-handed catcher would have in making the throw to third base, especially with a right-handed hitter at the plate.  A left-handed catcher would have a potentially more dangerous time tagging out a baserunner trying to score.  With the ball in the glove on the right hand, a left-handed catcher would have to turn his body to the left to tag a runner. In doing so, he can lose the opportunity to brace himself for an impending collision.  On the other hand, the Encyclopedia of Baseball Catchers states: 
One advantage is a left-handed catcher's ability to frame a right-handed pitcher's breaking balls. A right-handed catcher catches a right-hander's breaking ball across his body, with his glove moving out of the strike zone. A left-handed catcher would be able to catch the pitch moving into the strike zone and create a better target for the umpire.
- In four wall handball, typical strategy is to play along the left wall forcing the opponent to use their left hand to counter the attack and playing into the strength of a left-handed competitor.
- In handball, left-handed players have an advantage on the right side of the field when attacking, getting a better angle, and that defenders might be unused to them. Since few people are left-handed, there is a demand for such players.
- In water polo, the centre forward position has an advantage in turning to shoot on net when rotating the reverse direction as expected by the centre of the opposition defence and gain an improved position to score. Left-handed drivers are usually on the right side of the field, because they can get better angles to pass the ball or shoot for goal. typically uses a strategy in which a defence pairing includes one left-handed and one right-handed defender. A disproportionately large number of ice hockey players of all positions, 62 percent, shoot left, though this does not necessarily indicate left-handedness. 
- In American football, the handedness of a quarterback affects blocking patterns on the offensive line. Tight ends, when only one is used, typically line up on the same side as the throwing hand of the quarterback, while the offensive tackle on the opposite hand, which protects the quarterback's "blind side," is typically the most valued member of the offensive line. Receivers also have to adapt to the opposite spin.  While uncommon, there have been several notable left-handed quarterbacks.
- In bowling, the oil pattern used on the bowling lane breaks down faster the more times a ball is rolled down the lane. Bowlers must continually adjust their shots to compensate for the ball's change in rotation as the game or series is played and the oil is altered from its original pattern. A left-handed bowler competes on the opposite side of the lane from the right-handed bowler and therefore deals with less breakdown of the original oil placement. This means left-handed bowlers have to adjust their shot less frequently than right-handed bowlers in team events or qualifying rounds where there are possibly 4-10 people per set of two lanes. This can allow them to stay more consistent. However, this advantage is not present in bracket rounds and tournament finals where matches are 1v1 on a pair of lanes.
According to a meta-analysis of 144 studies, totaling 1,787,629 participants, the best estimate for the male to female odds ratio was 1.23, indicating that men are 23% likelier to be left-handed. In terms of proportions this odds ratio implies that if the incidence of left-handedness for females was 10%, then the incidence of male left-handedness would be 12%.  [ clarification needed ]
Sexuality and gender identity Edit
A number of studies examining the relationship between handedness and sexual orientation have reported that a disproportionate minority of homosexual people exhibit left-handedness,  though findings are mixed.   
A 2001 study also found that children who were assigned male at birth and whose gender identity is not male were more than twice as likely to be left-handed than a clinical control group (19.5% vs. 8.3%, respectively). 
Paraphilias (atypical sexual interests) have also been linked to higher rates of left-handedness. A 2008 study analyzing the sexual fantasies of 200 males found "elevated paraphilic interests were correlated with elevated non-right handedness".  Greater rates of left-handedness has also been documented among pedophiles.    
A 2014 study attempting to analyze the biological markers of asexuality asserts that non-sexual men and women were 2.4 and 2.5 times, respectively, more likely to be left-handed than their heterosexual counterparts. 
Mortality rates in combat Edit
A study at Durham University — which examined mortality data for cricketers whose handedness was a matter of public record — found that left-handed men were almost twice as likely to die in war as their right-handed contemporaries.  The study theorised that this was because weapons and other equipment was designed for the right-handed. “I can sympathise with all those left-handed cricketers who have gone to an early grave trying desperately to shoot straight with a right-handed Lee Enfield .303,” wrote a journalist reviewing the study in the cricket press.  The findings echo those of previous American studies, which found that left-handed US sailors were 34% more likely to have a serious accident than their right-handed counterparts. 
Episodic memory etc Edit
A high level of handedness (whether strongly favoring right or left) is associated with poorer episodic memory,   and with poorer communication between brain hemispheres,  which may give poorer emotional processing, although bilateral stimulation may reduce such effects.  
Corpus callosum Edit
A high level of handedness is associated with a smaller corpus callosum whereas low handedness with a larger one. 
Many tools and procedures are designed to facilitate use by right-handed people, often without realizing the difficulties incurred by the left-handed. John W. Santrock has written, "For centuries, left-handers have suffered unfair discrimination in a world designed for right-handers." 
As a child British King George VI (1895-1952) was naturally left-handed. He was forced to write with his right hand, as was common practice at the time. He was not expected to become king, so that was not a factor.  McManus noted that, as the Industrial Revolution spread across Western Europe and the United States in the 19th century, workers needed to operate complex machines that were designed with right-handers in mind. This would have made left-handers more visible and at the same time appear less capable and more clumsy. During this era, children were taught to write with a dip pen. While a right-hander could smoothly drag the pen across paper from left to right, a dip pen could not easily be pushed across by the left hand without digging into the paper and making blots and stains. 
Negative connotations and discrimination Edit
Moreover, apart from inconvenience, left-handed people have historically been considered unlucky or even malicious for their difference by the right-handed majority. In many European languages, including English, the word for the direction "right" also means "correct" or "proper". Throughout history, being left-handed was considered negative, or evil even into the 20th century, left-handed children were beaten by schoolteachers for writing with their left hand.
The Latin adjective sinister or sinistra (as applied to male or female nouns — Latin nouns are gender specific) means "left" as well as "unlucky", and this double meaning survives in European derivatives of Latin, including the English words "sinister" (meaning both 'evil' and 'on the bearer's left on a coat of arms') and "ambisinister" meaning 'awkward or clumsy with both or either hand'.
There are many negative connotations associated with the phrase "left-handed": clumsy, awkward, unlucky, insincere, sinister, malicious, and so on. A "left-handed compliment" is one that has two meanings, one of which is unflattering to the recipient. In French, gauche means both "left" and "awkward" or "clumsy", while droit(e) (cognate to English direct and related to "adroit") means both "right" and "straight", as well as "law" and the legal sense of "right". The name "Dexter" derives from the Latin for "right", as does the word "dexterity" meaning manual skill. As these are all very old words, they would tend to support theories indicating that the predominance of right-handedness is an extremely old phenomenon.
Black magic is sometimes referred to as the "left-hand path".
Until very recently in Taiwan (and still in Mainland China, Japan and both North and South Korea), left-handed people were forced to switch to being right-handed, or at least switch to writing with the right hand. Due to the importance of stroke order, developed for the comfortable use of right-handed people, it is considered more difficult to write legible Chinese characters with the left hand than it is to write Latin letters, though difficulty is subjective and depends on the writer.  Because writing when moving one's hand away from its side towards the other side of the body can cause smudging if the outward side of the hand is allowed to drag across the writing, writing in the Latin alphabet might possibly be less feasible with the left hand than the right under certain circumstances. Conversely, right-to-left alphabets, such as the Arabic and Hebrew, are generally considered easier to write with the left hand in general. [ citation needed ] Depending on the position and inclination of the writing paper, and the writing method, the left-handed writer can write as neatly and efficiently or as messily and slowly as right-handed writers. Usually the left-handed child needs to be taught how to write correctly with the left hand, since discovering a comfortable left-handed writing method on one's own may not be straightforward.  
In the Soviet Union, all left-handed children were forced to write with their right hand in the Soviet school system.  
International Left-Handers Day Edit
International Left-Handers Day is held annually every August 13.  It was founded by the Left-Handers Club in 1992, with the club itself having been founded in 1990.  International Left-Handers Day is, according to the club, "an annual event when left-handers everywhere can celebrate their sinistrality (left-handedness) and increase public awareness of the advantages and disadvantages of being left-handed."  It celebrates their uniqueness and differences, who are from seven to ten percent of the world's population. Thousands of left-handed people in today's society have to adapt to use right-handed tools and objects. Again according to the club, "in the U.K. alone there were over 20 regional events to mark the day in 2001 – including left-v-right sports matches, a left-handed tea party, pubs using left-handed corkscrews where patrons drank and played pub games with the left hand only, and nationwide 'Lefty Zones' where left-handers' creativity, adaptability and sporting prowess were celebrated, whilst right-handers were encouraged to try out everyday left-handed objects to see just how awkward it can feel using the wrong equipment!" 
Kangaroos and other macropod marsupials show a left-hand preference for everyday tasks in the wild. 'True' handedness is unexpected in marsupials however, because unlike placental mammals, they lack a corpus callosum. Left-handedness was particularly apparent in the red kangaroo (Macropus rufus) and the eastern gray kangaroo (Macropus giganteus). Red-necked (Bennett's) wallabies (Macropus rufogriseus) preferentially use their left hand for behaviours that involve fine manipulation, but the right for behaviours that require more physical strength. There was less evidence for handedness in arboreal species.  Studies of dogs, horses, and domestic cats have shown that females of those species tend to be right-handed, while males tend to be left-handed. 
The mind–body problem in philosophy examines the relationship between mind and matter, and in particular the relationship between consciousness and the brain. A variety of approaches have been proposed. Most are either dualist or monist. Dualism maintains a rigid distinction between the realms of mind and matter. Monism maintains that there is only one kind of stuff, and that mind and matter are both aspects of it. The problem was addressed by pre-Aristotelian philosophers,   and was famously addressed by René Descartes in the 17th century, resulting in Cartesian dualism. Descartes believed that humans only, and not other animals have this non-physical mind.
The rejection of the mind–body dichotomy is found in French Structuralism, and is a position that generally characterized post-war French philosophy.  The absence of an empirically identifiable meeting point between the non-physical mind and its physical extension has proven problematic to dualism and many modern philosophers of mind maintain that the mind is not something separate from the body.  These approaches have been particularly influential in the sciences, particularly in the fields of sociobiology, computer science, evolutionary psychology, and the neurosciences.    
Epiphenomenalism is the theory in philosophy of mind that mental phenomena are caused by physical processes in the brain or that both are effects of a common cause, as opposed to mental phenomena driving the physical mechanics of the brain. The impression that thoughts, feelings, or sensations cause physical effects, is therefore to be understood as illusory to some extent. For example, it is not the feeling of fear that produces an increase in heart beat, both are symptomatic of a common physiological origin, possibly in response to a legitimate external threat. 
The history of epiphenomenalism goes back to the post-Cartesian attempt to solve the riddle of Cartesian dualism, i.e., of how mind and body could interact. La Mettrie, Leibniz and Spinoza all in their own way began this way of thinking. The idea that even if the animal were conscious nothing would be added to the production of behavior, even in animals of the human type, was first voiced by La Mettrie (1745), and then by Cabanis (1802), and was further explicated by Hodgson (1870) and Huxley (1874).   Huxley (1874) likened mental phenomena to the whistle on a steam locomotive. However, epiphenomenalism flourished primarily as it found a niche among methodological or scientific behaviorism. In the early 1900s scientific behaviorists such as Ivan Pavlov, John B. Watson, and B. F. Skinner began the attempt to uncover laws describing the relationship between stimuli and responses, without reference to inner mental phenomena. Instead of adopting a form of eliminativism or mental fictionalism, positions that deny that inner mental phenomena exist, a behaviorist was able to adopt epiphenomenalism in order to allow for the existence of mind. However, by the 1960s, scientific behaviourism met substantial difficulties and eventually gave way to the cognitive revolution. Participants in that revolution, such as Jerry Fodor, reject epiphenomenalism and insist upon the efficacy of the mind. Fodor even speaks of "epiphobia"—fear that one is becoming an epiphenomenalist.
Thomas Henry Huxley defends in an essay titled On the Hypothesis that Animals are Automata, and its History an epiphenomenalist theory of consciousness according to which consciousness is a causally inert effect of neural activity—"as the steam-whistle which accompanies the work of a locomotive engine is without influence upon its machinery".  To this William James objects in his essay Are We Automata? by stating an evolutionary argument for mind-brain interaction implying that if the preservation and development of consciousness in the biological evolution is a result of natural selection, it is plausible that consciousness has not only been influenced by neural processes, but has had a survival value itself and it could only have had this if it had been efficacious.   Karl Popper develops in the book The Self and Its Brain a similar evolutionary argument. 
Animal ethics Edit
Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain.  In his interactions with scientists and other veterinarians, Rollin was regularly asked to prove animals are conscious and provide scientifically acceptable grounds for claiming they feel pain.  The denial of animal consciousness by scientists has been described as mentophobia by Donald Griffin.  Academic reviews of the topic are equivocal, noting that the argument that animals have at least simple conscious thoughts and feelings has strong support,  but some critics continue to question how reliably animal mental states can be determined.   A refereed journal Animal Sentience  launched in 2015 by the Institute of Science and Policy of The Humane Society of the United States is devoted to research on this and related topics.
Consciousness is an elusive concept that presents many difficulties when attempts are made to define it.   Its study has progressively become an interdisciplinary challenge for numerous researchers, including ethologists, neurologists, cognitive neuroscientists, philosophers, psychologists and psychiatrists.  
In 1976 Richard Dawkins wrote, "The evolution of the capacity to simulate seems to have culminated in subjective consciousness. Why this should have happened is, to me, the most profound mystery facing modern biology".  In 2004, eight neuroscientists felt it was still too soon for a definition. They wrote an apology in "Human Brain Function": 
"We have no idea how consciousness emerges from the physical activity of the brain and we do not know whether consciousness can emerge from non-biological systems, such as computers. At this point the reader will expect to find a careful and precise definition of consciousness. You will be disappointed. Consciousness has not yet become a scientific term that can be defined in this way. Currently we all use the term consciousness in many different and often ambiguous ways. Precise definitions of different aspects of consciousness will emerge . but to make precise definitions at this stage is premature."
Consciousness is sometimes defined as the quality or state of being aware of an external object or something within oneself.   It has been defined somewhat vaguely as: subjectivity, awareness, sentience, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind.  Despite the difficulty in definition, many philosophers believe that there is a broadly shared underlying intuition about what consciousness is.  Max Velmans and Susan Schneider wrote in The Blackwell Companion to Consciousness: "Anything that we are aware of at a given moment forms part of our consciousness, making conscious experience at once the most familiar and most mysterious aspect of our lives." 
Related terms, also often used in vague or ambiguous ways, are:
- : the state or ability to perceive, to feel, or to be conscious of events, objects, or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of something. In biological psychology, awareness is defined as a human's or an animal's perception and cognitive reaction to a condition or event. : the capacity for introspection and the ability to reconcile oneself as an individual separate from the environment and other individuals. : an acute sense of self-awareness. It is a preoccupation with oneself, as opposed to the philosophical state of self-awareness, which is the awareness that one exists as an individual being although some writers use both terms interchangeably or synonymously.  : the ability to be aware (feel, perceive, or be conscious) of one's surroundings or to have subjective experiences. Sentience is a minimalistic way of defining consciousness, which is otherwise commonly used to collectively describe sentience plus other characteristics of the mind. : often defined as wisdom, or the ability of an organism or entity to act with appropriate judgment, a mental faculty which is a component of intelligence or alternatively may be considered an additional faculty, apart from intelligence, with its own properties. : individual instances of subjective, conscious experience.
Sentience (the ability to feel, perceive, or to experience subjectivity) is not the same as self-awareness (being aware of oneself as an individual). The mirror test is sometimes considered to be an operational test for self-awareness, and the handful of animals that have passed it are often considered to be self-aware.   It remains debatable whether recognition of one's mirror image can be properly construed to imply full self-awareness,  particularly given that robots are being constructed which appear to pass the test.  
Much has been learned in neuroscience about correlations between brain activity and subjective, conscious experiences, and many suggest that neuroscience will ultimately explain consciousness ". consciousness is a biological process that will eventually be explained in terms of molecular signaling pathways used by interacting populations of nerve cells. ".  However, this view has been criticized because consciousness has yet to be shown to be a process,  and the so-called "hard problem" of relating consciousness directly to brain activity remains elusive. 
Since Descartes's proposal of dualism, it became a general consensus that the mind had become a matter of philosophy and that science was not able to penetrate the issue of consciousness – that consciousness was outside of space and time. However, in recent decades many scholars have begun to move toward a science of consciousness. Antonio Damasio and Gerald Edelman are two neuroscientists who have led the move to neural correlates of the self and of consciousness. Damasio has demonstrated that emotions and their biological foundation play a critical role in high level cognition,   and Edelman has created a framework for analyzing consciousness through a scientific outlook. The current problem consciousness researchers face involves explaining how and why consciousness arises from neural computation.   In his research on this problem, Edelman has developed a theory of consciousness, in which he has coined the terms primary consciousness and secondary consciousness.  
Eugene Linden, author of The Parrot's Lament suggests there are many examples of animal behavior and intelligence that surpass what people would suppose to be the boundary of animal consciousness. Linden contends that in many of these documented examples, a variety of animal species exhibit behavior that can only be attributed to emotion, and to a level of consciousness that we would normally ascribe only to our own species. 
Philosopher Daniel Dennett counters that:
Consciousness requires a certain kind of informational organization that does not seem to be 'hard-wired' in humans, but is instilled by human culture. Moreover, consciousness is not a black-or-white, all-or-nothing type of phenomenon, as is often assumed. The differences between humans and other species are so great that speculations about animal consciousness seem ungrounded. Many authors simply assume that an animal like a bat has a point of view, but there seems to be little interest in exploring the details involved. 
Consciousness in mammals (including humans) is an aspect of the mind generally thought to comprise qualities such as subjectivity, sentience, and the ability to perceive the relationship between oneself and one's environment. It is a subject of much research in philosophy of mind, psychology, neuroscience, and cognitive science. Some philosophers divide consciousness into phenomenal consciousness, which is subjective experience itself, and access consciousness, which refers to the global availability of information to processing systems in the brain.  Phenomenal consciousness has many different experienced qualities, often referred to as qualia. Phenomenal consciousness is usually consciousness of something or about something, a property known as intentionality in philosophy of mind. 
In humans, there are three common methods of studying consciousness, i.e. verbal report, behavioural demonstrations, and neural correlation with conscious activity. Unfortunately these can only be generalized to non-human taxa with varying degrees of difficulty.  While animals cannot speak their minds, a new study employed a very unique way that enabled neuroscientists to separate conscious awareness from non-conscious perception in animals.  In this study conducted in rhesus monkeys, Ben-Haim and his team used a process dissociation approach that predicted opposite behavioral outcomes towards the two modes of perception. They found that monkeys displayed the very same opposite behavioral outcomes as did humans when they were aware vs. unaware of the stimuli presented.
Mirror test Edit
The sense in which animals (or human infants) can be said to have consciousness or a self-concept has been hotly debated it is often referred to as the debate over animal minds. The best known research technique in this area is the mirror test devised by Gordon G. Gallup, in which the skin of an animal (or human infant) is marked, while it is asleep or sedated, with a mark that cannot be seen directly but is visible in a mirror. The animal is then allowed to see its reflection in a mirror if the animal spontaneously directs grooming behaviour towards the mark, that is taken as an indication that it is aware of itself.   Over the past 30 years, many studies have found evidence that animals recognise themselves in mirrors. Self-awareness by this criterion has been reported for:
- Land mammals: apes (chimpanzees, bonobos, orangutans and gorillas)  and elephants. 
- Cetaceans: bottlenose dolphins, killer whales and possibly false killer whales. 
- Birds: magpies, pigeons (can pass the mirror test after training in the prerequisite behaviors). 
Until recently it was thought that self-recognition was absent from animals without a neocortex, and was restricted to mammals with large brains and well developed social cognition. However, in 2008 a study of self-recognition in corvids reported significant results for magpies. Mammals and birds inherited the same brain components from their last common ancestor nearly 300 million years ago, and have since independently evolved and formed significantly different brain types. The results of the mirror and mark tests showed that neocortex-less magpies are capable of understanding that a mirror image belongs to their own body. The findings show that magpies respond in the mirror and mark test in a manner similar to apes, dolphins and elephants. This is a remarkable capability that, although not fully concrete in its determination of self-recognition, is at least a prerequisite of self-recognition. This is not only of interest regarding the convergent evolution of social intelligence it is also valuable for an understanding of the general principles that govern cognitive evolution and their underlying neural mechanisms. The magpies were chosen to study based on their empathy/lifestyle, a possible precursor for their ability of self-awareness.  However even in the chimpanzee, the species most studied and with the most convincing findings, clear-cut evidence of self-recognition is not obtained in all individuals tested. Occurrence is about 75% in young adults and considerably less in young and old individuals.  For monkeys, non-primate mammals, and in a number of bird species, exploration of the mirror and social displays were observed. However, hints at mirror-induced self-directed behavior have been obtained. 
The mirror test has attracted controversy among some researchers because it is entirely focused on vision, the primary sense in humans, while other species rely more heavily on other senses such as the olfactory sense in dogs.    A study in 2015 showed that the "sniff test of self-recognition (STSR)" provides evidence of self-awareness in dogs. 
Another approach to determine whether a non-human animal is conscious derives from passive speech research with a macaw (see Arielle). Some researchers propose that by passively listening to an animal's voluntary speech, it is possible to learn about the thoughts of another creature and to determine that the speaker is conscious. This type of research was originally used to investigate a child's crib speech by Weir (1962) and in investigations of early speech in children by Greenfield and others (1976).
Zipf's law might be able to be used to indicate if a given dataset of animal communication indicate an intelligent natural language. Some researchers have used this algorithm to study bottlenose dolphin language. 
Pain or suffering Edit
Further arguments revolve around the ability of animals to feel pain or suffering. Suffering implies consciousness. If animals can be shown to suffer in a way similar or identical to humans, many of the arguments against human suffering could then, presumably, be extended to animals. Others have argued that pain can be demonstrated by adverse reactions to negative stimuli that are non-purposeful or even maladaptive.  One such reaction is transmarginal inhibition, a phenomenon observed in humans and some animals akin to mental breakdown.
Carl Sagan, the American cosmologist, points to reasons why humans have had a tendency to deny animals can suffer:
Humans – who enslave, castrate, experiment on, and fillet other animals – have had an understandable penchant for pretending animals do not feel pain. A sharp distinction between humans and 'animals' is essential if we are to bend them to our will, make them work for us, wear them, eat them – without any disquieting tinges of guilt or regret. It is unseemly of us, who often behave so unfeelingly toward other animals, to contend that only humans can suffer. The behavior of other animals renders such pretensions specious. They are just too much like us. 
John Webster, a professor of animal husbandry at Bristol, argues:
People have assumed that intelligence is linked to the ability to suffer and that because animals have smaller brains they suffer less than humans. That is a pathetic piece of logic, sentient animals have the capacity to experience pleasure and are motivated to seek it, you only have to watch how cows and lambs both seek and enjoy pleasure when they lie with their heads raised to the sun on a perfect English summer's day. Just like humans. 
However, there is no agreement where the line should be drawn between organisms that can feel pain and those that cannot. Justin Leiber, a philosophy professor at Oxford University writes that:
Montaigne is ecumenical in this respect, claiming consciousness for spiders and ants, and even writing of our duties to trees and plants. Singer and Clarke agree in denying consciousness to sponges. Singer locates the distinction somewhere between the shrimp and the oyster. He, with rather considerable convenience for one who is thundering hard accusations at others, slides by the case of insects and spiders and bacteria, they pace Montaigne, apparently and rather conveniently do not feel pain. The intrepid Midgley, on the other hand, seems willing to speculate about the subjective experience of tapeworms . Nagel . appears to draw the line at flounders and wasps, though more recently he speaks of the inner life of cockroaches. 
There are also some who reject the argument entirely, arguing that although suffering animals feel anguish, a suffering plant also struggles to stay alive (albeit in a less visible way). In fact, no living organism 'wants' to die for another organism's sustenance. In an article written for The New York Times, Carol Kaesuk Yoon argues that:
When a plant is wounded, its body immediately kicks into protection mode. It releases a bouquet of volatile chemicals, which in some cases have been shown to induce neighboring plants to pre-emptively step up their own chemical defenses and in other cases to lure in predators of the beasts that may be causing the damage to the plants. Inside the plant, repair systems are engaged and defenses are mounted, the molecular details of which scientists are still working out, but which involve signaling molecules coursing through the body to rally the cellular troops, even the enlisting of the genome itself, which begins churning out defense-related proteins . If you think about it, though, why would we expect any organism to lie down and die for our dinner? Organisms have evolved to do everything in their power to avoid being extinguished. How long would any lineage be likely to last if its members effectively didn't care if you killed them? 
Cognitive bias and emotion Edit
Cognitive bias in animals is a pattern of deviation in judgment, whereby inferences about other animals and situations may be drawn in an illogical fashion.  Individuals create their own "subjective social reality" from their perception of the input.  It refers to the question "Is the glass half empty or half full?", used as an indicator of optimism or pessimism. Cognitive biases have been shown in a wide range of species including rats, dogs, rhesus macaques, sheep, chicks, starlings and honeybees.   
The neuroscientist Joseph LeDoux advocates avoiding terms derived from human subjective experience when discussing brain functions in animals.  For example, the common practice of calling brain circuits that detect and respond to threats "fear circuits" implies that these circuits are responsible for feelings of fear. LeDoux argues that Pavlovian fear conditioning should be renamed Pavlovian threat conditioning to avoid the implication that "fear" is being acquired in rats or humans.  Key to his theoretical change is the notion of survival functions mediated by survival circuits, the purpose of which is to keep organisms alive rather than to make emotions. For example, defensive survival circuits exist to detect and respond to threats. While all organisms can do this, only organisms that can be conscious of their own brain's activities can feel fear. Fear is a conscious experience and occurs the same way as any other kind of conscious experience: via cortical circuits that allow attention to certain forms of brain activity. LeDoux argues the only differences between an emotional and non-emotion state of consciousness are the underlying neural ingredients that contribute to the state.  
Neuroscience is the scientific study of the nervous system.  It is a highly active interdisciplinary science that collaborates with many other fields. The scope of neuroscience has broadened recently to include molecular, cellular, developmental, structural, functional, evolutionary, computational, and medical aspects of the nervous system. Theoretical studies of neural networks are being complemented with techniques for imaging sensory and motor tasks in the brain. According to a 2008 paper, neuroscience explanations of psychological phenomena currently have a "seductive allure", and "seem to generate more public interest" than explanations which do not contain neuroscientific information.  They found that subjects who were not neuroscience experts "judged that explanations with logically irrelevant neuroscience information were more satisfying than explanations without. 
Neural correlates Edit
The neural correlates of consciousness constitute the minimal set of neuronal events and mechanisms sufficient for a specific conscious percept.  Neuroscientists use empirical approaches to discover neural correlates of subjective phenomena.  The set should be minimal because, if the brain is sufficient to give rise to any given conscious experience, the question is which of its components is necessary to produce it.
Visual sense and representation was reviewed in 1998 by Francis Crick and Christof Koch. They concluded sensory neuroscience can be used as a bottom-up approach to studying consciousness, and suggested experiments to test various hypotheses in this research stream. 
A feature that distinguishes humans from most animals is that we are not born with an extensive repertoire of behavioral programs that would enable us to survive on our own ("physiological prematurity"). To compensate for this, we have an unmatched ability to learn, i.e., to consciously acquire such programs by imitation or exploration. Once consciously acquired and sufficiently exercised, these programs can become automated to the extent that their execution happens beyond the realms of our awareness. Take, as an example, the incredible fine motor skills exerted in playing a Beethoven piano sonata or the sensorimotor coordination required to ride a motorcycle along a curvy mountain road. Such complex behaviors are possible only because a sufficient number of the subprograms involved can be executed with minimal or even suspended conscious control. In fact, the conscious system may actually interfere somewhat with these automated programs. 
The growing ability of neuroscientists to manipulate neurons using methods from molecular biology in combination with optical tools depends on the simultaneous development of appropriate behavioural assays and model organisms amenable to large-scale genomic analysis and manipulation.  A combination of such fine-grained neuronal analysis in animals with ever more sensitive psychophysical and brain imaging techniques in humans, complemented by the development of a robust theoretical predictive framework, will hopefully lead to a rational understanding of consciousness.
The neocortex is a part of the brain of mammals. It consists of the grey matter, or neuronal cell bodies and unmyelinated fibers, surrounding the deeper white matter (myelinated axons) in the cerebrum. The neocortex is smooth in rodents and other small mammals, whereas in primates and other larger mammals it has deep grooves and wrinkles. These folds increase the surface area of the neocortex considerably without taking up too much more volume. Also, neurons within the same wrinkle have more opportunity for connectivity, while neurons in different wrinkles have less opportunity for connectivity, leading to compartmentalization of the cortex. The neocortex is divided into frontal, parietal, occipital, and temporal lobes, which perform different functions. For example, the occipital lobe contains the primary visual cortex, and the temporal lobe contains the primary auditory cortex. Further subdivisions or areas of neocortex are responsible for more specific cognitive processes. The neocortex is the newest part of the cerebral cortex to evolve (hence the prefix "neo") the other parts of the cerebral cortex are the paleocortex and archicortex, collectively known as the allocortex. In humans, 90% of the cerebral cortex is neocortex.
Researchers have argued that consciousness in mammals arises in the neocortex, and therefore cannot arise in animals which lack a neocortex. For example, Rose argued in 2002 that the "fishes have nervous systems that mediate effective escape and avoidance responses to noxious stimuli, but, these responses must occur without a concurrent, human-like awareness of pain, suffering or distress, which depend on separately evolved neocortex."  Recently that view has been challenged, and many researchers now believe that animal consciousness can arise from homologous subcortical brain networks. 
Attention is the cognitive process of selectively concentrating on one aspect of the environment while ignoring other things. Attention has also been referred to as the allocation of processing resources.  Attention also has variations amongst cultures. Voluntary attention develops in specific cultural and institutional contexts through engagement in cultural activities with more competent community members. 
Most experiments show that one neural correlate of attention is enhanced firing. If a neuron has a certain response to a stimulus when the animal is not attending to the stimulus, then when the animal does attend to the stimulus, the neuron's response will be enhanced even if the physical characteristics of the stimulus remain the same. In many cases attention produces changes in the EEG. Many animals, including humans, produce gamma waves (40–60 Hz) when focusing attention on a particular object or activity. 
Extended consciousness Edit
Extended consciousness is an animal's autobiographical self-perception. It is thought to arise in the brains of animals which have a substantial capacity for memory and reason. It does not necessarily require language. The perception of a historic and future self arises from a stream of information from the immediate environment and from neural structures related to memory. The concept was popularised by Antonio Damasio and is used in biological psychology. Extended consciousness is said to arise in structures in the human brain described as image spaces and dispositional spaces. Image spaces imply areas where sensory impressions of all types are processed, including the focused awareness of the core consciousness. Dispositional spaces include convergence zones, which are networks in the brain where memories are processed and recalled, and where knowledge is merged with immediate experience.  
Metacognition is defined as "cognition about cognition", or "knowing about knowing."  It can take many forms it includes knowledge about when and how to use particular strategies for learning or for problem solving.  It has been suggested that metacognition in some animals provides evidence for cognitive self-awareness.  There are generally two components of metacognition: knowledge about cognition, and regulation of cognition.  Writings on metacognition can be traced back at least as far as De Anima and the Parva Naturalia of the Greek philosopher Aristotle.  Metacognologists believe that the ability to consciously think about thinking is unique to sapient species and indeed is one of the definitions of sapience. [ citation needed ] There is evidence that rhesus monkeys and apes can make accurate judgments about the strengths of their memories of fact and monitor their own uncertainty,  while attempts to demonstrate metacognition in birds have been inconclusive.  A 2007 study provided some evidence for metacognition in rats,    but further analysis suggested that they may have been following simple operant conditioning principles,  or a behavioral economic model. 
Mirror neurons Edit
Mirror neurons are neurons that fire both when an animal acts and when the animal observes the same action performed by another.    Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in primate and other species including birds. The function of the mirror system is a subject of much speculation. Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception action coupling (see the common coding theory).  They argue that mirror neurons may be important for understanding the actions of other people, and for learning new skills by imitation. Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills,   while others relate mirror neurons to language abilities.  Neuroscientists such as Marco Iacoboni (UCLA) have argued that mirror neuron systems in the human brain help us understand the actions and intentions of other people. In a study published in March 2005 Iacoboni and his colleagues reported that mirror neurons could discern if another person who was picking up a cup of tea planned to drink from it or clear it from the table. In addition, Iacoboni and a number of other researchers have argued that mirror neurons are the neural basis of the human capacity for emotions such as empathy.   Vilayanur S. Ramachandran has speculated that mirror neurons may provide the neurological basis of self-awareness.  
Evolutionary psychology Edit
Consciousness is likely an evolved adaptation since it meets George Williams' criteria of species universality, complexity,  and functionality, and it is a trait that apparently increases fitness.  Opinions are divided as to where in biological evolution consciousness emerged and about whether or not consciousness has survival value. It has been argued that consciousness emerged (i) exclusively with the first humans, (ii) exclusively with the first mammals, (iii) independently in mammals and birds, or (iv) with the first reptiles.  Donald Griffin suggests in his book Animal Minds a gradual evolution of consciousness.  Each of these scenarios raises the question of the possible survival value of consciousness.
In his paper "Evolution of consciousness," John Eccles argues that special anatomical and physical adaptations of the mammalian cerebral cortex gave rise to consciousness.  In contrast, others have argued that the recursive circuitry underwriting consciousness is much more primitive, having evolved initially in pre-mammalian species because it improves the capacity for interaction with both social and natural environments by providing an energy-saving "neutral" gear in an otherwise energy-expensive motor output machine.  Once in place, this recursive circuitry may well have provided a basis for the subsequent development of many of the functions that consciousness facilitates in higher organisms, as outlined by Bernard J. Baars.  Richard Dawkins suggested that humans evolved consciousness in order to make themselves the subjects of thought.  Daniel Povinelli suggests that large, tree-climbing apes evolved consciousness to take into account one's own mass when moving safely among tree branches.  Consistent with this hypothesis, Gordon Gallup found that chimps and orangutans, but not little monkeys or terrestrial gorillas, demonstrated self-awareness in mirror tests. 
The concept of consciousness can refer to voluntary action, awareness, or wakefulness. However, even voluntary behaviour involves unconscious mechanisms. Many cognitive processes take place in the cognitive unconscious, unavailable to conscious awareness. Some behaviours are conscious when learned but then become unconscious, seemingly automatic. Learning, especially implicitly learning a skill, can take place outside of consciousness. For example, plenty of people know how to turn right when they ride a bike, but very few can accurately explain how they actually do so. 
Neural Darwinism Edit
Neural Darwinism is a large scale theory of brain function initially proposed in 1978 by the American biologist Gerald Edelman.  Edelman distinguishes between what he calls primary and secondary consciousness:
- : is the ability, found in humans and some animals, to integrate observed events with memory to create an awareness of the present and immediate past of the world around them. This form of consciousness is also sometimes called "sensory consciousness". Put another way, primary consciousness is the presence of various subjective sensory contents of consciousness such as sensations, perceptions, and mental images. For example, primary consciousness includes a person's experience of the blueness of the ocean, a bird's song, and the feeling of pain. Thus, primary consciousness refers to being mentally aware of things in the world in the present without any sense of past and future it is composed of mental images bound to a time around the measurable present.  : is an individual's accessibility to their history and plans. The concept is also loosely and commonly associated with having awareness of one's own consciousness. The ability allows its possessors to go beyond the limits of the remembered present of primary consciousness. 
Primary consciousness can be defined as simple awareness that includes perception and emotion. As such, it is ascribed to most animals. By contrast, secondary consciousness depends on and includes such features as self-reflective awareness, abstract thinking, volition and metacognition.  
Edelman's theory focuses on two nervous system organizations: the brainstem and limbic systems on one side and the thalamus and cerebral cortex on the other side. The brain stem and limbic system take care of essential body functioning and survival, while the thalamocortical system receives signals from sensory receptors and sends out signals to voluntary muscles such as those of the arms and legs. The theory asserts that the connection of these two systems during evolution helped animals learn adaptive behaviors. 
Other scientists have argued against Edelman's theory, instead suggesting that primary consciousness might have emerged with the basic vegetative systems of the brain. That is, the evolutionary origin might have come from sensations and primal emotions arising from sensors and receptors, both internal and surface, signaling that the well-being of the creature was immediately threatened—for example, hunger for air, thirst, hunger, pain, and extreme temperature change. This is based on neurological data showing the thalamic, hippocampal, orbitofrontal, insula, and midbrain sites are the key to consciousness of thirst.  These scientists also point out that the cortex might not be as important to primary consciousness as some neuroscientists have believed.  Evidence of this lies in the fact that studies show that systematically disabling parts of the cortex in animals does not remove consciousness. Another study found that children born without a cortex are conscious. Instead of cortical mechanisms, these scientists emphasize brainstem mechanisms as essential to consciousness.  Still, these scientists concede that higher order consciousness does involve the cortex and complex communication between different areas of the brain.
While animals with primary consciousness have long-term memory, they lack explicit narrative, and, at best, can only deal with the immediate scene in the remembered present. While they still have an advantage over animals lacking such ability, evolution has brought forth a growing complexity in consciousness, particularly in mammals. Animals with this complexity are said to have secondary consciousness. Secondary consciousness is seen in animals with semantic capabilities, such as the four great apes. It is present in its richest form in the human species, which is unique in possessing complex language made up of syntax and semantics. In considering how the neural mechanisms underlying primary consciousness arose and were maintained during evolution, it is proposed that at some time around the divergence of reptiles into mammals and then into birds, the embryological development of large numbers of new reciprocal connections allowed rich re-entrant activity to take place between the more posterior brain systems carrying out perceptual categorization and the more frontally located systems responsible for value-category memory.  The ability of an animal to relate a present complex scene to its own previous history of learning conferred an adaptive evolutionary advantage. At much later evolutionary epochs, further re-entrant circuits appeared that linked semantic and linguistic performance to categorical and conceptual memory systems. This development enabled the emergence of secondary consciousness.  
Ursula Voss of the Universität Bonn believes that the theory of protoconsciousness  may serve as adequate explanation for self-recognition found in birds, as they would develop secondary consciousness during REM sleep.  She added that many types of birds have very sophisticated language systems. Don Kuiken of the University of Alberta finds such research interesting as well as if we continue to study consciousness with animal models (with differing types of consciousness), we would be able to separate the different forms of reflectiveness found in today's world. 
For the advocates of the idea of a secondary consciousness, self-recognition serves as a critical component and a key defining measure. What is most interesting then, is the evolutionary appeal that arises with the concept of self-recognition. In non-human species and in children, the mirror test (see above) has been used as an indicator of self-awareness.
In 2012, a group of neuroscientists attending a conference on "Consciousness in Human and non-Human Animals" at the University of Cambridge in the UK, signed the Cambridge Declaration on Consciousness (see box on the right).  
In the accompanying text they "unequivocally" asserted: 
- "The field of Consciousness research is rapidly evolving. Abundant new techniques and strategies for human and non-human animal research have been developed. Consequently, more data is becoming readily available, and this calls for a periodic reevaluation of previously held preconceptions in this field. Studies of non-human animals have shown that homologous brain circuits correlated with conscious experience and perception can be selectively facilitated and disrupted to assess whether they are in fact necessary for those experiences. Moreover, in humans, new non-invasive techniques are readily available to survey the correlates of consciousness." 
- "The neural substrates of emotions do not appear to be confined to cortical structures. In fact, subcortical neural networks aroused during affective states in humans are also critically important for generating emotional behaviors in animals. Artificial arousal of the same brain regions generates corresponding behavior and feeling states in both humans and non-human animals. Wherever in the brain one evokes instinctual emotional behaviors in non-human animals, many of the ensuing behaviors are consistent with experienced feeling states, including those internal states that are rewarding and punishing. Deep brain stimulation of these systems in humans can also generate similar affective states. Systems associated with affect are concentrated in subcortical regions where neural homologies abound. Young human and non-human animals without neocortices retain these brain-mind functions. Furthermore, neural circuits supporting behavioral/electrophysiological states of attentiveness, sleep and decision making appear to have arisen in evolution as early as the invertebrate radiation, being evident in insects and cephalopod mollusks (e.g., octopus)." 
- "Birds appear to offer, in their behavior, neurophysiology, and neuroanatomy a striking case of parallel evolution of consciousness. Evidence of near human-like levels of consciousness has been most dramatically observed in grey parrots. Mammalian and avian emotional networks and cognitive microcircuitries appear to be far more homologous than previously thought. Moreover, certain species of birds have been found to exhibit neural sleep patterns similar to those of mammals, including REM sleep and, as was demonstrated in zebra finches, neurophysiological patterns previously thought to require a mammalian neocortex. Magpies in particular have been shown to exhibit striking similarities to humans, great apes, dolphins, and elephants in studies of mirror self-recognition." 
- "In humans, the effect of certain hallucinogens appears to be associated with a disruption in cortical feedforward and feedback processing. Pharmacological interventions in non-human animals with compounds known to affect conscious behavior in humans can lead to similar perturbations in behavior in non-human animals. In humans, there is evidence to suggest that awareness is correlated with cortical activity, which does not exclude possible contributions by subcortical or early cortical processing, as in visual awareness. Evidence that human and non-human animal emotional feelings arise from homologous subcortical brain networks provide compelling evidence for evolutionarily shared primal affective qualia." 
A common image is the scala naturae, the ladder of nature on which animals of different species occupy successively higher rungs, with humans typically at the top.  A more useful approach has been to recognize that different animals may have different kinds of cognitive processes, which are better understood in terms of the ways in which they are cognitively adapted to their different ecological niches, than by positing any kind of hierarchy.  
Dogs were previously listed as non-self-aware animals. Traditionally, self-consciousness was evaluated via the mirror test. But dogs and many other animals, are not (as) visually oriented.   A 2015 study claims that the "sniff test of self-recognition" (STSR) provides significant evidence of self-awareness in dogs, and could play a crucial role in showing that this capacity is not a specific feature of only great apes, humans and a few other animals, but it depends on the way in which researchers try to verify it. According to the biologist Roberto Cazzolla Gatti (who published the study), "the innovative approach to test the self-awareness with a smell test highlights the need to shift the paradigm of the anthropocentric idea of consciousness to a species-specific perspective".   This study has been confirmed by another study. 
Grey parrots Edit
Research with captive grey parrots, especially Irene Pepperberg's work with an individual named Alex, has demonstrated they possess the ability to associate simple human words with meanings, and to intelligently apply the abstract concepts of shape, colour, number, zero-sense, etc. According to Pepperberg and other scientists, they perform many cognitive tasks at the level of dolphins, chimpanzees, and even human toddlers.  Another notable African grey is N'kisi, which in 2004 was said to have a vocabulary of over 950 words which she used in creative ways.  For example, when Jane Goodall visited N'kisi in his New York home, he greeted her with "Got a chimp?" because he had seen pictures of her with chimpanzees in Africa. 
In 2011, research led by Dalila Bovet of Paris West University Nanterre La Défense, demonstrated grey parrots were able to coordinate and collaborate with each other to an extent. They were able to solve problems such as two birds having to pull strings at the same time to obtain food. In another example, one bird stood on a perch to release a food-laden tray, while the other pulled the tray out from the test apparatus. Both would then feed. The birds were observed waiting for their partners to perform the necessary actions so their behaviour could be synchronized. The parrots appeared to express individual preferences as to which of the other test birds they would work with. 
It was recently thought that self-recognition was restricted to mammals with large brains and highly evolved social cognition, but absent from animals without a neocortex. However, in 2008, an investigation of self-recognition in corvids was conducted revealing the ability of self-recognition in the magpie. Mammals and birds inherited the same brain components from their last common ancestor nearly 300 million years ago, and have since independently evolved and formed significantly different brain types. The results of the mirror test showed that although magpies do not have a neocortex, they are capable of understanding that a mirror image belongs to their own body. The findings show that magpies respond in the mirror test in a manner similar to apes, dolphins, killer whales, pigs and elephants. This is a remarkable capability that, although not fully concrete in its determination of self-recognition, is at least a prerequisite of self-recognition. This is not only of interest regarding the convergent evolution of social intelligence, it is also valuable for an understanding of the general principles that govern cognitive evolution and their underlying neural mechanisms. The magpies were chosen to study based on their empathy/lifestyle, a possible precursor for their ability of self-awareness. 
A 2020 study found that carrion crows show a neuronal response that correlates with their perception of a stimulus, which they argue to be an empirical marker of (avian) sensory consciousness – the conscious perception of sensory input – in the crows which do not have a cerebral cortex. The study thereby substantiates the theory that conscious perception does not require a cerebral cortex and that the basic foundations for it – and possibly for human-type consciousness – may have evolved before the last common ancestor >320 Mya or independently in birds.   A related study showed that the birds' pallium's neuroarchitecture is reminiscent of the mammalian cortex. 
Octopuses are highly intelligent, possibly more so than any other order of invertebrates. The level of their intelligence and learning capability are debated,     but maze and problem-solving studies show they have both short- and long-term memory. Octopus have a highly complex nervous system, only part of which is localized in their brain. Two-thirds of an octopus's neurons are found in the nerve cords of their arms. Octopus arms show a variety of complex reflex actions that persist even when they have no input from the brain.  Unlike vertebrates, the complex motor skills of octopuses are not organized in their brain using an internal somatotopic map of their body, instead using a non-somatotopic system unique to large-brained invertebrates.  Some octopuses, such as the mimic octopus, move their arms in ways that emulate the shape and movements of other sea creatures.
In laboratory studies, octopuses can easily be trained to distinguish between different shapes and patterns. They reportedly use observational learning,  although the validity of these findings is contested.   Octopuses have also been observed to play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them.  Octopuses often escape from their aquarium and sometimes enter others. They have boarded fishing boats and opened holds to eat crabs.  At least four specimens of the veined octopus (Amphioctopus marginatus) have been witnessed retrieving discarded coconut shells, manipulating them, and then reassembling them to use as shelter.  
Traditional shamanistic cultures speak of animal spirits and the consciousness of animals.   In India, Jains consider all the jivas (living organisms including plants, animals and insects) as conscious. According to Jain scriptures, even nigoda (microscopic creatures) possess high levels of consciousness and have decision making abilities. [ citation needed ] While pre-modern Christianity held that animals have no souls,   in modern times most Christian denominations believe that animals do indeed have souls. 
Applying the term homosexual to animals
The term homosexual was coined by Karl-Maria Kertbeny in 1868 to describe same-sex sexual attraction and sexual behavior in humans.  Its use in animal studies has been controversial for two main reasons: animal sexuality and motivating factors have been and remain poorly understood, and the term has strong cultural implications in western society that are irrelevant for species other than humans.  Thus homosexual behavior has been given a number of terms over the years. According to Bruce Bagemihl, when describing animals, the term homosexual is preferred over gay, lesbian, and other terms currently in use, as these are seen as even more bound to human homosexuality. 
Homosexual: in animals, this has been used to refer to same-sex behavior that is not sexual in character (e.g. ‘homosexual tandem running’ in termites), same-sex courtship or copulatory behavior occurring over a short period of time (e.g. ‘homosexual mounting’ in cockroaches and rams) or long-term pair bonds between same-sex partners that might involve any combination of courting, copulating, parenting and affectional behaviors (e.g. ‘homosexual pair bonds’ in gulls). In humans, the term is used to describe individual sexual behaviors as well as long-term relationships, but in some usages connotes a gay or lesbian social identity. Scientific writing would benefit from reserving this anthropomorphic term for humans and not using it to describe behavior in other animals, because of its deeply rooted context in human society.
Animal preference and motivation is always inferred from behavior. In wild animals, researchers will as a rule not be able to map the entire life of an individual, and must infer from frequency of single observations of behavior. The correct usage of the term homosexual is that an animal exhibits homosexual behavior or even same-sex sexual behavior however, this article conforms to the usage by modern research,     [ page needed ]  applying the term homosexuality to all sexual behavior (copulation, genital stimulation, mating games and sexual display behavior) between animals of the same sex. In most instances, it is presumed that the homosexual behavior is but part of the animal's overall sexual behavioral repertoire, making the animal "bisexual" rather than "homosexual" as the terms are commonly understood in humans.  [ page needed ]
The observation of homosexual behavior in animals can be seen as both an argument for and against the acceptance of homosexuality in humans, and has been used especially against the claim that it is a peccatum contra naturam ("sin against nature"). For instance, homosexuality in animals was cited by the American Psychiatric Association and other groups in their amici curiae brief to the United States Supreme Court in Lawrence v. Texas, which ultimately struck down the sodomy laws of 14 states.  
A majority of the research available concerning homosexual behavior in animals lacks specification between animals that exclusively exhibit same-sex tendencies and those that participate in heterosexual and homosexual mating activities interchangeably. This lack of distinction has led to differing opinions and conflicting interpretations of collected data amongst scientists and researchers. For instance, Bruce Bagemihl, author of the book Biological Exuberence: Animal Homosexuality and Natural Diversity, emphasizes that there are no anatomical or endocrinological differences between exclusively homosexual and exclusively heterosexual animal pairs.  [ page needed ] However, if the definition of "homosexual behavior" is made to include animals that participate in both same-sex and opposite-sex mating activities, hormonal differences have been documented among key sex hormones, such as testosterone and estradiol, when compared to those who participate solely in heterosexual mating. 
Many of the animals used in laboratory-based studies of homosexuality do not appear to spontaneously exhibit these tendencies often in the wild. Such behavior is often elicited and exaggerated by the researcher during experimentation through the destruction of a portion of brain tissue, or by exposing the animal to high levels of steroid hormones prenatally.  [ page needed ] Information gathered from these studies is limited when applied to spontaneously occurring same-sex behavior in animals outside of the laboratory. 
Homosexual behaviour in animals has been discussed since classical antiquity. The earliest written mention of animal homosexuality appears to date back to 2,300 years ago, when Aristotle (384–322 BC) described copulation between pigeons, partridges and quails of the same sex.  The Hieroglyphics of Horapollo, written in the 4th century AD by the Egyptian writer Horapollo, mentions "hermaphroditism" in hyenas and homosexuality in partridges.  The first review of animal homosexuality was written by the zoologist Ferdinand Karsch-Haack in 1900. 
Until recent times [ when? ] , the presence of same-sex sexual behavior was not "officially" observed on a large scale, possibly due to observer bias caused by social attitudes to same-sex sexual behavior,  innocent confusion, lack of interest, distaste, scientists fearing loss of their grants or even from a fear of "being ridiculed by their colleagues".   Georgetown University biologist Janet Mann states "Scientists who study the topic are often accused of trying to forward an agenda, and their work can come under greater scrutiny than that of their colleagues who study other topics."  They also noted "Not every sexual act has a reproductive function . that's true of humans and non-humans."  Studies have demonstrated homosexual behavior in a number of species, but the true extent of homosexuality in animals is not known.
Some researchers believe this behavior to have its origin in male social organization and social dominance, similar to the dominance traits shown in prison sexuality. Others, particularly Bagemihl, Joan Roughgarden, Thierry Lodé  and Paul Vasey suggest the social function of sex (both homosexual and heterosexual) is not necessarily connected to dominance, but serves to strengthen alliances and social ties within a flock. While reports on many such mating scenarios are still only anecdotal, a growing body of scientific work confirms that permanent homosexuality occurs not only in species with permanent pair bonds,  but also in non-monogamous species like sheep. One report on sheep found that 8% of rams exhibited homosexual preferences—that is, even when given a choice, they chose male over female partners.  In fact, apparent homosexual individuals are known from all of the traditional domestic species, from sheep, cattle and horses to cats, dogs and budgerigars.  [ page needed ]
A definite physiological explanation or reason for homosexual activity in animal species has not been agreed upon by researchers in the field. Numerous scholars are of the opinion that varying levels (either higher or lower) of the sex hormones in the animal,  in addition to the size of the animal's gonads,  play a direct role in the sexual behavior and preference exhibited by that animal. Others firmly argue no evidence to support these claims exists when comparing animals of a specific species exhibiting homosexual behavior exclusively and those that do not. Ultimately, empirical support from comprehensive endocrinological studies exist for both interpretations.   Researchers found no evidence of differences in the measurements of the gonads, or the levels of the sex hormones of exclusively homosexual western gulls and ring-billed gulls. 
Additional studies pertaining to hormone involvement in homosexual behavior indicate that when administering treatments of testosterone and estradiol to female heterosexual animals, the elevated hormone levels increase the likelihood of homosexual behavior. Additionally, boosting the levels of sex hormones during an animal's pregnancy appears to increase the likelihood of it birthing a homosexual offspring. 
Researchers found that disabling the fucose mutarotase (FucM) gene in laboratory mice – which influences the levels of estrogen to which the brain is exposed – caused the female mice to behave as if they were male as they grew up. "The mutant female mouse underwent a slightly altered developmental programme in the brain to resemble the male brain in terms of sexual preference" said Professor Chankyu Park of the Korea Advanced Institute of Science and Technology in Daejon, South Korea, who led the research. His findings were published in the BMC Genetics journal on July 7, 2010.   Another study found that by manipulating a gene in fruit flies (Drosophila), homosexual behavior appeared to have been induced. However, in addition to homosexual behavior, several abnormal behaviors were also exhibited apparently due to this mutation. 
In March 2011, research showed that serotonin is involved in the mechanism of sexual orientation of mice.   A study conducted on fruit flies found that inhibiting the dopamine neurotransmitter inhibited lab-induced homosexual behavior. 
An estimated one-quarter of all black swan pairings are of males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. The males spent time in each other's society, guarded the common territory, performed greeting ceremonies before each other, and (in the reproductive period) pre-marital rituals, and if one of the birds tried to sit on the other, an intense fight began.   More of their cygnets survive to adulthood than those of different-sex pairs, possibly due to their superior ability to defend large portions of land. The same reasoning has been applied to male flamingo pairs raising chicks.  
Female albatross, on the north-western tip of the island of Oahu, Hawaii, form pairs for co-growing offspring. On the observed island, the number of females considerably exceeds the number of males (59% N=102/172), so 31% of females, after mating with males, create partnerships for hatching and feeding chicks. Compared to male-female couples female partnerships have a lower hatching rate (41% vs 87%) and lower overall reproductive success (31% vs. 67%). 
Research has shown that the environmental pollutant methylmercury can increase the prevalence of homosexual behavior in male American white ibis. The study involved exposing chicks in varying dosages to the chemical and measuring the degree of homosexual behavior in adulthood. The results discovered was that as the dosage was increased the likelihood of homosexual behavior also increased. The endocrine blocking feature of mercury has been suggested as a possible cause of sexual disruption in other bird species.  
Mallards form male-female pairs only until the female lays eggs, at which time the male leaves the female. Mallards have rates of male-male sexual activity that are unusually high for birds, in some cases, as high as 19% of all pairs in a population.  [ page needed ] Kees Moeliker of the Natural History Museum Rotterdam has observed one male mallard engage in homosexual necrophilia. 
Penguins have been observed to engage in homosexual behaviour since at least as early as 1911. George Murray Levick, who documented this behaviour in Adélie penguins at Cape Adare, described it as "depraved". The report was considered too shocking for public release at the time, and was suppressed. The only copies that were made available privately to researchers were translated into Greek, to prevent this knowledge becoming more widely known. The report was unearthed only a century later, and published in Polar Record in June 2012.  
In early February 2004, The New York Times reported that Roy and Silo, a male pair of chinstrap penguins in the Central Park Zoo in New York City, had successfully hatched and fostered a female chick from a fertile egg they had been given to incubate.  Other penguins in New York zoos have also been reported to have formed same-sex pairs.  
In Odense Zoo in Denmark, a pair of male king penguins adopted an egg that had been abandoned by a female, proceeding to incubate it and raise the chick.   Zoos in Japan and Germany have also documented homosexual male penguin couples.   The couples have been shown to build nests together and use a stone as a substitute for an egg. Researchers at Rikkyo University in Tokyo found 20 homosexual pairs at 16 major aquariums and zoos in Japan.
The Bremerhaven Zoo in Germany attempted to encourage reproduction of endangered Humboldt penguins by importing females from Sweden and separating three male pairs, but this was unsuccessful. The zoo's director said that the relationships were "too strong" between the homosexual pairs.  German gay groups protested at this attempt to break up the male-male pairs  but the zoo's director was reported as saying "We don't know whether the three male pairs are really homosexual or whether they have just bonded because of a shortage of females . nobody here wants to forcibly separate homosexual couples." 
A pair of male Magellanic penguins who had shared a burrow for six years at the San Francisco Zoo and raised a surrogate chick, split when the male of a pair in the next burrow died and the female sought a new mate. 
Buddy and Pedro, a pair of male African penguins, were separated by the Toronto Zoo to mate with female penguins.   Buddy has since paired off with a female. 
Suki and Chupchikoni are two female African penguins that pair bonded at the Ramat Gan Safari in Israel. Chupchikoni was assumed to be male until her blood was tested. 
In 2014 Jumbs and Hurricane, two Humboldt penguins at Wingham Wildlife Park became the center of international media attention as two male penguins who had pair bonded a number of years earlier and then successfully hatched and reared an egg given to them as surrogate parents after the mother abandoned it halfway through incubation. 
In 2018, two female King penguins at Kelly Tarltons in Auckland, New Zealand, called Thelma and Louise (named after the 1991 film) have been in a relationship for eight years, when most of the other eligible penguins switch partners each mating season, regardless of their orientation. The two penguins were both taking care of an egg that Thelma hatched, but is unknown whether it was fertilized. 
In 1998 two male griffon vultures named Dashik and Yehuda, at the Jerusalem Biblical Zoo, engaged in "open and energetic sex" and built a nest. The keepers provided the couple with an artificial egg, which the two parents took turns incubating, and 45 days later, the zoo replaced the egg with a baby vulture. The two male vultures raised the chick together.  A few years later, however, Yehuda became interested in a female vulture that was brought into the aviary. Dashik became depressed, and was eventually moved to the zoological research garden at Tel Aviv University where he too set up a nest with a female vulture. 
Two male vultures at the Allwetter Zoo in Muenster built a nest together, although they were picked on and their nest materials were often stolen by other vultures. They were eventually separated to try to promote breeding by placing one of them with female vultures, despite the protests of German homosexual groups. 
Both male and female pigeons sometimes exhibit homosexual behavior. In addition to sexual behavior, same-sex pigeon pairs will build nests, and hens will lay (infertile) eggs and attempt to incubate them. 
The Amazon river dolphin or boto has been reported to form up in bands of 3–5 individuals engaging in sexual activity. The groups usually comprise young males and sometimes one or two females. Sex is often performed in non-reproductive ways, using snout, flippers and genital rubbing, without regard to gender.  In captivity, they have been observed to sometimes perform homosexual and heterosexual penetration of the blowhole, a hole homologous with the nostril of other mammals, making this the only known example of nasal sex in the animal kingdom.   The males will sometimes also perform sex with males from the tucuxi species, a type of small porpoise. 
Courtship, mounting, and full anal penetration between bulls has been noted to occur among American bison. The Mandan nation Okipa festival concludes with a ceremonial enactment of this behavior, to "ensure the return of the buffalo in the coming season".  Also, mounting of one female by another (known as "bulling") is extremely common among cattle. The behaviour is hormone driven and synchronizes with the emergence of estrus (heat), particularly in the presence of a bull.
More than 20 species of bat have been documented to engage in homosexual behavior.   Bat species that have been observed engaging in homosexual behavior in the wild include: 
- the grey-headed flying fox (Pteropus poliocephalus)
- the Bonin flying fox (Pteropus pselaphon) 
- the Indian flying fox (Pteropus giganteus) (Corynorhinus rafinesquii)
- the common bent-wing bat (Miniopterus schreibersii)
- the serotine bat (Eptesicus serotinus) (Myotis bechsteinii)
- the long-fingered bat (Myotis capaccinii) (Myotis daubentonii)
- the little brown bat (Myotis lucifugus)
- the greater mouse-eared bat (Myotis myotis)
- the whiskered bat (Myotis mystacinus) (Myotis nattereri)
- the common noctule (Nyctalus noctula) (Nyctalus leisleri)
- the common pipistrelle (Pipistrellus pipistrellus)
- the brown long-eared bat (Plecotus auritus)
- the barbastelle (Barbastella barbastellus)
- the greater horseshoe bat (Rhinolophus ferrumequinum)
- the lesser horseshoe bat (Rhinolophus hipposideros)
Bat species that have been observed engaging in homosexual behavior in captivity include the Comoro flying fox (Pteropus livingstonii), the Rodrigues flying fox (Pteropus rodricensis) and the common vampire bat (Desmodus rotundus). 
Homosexual behavior in bats has been categorized into 6 groups: mutual homosexual grooming and licking, homosexual masturbation, homosexual play, homosexual mounting, coercive sex, and cross-species homosexual sex.  
In the wild, the grey-headed flying fox (Pteropus poliocephalus) engages in allogrooming wherein one partner licks and gently bites the chest and wing membrane of the other partner. Both sexes display this form of mutual homosexual grooming and it is more common in males. Males often have erect penises while they are mutually grooming each other. Like opposite-sex grooming partners, same-sex grooming partners continuously utter a “pre-copulation call”, which is described as a "pulsed grating call", while engaged in this activity.  
In wild Bonin flying foxes (Pteropus pselaphon), males perform fellatio or 'male-male genital licking' on other males. Male–male genital licking events occur repeatedly several times in the same pair, and reciprocal genital licking also occurs. The male-male genital licking in these bats is considered a sexual behavior. Allogrooming in Bonin flying foxes has never been observed, hence the male-male genital licking in this species does not seem to be a byproduct of allogrooming, but rather a behavior of directly licking the male genital area, independent of allogrooming.  In captivity, same-sex genital licking has been observed among males of the Comoro flying fox (Pteropus livingstonii) as well as among males of the common vampire bat (Desmodus rotundus).  
In wild Indian flying foxes (Pteropus giganteus), males often mount one another, with erections and thrusting, while play-wrestling.  Males of the long-fingered bat (Myotis capaccinii) have been observed in the same position of male-female mounting, with one gripping the back of the other's fur. A similar behavior was also observed in the common bent-wing bat (Miniopterus schreibersii). 
In wild little brown bats (Myotis lucifugus), males often mount other males (and females) during late autumn and winter, when many of the mounted individuals are torpid.  35% of matings during this period are homosexual.  These coercive copulations usually include ejaculation and the mounted bat often makes a typical copulation call consisting of a long squawk.  Similarly, in hibernacula of the common noctule (Nyctalus noctula), active males were observed to wake up from lethargy on a warm day and engage in mating with lethargic males and (active or lethargic) females. The lethargic males, like females, called out loudly and presented their buccal glands with opened mouth during copulation. 
Vesey-Fitzgerald (1949) observed homosexual behaviours in all 12 British bat species known at the time: “Homosexuality is common in the spring in all species, and, since the males are in full possession of their powers, I suspect throughout the summer. I have even seen homosexuality between Natterer's and Daubenton's bats (Myotis nattereri and M. daubentonii)." 
Dolphins of several species engage in homosexual acts, though it is best studied in the bottlenose dolphins.  [ page needed ] Sexual encounters between females take the shape of "beak-genital propulsion", where one female inserts her beak in the genital opening of the other while swimming gently forward.  Between males, homosexual behaviour includes rubbing of genitals against each other, which sometimes leads to the males swimming belly to belly, inserting the penis in the other's genital slit and sometimes anus. 
Janet Mann, Georgetown University professor of biology and psychology, argues that the strong personal behavior among male dolphin calves is about bond formation and benefits the species in an evolutionary context.  She cites studies showing that these dolphins later in life as adults are in a sense bisexual, and the male bonds forged earlier in life work together for protection as well as locating females to reproduce with. Confrontations between flocks of bottlenose dolphins and the related species Atlantic spotted dolphin will sometimes lead to cross-species homosexual behaviour between the males rather than combat. 
African and Asian male elephants will engage in same-sex bonding and mounting. Such encounters are often associated with affectionate interactions, such as kissing, trunk intertwining, and placing trunks in each other's mouths. Male elephants, who often live apart from the general herd, often form "companionships", consisting of an older individual and one or sometimes two younger males with sexual behavior being an important part of the social dynamic. Unlike heterosexual relations, which are always of a fleeting nature, the relationships between males may last for years. The encounters are analogous to heterosexual bouts, one male often extending his trunk along the other's back and pushing forward with his tusks to signify his intention to mount. Same-sex relations are common and frequent in both sexes, with Asiatic elephants in captivity devoting roughly 45% of sexual encounters to same-sex activity. 
Male giraffes have been observed to engage in remarkably high frequencies of homosexual behavior. After aggressive "necking", it is common for two male giraffes to caress and court each other, leading up to mounting and climax. Such interactions between males have been found to be more frequent than heterosexual coupling.  In one study, up to 94% of observed mounting incidents took place between two males. The proportion of same sex activities varied between 30 and 75%, and at any given time one in twenty males were engaged in non-combative necking behavior with another male. Only 1% of same-sex mounting incidents occurred between females. 
Homosexual behavior is quite common in wild marmots.  In Olympic marmots (Marmota olympus) and hoary marmots (Marmota caligata), females often mount other females as well as engage in other affectionate and sexual behaviors with females of the same species.  They display a high frequency of these behaviors especially when they are in heat.   A homosexual encounter often begins with a greeting interaction in which one female nuzzles her nose on the other female's cheek or mouth, or both females touch noses or mouths. Additionally, a female may gently chew on the ear or neck of her partner, who responds by raising her tail. The first female may sniff the other's genital region or nuzzle that region with her mouth. She may then proceed to mount the other female, during which the mounting female gently grasps the mounted female's dorsal neck fur in her jaws while thrusting. The mounted female arches her back and holds her tail to one side to facilitate their sexual interaction.  
Both male and female lions have been seen to interact homosexually.   Male lions pair-bond for a number of days and initiate homosexual activity with affectionate nuzzling and caressing, leading to mounting and thrusting. About 8% of mountings have been observed to occur with other males. Pairings between females are held to be fairly common in captivity but have not been observed in the wild.
European polecats (Mustela putorius) were found to engage homosexually with non-sibling animals. Exclusive homosexuality with mounting and anal penetration in this solitary species serves no apparent adaptive function.  [ page needed ]
Bonobos form a matriarchal society, unusual among apes. They are fully bisexual: both males and females engage in hetero- and homosexual behavior, being noted for female–female sex in particular,  including between juveniles and adults.  Roughly 60% of all bonobo sexual activity occurs between two or more females. While the homosexual bonding system in bonobos represents the highest frequency of homosexuality known in any primate species, homosexuality has been reported for all great apes (a group which includes humans), as well as a number of other primate species.     
Dutch primatologist Frans de Waal, who extensively observed and filmed bonobos, believed that sexual activity is the bonobo's way of avoiding conflict. Anything that arouses the interest of more than one bonobo at a time, not just food, tends to result in sexual contact. If two bonobos approach a cardboard box thrown into their enclosure, they will briefly mount each other before playing with the box. Such situations lead to squabbles in most other species. But bonobos are quite tolerant, perhaps because they use sex to divert attention and to defuse tension.  
Bonobo sex often occurs in aggressive contexts totally unrelated to food. A jealous male might chase another away from a female, after which the two males reunite and engage in scrotal rubbing. Or after a female hits a juvenile, the latter's mother may lunge at the aggressor, an action that is immediately followed by genital rubbing between the two adults. 
Homosexual behavior among male gorillas has been studied.  This behavior occurs more often in all-male bachelor packs in the wild and it is believed to play a role in social bonding. Homosexual behavior among female mountain gorillas has also been documented. 
With the Japanese macaque, also known as the "snow monkey", same-sex relations are frequent, though rates vary between troops. Females will form "consortships" characterized by affectionate social and sexual activities. In some troops up to one quarter of the females form such bonds, which vary in duration from a few days to a few weeks. Often, strong and lasting friendships result from such pairings. Males also have same-sex relations, typically with multiple partners of the same age. Affectionate and playful activities are associated with such relations. 
Homosexual behavior forms part of the natural repertoire of sexual or sociosexual behavior of orangutans. Male homosexual behavior occurs both in the wild and in captivity, and it occurs in both adolescent and mature individuals. Homosexual behavior in orangutans is not an artifact of captivity or contact with humans. 
Among monkeys [ clarification needed ] , Lionel Tiger and Robin Fox conducted a study on how Depo-Provera contraceptives lead to decreased male attraction to females. 
Ovis aries has attracted much attention due to the fact that around 8–10% of rams have an exclusive homosexual orientation.       Such rams prefer to court and mount other rams only, even in the presence of estrous ewes.  Moreover, around 18–22% of rams are bisexual. 
Several observations indicate that male–male sexual preference in rams is sexually motivated. Rams routinely perform the same courtship behaviors (including foreleg kicks, nudges, vocalizations, anogenital sniffs and flehmen) prior to mounting other males as observed when other rams court and mount estrous females. Furthermore, pelvic thrusting and ejaculation often accompany same-sex mounts by rams. 
A number of studies have reported differences in brain structure and function between male-oriented and female-oriented rams, suggesting that sexual partner preferences are neurologically hard-wired.  A 2003 study by Dr. Charles E. Roselli et al. (Oregon Health and Science University), states that homosexuality in male sheep is associated with a region in the rams' brains which the authors call the "ovine Sexually Dimorphic Nucleus" (oSDN) which is half the size of the corresponding region in heterosexual male sheep.  Scientists found that, "The oSDN in rams that preferred females was significantly larger and contained more neurons than in male-oriented rams and ewes. In addition, the oSDN of the female-oriented rams expressed higher levels of aromatase, a substance that converts testosterone to estradiol, a form of estrogen which is believed to facilitate typical male sexual behaviors. Aromatase expression was no different between male-oriented rams and ewes [. ] The dense cluster of neurons that comprise the oSDN express cytochrome P450 aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression." These results suggest that ". naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and its capacity for estrogen synthesis."  As noted before, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. It should also be noted that the results of this particular study have not been confirmed by other studies.
The Merck Manual of Veterinary Medicine appears to consider homosexuality among sheep as a routine occurrence and an issue to be dealt with as a problem of animal husbandry. 
Studies have failed to identify any compelling social factors that can predict or explain the variations in sexual partner preferences of domestic rams.  Homosexual orientation and same-sex mounting in rams is not related to dominance, social rank or competitive ability. Indeed, male-oriented rams are not more or less dominant than female-oriented rams.   Homosexual orientation in rams is also not affected by rearing conditions, i.e., rearing males in all-male groups, rearing male and female lambs together, early exposure of adolescent males to females and early social experiences with females do not promote or prevent homosexual orientation in rams.   Male-oriented partner preference also does not appear to be an artifact caused by captivity or human management of sheep. 
Homosexual courtship and sexual activity routinely occur among rams of wild sheep species, such as bighorn sheep (Ovis canadensis), thinhorn sheep (Ovis dalli), mouflons and urials (Ovis orientalis).  Usually a higher ranking older male courts a younger male using a sequence of stylized movements. To initiate homosexual courtship, a courting male approaches the other male with his head and neck lowered and extended far forward in what is called the 'low-stretch' posture. He may combine this with the 'twist,' in which the courting male sharply rotates his head and points his muzzle toward the other male, often while flicking his tongue and making grumbling sounds. The courting male also often performs a 'foreleg kick', in which he snaps his front leg up against the other male's belly or between his hind legs. He also occasionally sniffs and nuzzles the other male's genital area and may perform the flehmen response. Thinhorn rams additionally lick the penis of the male they are courting. In response, the male being courted may rub his cheeks and forehead on the courting male's face, nibble and lick him, rub his horns on the courting male's neck, chest, or shoulders, and develop an erection. Males of another wild sheep species, the Asiatic mouflons, perform similar courtship behaviors towards fellow males. 
Sexual activity between wild males typically involves mounting and anal intercourse. In Thinhorn sheep, genital licking also occurs. During mounting, the larger male usually mounts the smaller male by rearing up on his hind legs and placing his front legs on his partner's flanks. The mounting male usually has an erect penis and accomplishes full anal penetration while performing pelvic thrusts that may lead to ejaculation. The mounted male arches his back to facilitate the copulation. Homosexual courtship and sexual activity can also take place in groups composed of three to ten wild rams clustered together in a circle. These non-aggressive groups are called 'huddles' and involve rams rubbing, licking, nuzzling, horning, and mounting each other. Female Mountain sheep also engage in occasional courtship activities with one another and in sexual activities such as licking each other's genitals and mounting. 
The family structure of the spotted hyena is matriarchal, and dominance relationships with strong sexual elements are routinely observed between related females. Due largely to the female spotted hyena's unique urogenital system, which looks more like a penis rather than a vagina, early naturalists thought hyenas were hermaphroditic males who commonly practiced homosexuality.  [ failed verification ] Early writings such as Ovid's Metamorphoses and the Physiologus suggested that the hyena continually changed its sex and nature from male to female and back again. In Paedagogus, Clement of Alexandria noted that the hyena (along with the hare) was "quite obsessed with sexual intercourse". Many Europeans associated the hyena with sexual deformity, prostitution, deviant sexual behavior, and even witchcraft.
The reality behind the confusing reports is the sexually aggressive behavior between the females, including mounting between females. Research has shown that "in contrast to most other female mammals, female Crocuta are male-like in appearance, larger than males, and substantially more aggressive,"  and they have "been masculinized without being defeminized".  [ failed verification ]
Study of this unique genitalia and aggressive behavior in the female hyena has led to the understanding that more aggressive females are better able to compete for resources, including food and mating partners.   Research has shown that "elevated levels of testosterone in utero"  contribute to extra aggressiveness both males and females mount members of both the same and opposite sex,   who in turn are possibly acting more submissive because of lower levels of testosterone in utero. 
Several species of whiptail lizard (especially in the genus Aspidoscelis) consist only of females that have the ability to reproduce through parthenogenesis.  Females engage in sexual behavior to stimulate ovulation, with their behavior following their hormonal cycles during low levels of estrogen, these (female) lizards engage in "masculine" sexual roles. Those animals with currently high estrogen levels assume "feminine" sexual roles. Some parthenogenetic lizards that perform the courtship ritual have greater fertility than those kept in isolation due to an increase in hormones triggered by the sexual behaviors. So, even though asexual whiptail lizards populations lack males, sexual stimuli still increase reproductive success. From an evolutionary standpoint, these females are passing their full genetic code to all of their offspring (rather than the 50% of genes that would be passed in sexual reproduction). Certain species of gecko also reproduce by parthenogenesis. 
Some species of sexually reproducing geckos have also been found to display homosexual behavior, e.g. the day geckos Phelsuma laticauda and Phelsuma cepediana. 
Jonathan, the world's oldest tortoise (an Aldabra giant tortoise), had been mating with another tortoise named Frederica since 1991. In 2017, it was discovered that Frederica was actually probably male all along, and was renamed Frederic. 
Insects and arachnids
There is evidence of same-sex sexual behavior in at least 110 species of insects and arachnids.  Scharf et al. says: "Males are more frequently involved in same-sex sexual (SSS) behavior in the laboratory than in the field, and isolation, high density, and exposure to female pheromones increase its prevalence. SSS behavior is often shorter than the equivalent heterosexual behavior. Most cases can be explained via mistaken identification by the active (courting/mounting) male. Passive males often resist courting/mating attempts". 
Scharf et al. continues: "SSS behavior has been reported in most insect orders, and Bagemihl (1999) provides a list of
100 species of insects demonstrating such behavior. Yet, this list lacks detailed descriptions, and a more comprehensive summary of its prevalence in invertebrates, as well as ethology, causes, implications, and evolution of this behavior, remains lacking". 
Male homosexuality has been inferred in several species of dragonflies (the order Odonata). The cloacal pinchers of male damselflies and dragonflies inflict characteristic head damage to females during sex. A survey of 11 species of damsel and dragonflies   has revealed such mating damages in 20 to 80% of the males too, indicating a fairly high occurrence of sexual coupling between males.
Male Drosophila melanogaster flies bearing two copies of a mutant allele in the fruitless gene court and attempt to mate exclusively with other males.  The genetic basis of animal homosexuality has been studied in the fly D. melanogaster.  Here, multiple genes have been identified that can cause homosexual courtship and mating.  These genes are thought to control behavior through pheromones as well as altering the structure of the animal's brains.   These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behavior.  
Male bed bugs (Cimex lectularius) are sexually attracted to any newly fed individual and this results in homosexual mounting. This occurs in heterosexual mounting by the traumatic insemination in which the male pierces the female abdomen with his needle-like penis. In homosexual mating this risks abdominal injuries as males lack the female counteradaptive spermalege structure. Males produce alarm pheromones to reduce such homosexual mating.
Are there differences in how individual orangutans build nests?
Yes. Although all wild orangutans use leaves to embellish their nests with leafy pillows, blankets or mattresses, the frequency with which each type of embellishment is used varies across populations and sites.
Differences can also stem from individual preference. For example, Batang makes very elaborate nests. A few days before she gave birth to Redd, she created an even larger and more elaborate nest! Since she has given birth, we have not formally documented her nesting behaviors or location preferences. However, the primate team has noticed that she has made larger and more well-cushioned nests since Redd’s arrival.
Redd has been observed making his own “practice” nest, but he always shares a night nest with Batang. He will continue to do so for many more years.
Bornean orangutan Batang (left) and Bornean-Sumatran orangutan Kiko (right) prepare their nests using enrichment materials.
Subjects and Setting. Hand preference data were initially collected for termite-fishing in a sample of 17 chimpanzees living in the Gombe National Park, Tanzania. Gombe is a small (35-km 2 ) park, located on the western border of Tanzania and is home to three communities of chimpanzees. Individuals from the Kasekela community, which has been studied for >40 years, were observed for this study (Table 1). The chimpanzees termite-fish year-round, but their efforts become intensely concentrated at the start of the rainy season, from October to December (46). For this study, E.V.L. and a Tanzanian research assistant (K. John) collected data during four periods of field work at Gombe National Park, from October through December in 1998 (35 days), 1999 (41 days), 2000 (43 days), and 2001 (44 days) on a total of 5 mothers and 14 offspring (8 males, 6 females) over the 4-year period. Seventeen of these individuals (5 mothers and 12 offspring) were analyzed for this study.
Procedure. All-day focal animal follows (47) were performed over four consecutive termite-fishing seasons on the females who had offspring <11 years of age. When a termite-fishing session occurred, a focal target was selected from a randomized sequence generated for each family (mother and offspring) and was videotaped for a 15-min bout before moving on to the next individual in the sequence. Unfinished 15-min bouts (e.g., only 9 min of data were collected before the session terminated when the family left the mound) were continued during the next session. By using this methodology, >67 h of video footage from termite-fishing sessions was collected.
Videotaped data were transferred to a digital format and copied onto compact diskettes to facilitate analyses by using the observer video-pro (Noldus Information Technology, Wageningen, The Netherlands), a software package for behavioral analysis. During termite-fishing bouts, each individual sequence of insertion and withdrawal was scored as one “dip.” Each dip was also scored as one of the following: (i)LL, inserted and withdrawn with left hand (ii) LR, inserted with left and withdrawn with right hand (iii) RL, inserted with right and withdrawn with left (iv) RR, inserted and withdrawn with right hand. Dips that were not completed (e.g. the individual inserted the tool and then left without withdrawing it) or that were not visible for the complete sequence of insert/withdraw were excluded in analyses.
For each chimpanzee, a handedness index (HI) was derived for each subject by subtracting the number of left-hand responses from the number of right-hand responses and dividing by the total number of responses. We combined the data from the different years because a split-half correlation coefficient between HI scores revealed a significantly positive association (r = 0.834, P < 0.01), indicating consistent hand use across observation periods. RL and LR responses were rare (<1% of dips) thus, HI scores were based only on the LL or RR response because they were the most frequent. Positive values reflected right-hand biases and negative values reflected left-hand biases. In addition, subjects were classified as left-, right- or ambiguously handed based on binomial z scores calculated on the basis of the frequency of left- and right-hand use. Chimpanzees with z scores above or below 1.96 were classified as right- or left-handed. All others were classified as ambiguously handed.
'Handedness' in scale-eating fish: Nature and nurture
The preference for using one side of the body over the other, as observed typically in human handedness, is referred to as behavioral laterality. Increasing numbers of studies are revealing that, aside from humans, other vertebrates, and even invertebrates, exhibit at least some degree of handedness.
Behavioral laterality is advantageous to foraging, defending against competitors, being vigilant against predators, or attending to prospective mates. While lateralized behaviors are thought to be strengthened over time, it remains unclear how they are acquired during development.
Yoichi Oda of Nagoya University and Yuichi Takeuchi of the University of Toyama have now revealed how and when behavioral laterality arises. They did this by using the scale-eating predator cichlid Perissodus microlepis (P. microlepis), found in Lake Tanganyika in Africa, at its developing stage. They recently published their findings in Scientific Reports.
"P. microlepis is an attractive model of behavioral laterality because the adult fish exhibits clear asymmetric mouth morphology and conspicuously lateralized predatory behaviour left- or right-sided attack of the prey fish," study corresponding author Takeuchi says. "Importantly, we successfully bred P. microlepis in our laboratory -- a challenging feat by any measure. This has allowed us to investigate how the development of behavioral laterality depends on experience."
The researchers found that naive juvenile P. microlepis with no prior scale-eating experience started attacking prey on both sides, but they gradually tended to attack the side that corresponded to the mouth opening direction during subsequent trials. These findings confirmed the researchers' previous results obtained from stomach content analysis: the stomach contents of early juveniles collected in the same location included scales from both sides, while the scales found in adults were almost all from one side of the prey.
Furthermore, the duo noted naive adult P. microlepis attacked bi-directionally, as did naive juveniles. This meant acquisition of the lateralized behavior is not age-dependent, but rather, an acquired trait.
Interestingly, the kinetics of body flexion, in terms of maximum amplitude and angular velocity, during a dominant side attack outperformed those of a non-dominant side attack. Thus, the lateral difference in behavioral kinetics is naturally determined.
"Our findings provide qualitative evidence to support the hypothesis that behavioral laterality is reinforced based on experience during development, as well as a new perspective that the stronger side is naturally determined," coauthor Oda says. "The motions and identifiable neural circuits to control the scale-eater's lateralized behavior may provide valuable material for studying the development of behavioral laterality and its underlying brain mechanisms in animals."
This is a very interesting report of apparent hunting differences in terms of prey preference in two neighbouring/overlapping bonobo groups. The paper is well-written and clear, as are the figures, and the methods and statistical analyses are rigorous, such that the authors' principal conclusions are well supported. While all reviewers enjoyed reading the article and were generally positive about the data (particularly in that they address the 'ecological' argument usually brought up by animal culture critics), some of the claims seem too strong and should be better substantiated by data presented directly in the main text.
We thank the anonymous reviewers, Reviewing Editor Erica Van de Waal, and Senior Editor Delef Weigel for their valuable and constructive comments which have helped to improve our manuscript. We have addressed all their comments by incorporating additional predictors into the model (i.e., the number of males or female party members, association patterns of party members), adding tables, figure supplement, and video, and by toning down cultural claims.
Revisions for this paper:
We ask the authors to revise their paper following the comments here below. If the authors can address our concerns, we agree that the paper should be published in eLife, but the claims on culture should be toned down in line with what the data show.
1) Reviewer 3 has in particular a strong concern regarding the duiker data, which he fails to understand with the way data are presented in the manuscript at the moment (the raw data do not appear to be in mat sup either). This is crucial, because what the authors present as 'cultural differences' may also be just one group having a certain preference for a given prey (anomalures: whether this in itself is enough to claim cultural differences can be debated, enough for some authors, and not for others). In addition, there is no evidence at present that this group difference results from social learning.
This is a valuable point which we failed to clarify in the initial submission. As suggested, we have added a table of all documented successful hunts (Supplementary file 1), together with information of party composition by group, and when available, the identity and sex of the individual who captured the prey and the identity of individuals that participated in the hunt. We also provide more information on duiker hunts in the manuscript and toned-down discussions of social learning.
2) Group composition (Sex differences and ID of hunters): we disagree with authority arguments of the type "because we do not expect it to have a difference on prey acquisition". Where does this come from, especially as it is well documented in the literature that there are sex differences regarding hunting in bonobos, which is often assumed to be female-led? It seems like it would be an important factor, in fact even more so for the identity of the main 'driver' of the hunt (or the Gilby 'impact hunter'). The results for the catcher also suggest that sex would have been a good factor to include in the analysis. What if females have a preference for anomalures and males for duikers? If there are more females in group A than group B, this can lead to apparent cultural differences because a given sex dominates the hunt in one group compared to the other. The authors need to provide more data to address this. In particular, it would be great to have a table that at least shows the different IDs of the 'catchers', as there is already a sex bias appearing with females more likely to catch the prey than males (which group do they belong to? Are the sex split between the two communities?). This will possibly already address the issue of personal preferences.
The reviewers are correct to suggest that sex differences may drive prey acquisition patterns, which may explain group specific prey preference. We have now incorporated additional information on the group identity and sex of the individuals that captured the prey within the main text and as supplementary tables (Supplementary files 1-2). Further, to account for potential sex-differences in prey acquisition in the statistical analysis, we now include the number of available hunters as two separate covariate predictors, the number of male or female party members. Neither of the two predictors showed a clear effect on the response nor altered the overall model results.
General comment for the Discussion: you found that the smaller group was mainly capturing anomalures, could it be not explained by the fact that smaller group are more cohesive and coordinated? Even if you checked for party size could you control for the composition of the party size? If in the larger group individuals are less bonded and stay more in the same party without successful anomalure hunters they might be less likely to hunt them? Whereas in the smaller group the individuals might be spending more time with all other group members?
This is a nice suggestion. To account for the idea that social cohesion may drive hunt success of different species, we have incorporated into the model the average dyadic association values of hunt party members (i.e., adult individuals present during the hunt). The average dyadic association value serves as a proxy for social cohesion, such that higher values represent parties of individuals that associate more frequently overall. By including this predictor, we are accounting for some of the variation in prey type that can potentially be explained by social familiarity (see Results).
3) Intergroup hunts: There is a lack of clarity on which hunt was an intergroup hunt in Table 1 and it is particularly concerning regarding the duiker hunt data. Out of 59 successful hunts, 9 were considered intergroup hunts. If 5 of the successful duiker hunts (as in, including meat sharing) involve members of both groups, why are they classified as 1 Ekalakala and 11 Kokoalongo hunts? Is it because the 'catcher' is from one of the groups only? But then if the ones who consume the food are from both groups, it's hard to argue that there would be 'culinary' cultural differences between the two groups (at best, the cultural difference is in the likelihood to engage in a certain type of hunting for a given prey). Thus shouldn't these hunts be taken out from the group analysis? The authors only state that 'the same pattern persisted during intergroup encounters'. The problem then is that whether these particular hunts are shared between both groups or removed, the possible duiker difference probably becomes minimal and largely irrelevant for lack of data. If that is the case, based on the remaining data, the authors can probably identify a strong preference in one group (Ekalakala for anomalure), but it gets harder to argue for cultural differences (particularly because there is no evidence provided for social learning processes).
Per the reviewers’ comment, we understand that there was unclarity regarding definitions of intergroup encounters, the group identity associated with the hunt, and the overall duiker hunt data in initial submission.
Hunts were classified as Ekalakala or Kokoalongo depending on the identity of the individuals that participated in the hunt, and in cases when both groups participated, the identity of the individual that caught the prey defined the group identity (see Discussion).
We defined a hunt as occurring during an intergroup encounter whenever individuals of both groups were observed in the same party, either during the hunt scan or during the immediate scan prior to the hunt. This was done to account for the high likelihood that the other group is nearby when observed in temporal and spatial vicinity to the hunt. We have added the intergroup encounter definition in the Results. By this approach, two of the nine hunts defined as intergroup encounters included only Kokoalongo individuals (two duiker hunts). We believe that this definition is reliable as in one of the two cases, an Ekalakala female immediately joined the Kokoalongo party once duiker distress calls were heard after the successful capture, and received a share of the meat (Supplementary file 1).
Although, as the reviewer pointed out, 45% of Kokoalongo duiker hunts occurred during intergroup encounters, the average number of Ekalakala individuals present during these hunts was 1.4 and none of them participated in the hunt. Further, while Kokoalongo individuals are frequently observed to hunt adult duikers (Video 1), the only duiker hunt observed in Ekalakala involved the capture of a duiker calf from its hiding place (involving a different hunting technique than what is needed for adult duikers). We have added this information in the main text (Results) and in the legend of the additional figure (Figure 1—figure supplement 1). Taken together, we argue that the duiker data is meaningful for the observed between-group prey difference.
The reviewers also suggest that between group meat sharing observations likely refute ‘culinary’ differences between the groups. However, here we argue that prey palatability differences may still be at the basis of the observed group differences because the costs associated with hunting for meat are very different (higher) than the costs associated with begging for meat. While palatability differences may dictate hunting decisions so to maximize the cost-to-benefit ratio, once prey is already captured the costs associated with access to meat are minimized. In sum, bonobos might not initiate a hunt on a less preferred prey due to hunt costs but will beg for the meat when others have caught it. We have added this as part of the Discussion.
4) Culture/social learning claims: The Discussion starts with the authors saying that the "findings demonstrate that social processes….". That is not true. The Materials and methods and Results do not actually test for any type of social influence or social learning on hunting. This sentence should thus be modified and in general, the discussion on specific kinds of social learning mechanism (emulation etc…) toned down or removed because there is nothing in the manuscript that can help with this question. It would be really nice to see (pending confirmation), that one community is more opportunistic in their hunting (duiker + squirrel) compared to the other that seems to go more collectively. However, the authors cannot take evidence for social learning for collective hunting from the literature (here, findings in dolphin hunting and a more general review in primates including foraging) as evidence for their own findings, which are quite distinct, starting with the study species. The authors' finding is that there is a clear difference in terms of (at least one) prey preference between two neighbouring/overlapping bonobo groups. Whether this is cultural can then be discussed in light of what the authors have shown (e.g. no effect of ecology). Hence it would be more correct to say: we have observed this difference, can it be cultural? In contrast, the last two paragraphs are good to keep, once the assumption that the difference is cultural is met.
We followed the reviewers’ comment and modified claims of cultural transmission or discussion of social learning and further emphasize our findings regarding the lack of ecological effects.
5) Unsuccessful hunts: you need to clarify your hypothesis about the unsuccessful hunts. Is the argument about the unsuccessful hunts (Discussion, second paragraph) really valid if it is based on only 6 months of data (compared to the hunt count that spans several years)? Also, while reading the results, it seemed that it was the entire dataset for unsuccessful hunts but that does not seem to be the case in fact. It should probably be introduced differently to avoid confusion then. In general, this specific paragraph in the Discussion needs to be reworked as well if it aims to address the possible "Gilby argument". It can only be properly addressed by offering a list of IDs that shows diversity in individual hunters/catchers, and not by making group pattern remarks.
Successful hunts have only been recorded starting July 2019, when LS initiated field work in Kokolopori. Thus, we unfortunately only have preliminary data regarding unsuccessful hunts. We have reworded the sentence to clarify this “Starting July 2019 we also collected data on unsuccessful hunts”. We have also decided to remove this section from the Discussion, as we agree with the reviewer that the results are preliminary and sample size is low.
Revisions expected in follow-up work:
You need to add a table (for the hunts where you were able to record the identity of all participants) with the list of the different individuals that have been observed hunting on all three types of prey in both group, with their individual characteristics (such as age, sex, rank, relatedness of participants), and if the hunt with successful or not.
We have added two tables (Supplementary files 1-2). One table includes the identities of all individuals per group with information on their sex and age categories (Supplementary file 2), and a second table with information of all successful hunt cases, including the identity of individuals present during the hunt by group membership, and when available the identity and sex of the individual who caught the prey and of those that participated in the hunt (Supplementary file 1).
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