Animal reproduction when resources are scarce

Animal reproduction when resources are scarce

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(This question is aimed at animals that take care of their young, e.g. dolphins.)

As I understand it, most animals reproduce if possible. The only way they don't is because they run out of resources. I'm wondering how this shows up in practice:

  1. Do they simply not have sex? If so, does this mean they don't have sex drive?
  2. Do they conceive and then let their children starve to death?
  3. Do they fail to conceive (that is, are there biological changes to a mother who's not getting enough food such that she is no longer fertile)?

Option #2 seems the least likely to me because presumably pregnancy involves a cost to the parent, and with resources already scarce this would be a waste; however the plot of White Fang does indeed involve this situation (I have no idea how scientifically accurate that book is however).

This is actually a comment, but was not allowed to comment.

I think what you're interested is more in population dynamics and I think what you're really asking about is more in line with what system of differential equations explains a network of animals and how their populations grow and decrease with the amount of resources and the species population.

I think the problem is that, there's actually no good model, because there are so many factors which seem to contribute to shifts.

@ MIT this year, they had a visiting prof from Tufts give a talk on this, and they're working on models, to try and describe this, and even HE- the prof said, yeah, it gets complicated even for two species.

I would recommend, that if you have time to read up on this? <--- a link to his publications

Also, this is probably more a question for the mathematics stack.

Animal Reproduction

Reproductive success in livestock is essential for the economic livelihood of producers and ultimately affects the consumer cost of meat and other animal products. In many livestock production systems, poor fertility is a major factor that limits productivity. NIFA provides funding for basic and applied research in animal reproduction, and supports extension and education programs that transfer new science-based knowledge to the field and classroom.

The ability of animals to reproduce efficiently is an integral component of animal agriculture. However, infertility is a problem to some degree in all animal production systems, including aquaculture species. Reproductive failure is one of the most significant factors that limit the productivity of animal production systems and result in millions of dollars in lost profits annually. A major challenge facing many producers is finding practical, cost-effective ways to improve reproductive performance without compromising the production of safe, high quality meat and milk products.

Inefficient reproduction in livestock may be caused by numerous factors including: abnormal or absent reproductive cycles failure to show estrus (heat) embryonic and fetal loss and mortality during the neonatal period failure to reach puberty at an optimum age or an inability of young females to conceive early in the breeding season environmental stressors such as temperature extremes or changes in photoperiod (day and night cycle), or production of sperm with a low potential for fertilization.

In some production systems, breeding programs designed to select for milk or meat traits have had deleterious effects on reproductive performance. In dairy cattle, intense genetic selection for increased milk production has been accompanied by significant reductions in fertility. Similarly, in broiler breeders (chickens bred specifically for meat production) reproductive ability decreases as body weight (meat production) increases. To address these problems, NIFA provides national leadership and funding opportunities to conduct basic and applied research on reproduction in agriculturally important animals.

Basic research increases our knowledge of the fundamental biology of reproduction, which in turn facilitates the development of state-of-the-art management strategies that optimize reproductive efficiency and minimize economic loss. Through partnerships with land-grant universities, NIFA also supports extension and outreach programs. These programs transfer research-based knowledge and reproductive management practices to livestock, poultry and aquaculture producers and professionals across the country. NIFA is also committed to educating the next generation of animal scientists and producers by providing educational funding opportunities for curriculum development, fellowships and training in reproductive biology.

Animals Get Energy From the Environment

Animals require energy to support the processes of life: movement, foraging, digestion, reproduction, growth, and work. Organisms can be categorized into one of the following groups:

  • Autotroph—an organism that obtains energy from sunlight (in the case of green plants) or inorganic compounds (in the case of sulfur bacteria)
  • Heterotroph—an organism that uses organic materials as a source of energy

Animals are heterotrophs, obtaining their energy from the ingestion of other organisms. When resources are scarce or environmental conditions limit the ability of animals to obtain food or go about their normal activities, animals' metabolic activity may decrease to conserve energy until better conditions prevail.

A component of an organism's environment, such as a nutrient, that is in short supply and therefore limits the organism's ability to reproduce in greater numbers is referred to as a limiting factor of the environment.

The different types of metabolic dormancy or responses include:

  • Torpor—a time of decreased metabolism and reduced body temperature in daily activity cycles
  • Hibernation—a time of decreased metabolism and reduced body temperature that may last weeks or months
  • Winter sleep—periods of inactivity during which body temperature does not fall substantially and from which animals can be awakened and become active quickly
  • Aestivation—a period of inactivity in animals that must sustain extended periods of drying

Environmental characteristics (temperature, moisture, food availability, and so on) vary over time and location so animals have adapted to a certain range of values for each characteristic.

The range of an environmental characteristic to which an animal is adapted is called its tolerance range for that characteristic. Within an animal's tolerance range is an optimal range of values at which the animal is most successful.

Resource allocation to reproduction in animals

The standard Dynamic Energy Budget (DEB) model assumes that a fraction κ of mobilised reserve is allocated to somatic maintenance plus growth, while the rest is allocated to maturity maintenance plus maturation (in embryos and juveniles) or reproduction (in adults). All DEB parameters have been estimated for 276 animal species from most large phyla and all chordate classes. The goodness of fit is generally excellent. We compared the estimated values of κ with those that would maximise reproduction in fully grown adults with abundant food. Only 13% of these species show a reproduction rate close to the maximum possible (assuming that κ can be controlled), another 4% have κ lower than the optimal value, and 83% have κ higher than the optimal value. Strong empirical support hence exists for the conclusion that reproduction is generally not maximised. We also compared the parameters of the wild chicken with those of races selected for meat and egg production and found that the latter indeed maximise reproduction in terms of κ, while surface-specific assimilation was not affected by selection. We suggest that small values of κ relate to the down-regulation of maximum body size, and large values to the down-regulation of reproduction. We briefly discuss the ecological context for these findings.

Animal Reproduction Science

Animal Reproduction Science publishes results from studies relating to reproduction and fertility in animals. This includes both fundamental research and applied studies, including management practices that increase our understanding of the biology and manipulation of reproduction. Manuscripts should.

Animal Reproduction Science publishes results from studies relating to reproduction and fertility in animals. This includes both fundamental research and applied studies, including management practices that increase our understanding of the biology and manipulation of reproduction. Manuscripts should go into depth in the mechanisms involved in the research reported, rather than give a mere description of findings. Results and conclusions should contribute to improving the management of an animal species or population, with regard to its fertility or reproductive efficiency. Results and conclusions should contribute to improving the management of an animal species or population, with regard to its fertility or reproductive efficiency. The focus is on animals that are useful to humans including food- and fibre-producing companion/recreational captive and endangered species including zoo animals, but excluding laboratory animals unless the results of the study provide new information that impacts the basic understanding of the biology or manipulation of reproduction.

The journal's scope includes the study of reproductive physiology and endocrinology, reproductive cycles, natural and artificial control of reproduction, preservation and use of gametes and embryos, pregnancy and parturition, infertility and sterility, diagnostic and therapeutic techniques.
The Editorial Board of Animal Reproduction Science has decided not to publish papers in which there is an exclusive examination of the in vitro development of oocytes and embryos however, there will be consideration of papers that include in vitro studies where the source of the oocytes and/or development of the embryos beyond the blastocyst stage is part of the experimental design.

Submission is encouraged of manuscripts that are focused on reproduction in aquatic animals. Manuscripts focused on reproduction in insects, however, do not fit the scope of the Journal and will be rejected without peer review.

Authors with any concerns are encouraged to contact the Editor-in-Chief to enquire about the suitability of the content of their paper for submission. There are no page charges for manuscripts published in Animal Reproduction Science and publication of papers only takes place after rigorous peer review.

Animal reproduction when resources are scarce - Biology

Deby Cassill discusses her innovative classification model while reviewing recorded ant videos in her lab.

(Aug. 28, 2019) – Why do some fish spawn millions of offspring but whales give birth to just one at a time? Why do elephants spend decades caring for their young, while sea turtles abandon their offspring after laying eggs? And what do ant colonies and human societies have in common?

These are the questions probed by Biology Professor Deby Cassill in a scientific paper that presents an innovative model to classify animal species.

Cassill’s unique classification model, which was recently published in the journal Scientific Reports, looks at how pressures from natural selection, such as predation and resource scarcity, influence how mothers invest in offspring quantity and quality. She argues fish invest in large numbers of offspring when the likelihood their young offspring will be killed by predators is high. On the other hand, mammal mothers like whales and elephants provide extensive care to just one or a few offspring when the likelihood their young will starve during seasonal droughts is high.

The paper is Cassill’s first in an ambitious series that presents unifying theories in ecology, evolution and economics.

In the paper, Cassill presents what she calls a “maternal risk management model,” and establishes four categories that all animals can be grouped into. They are predation selection, scarcity selection, weak selection and convergent selection.

Some species, such as whales and elephants, have few offspring but spend their energy making sure those offspring survive to adulthood. Other species, such as fish, turtles and ants, have hundreds or millions of offspring, most of which will perish while still young and sterile. The number of offspring produced and the quality of care given to those offspring determine which natural selection category each species falls into.

Cassill classified 87 animal species into the maternal risk management model.

Whales fall into the scarcity selection category which favors extended maternal care in family units, rearing a few offspring to adulthood. These species may experience resource scarcity but are unlikely to experience predation. Ants and humans, on the other hand, fall into the convergent selection category. The dual pressures of death by predators and starvation during seasonal periods of scarce resources have fused ant families and human families into permanent societies marked by divisions of labor. Cooperative interactions among members occurs more often than competitive interactions.

Cassill’s recent paper is the first in a series of papers setting forth unifying theories of ecology, evolution and economics.

Cassill began her career in biology as an ant expert. When the average person looks at an ant colony, he or she sees a bunch of pests, but for Cassill, these highly social insects are a lot more human-like than we tend to give them credit for.

“I see ants as a self-organizing social system, much like humans,” Cassill said. “For example, people have bosses, but how they get their jobs done depends on things like their personality, their mood, levels of hunger, sleep deprivation and local conditions. Ants do the same thing.”

For example, in a yet to be published study, Cassill found that some ants are workaholics while others sleep the day away.

While studying fire ant colonies in her lab, Cassill began to think differently about ant societies. Most ant biologists wondered why some ants are sterile altruists, giving up their ability to reproduce so they can feed and protect their selfishly fertile siblings, which leave the colony, mate and become queens of their own colonies.

Cassill asked a different question, which spurred her ambitious series of papers: “Why does a queen-mother produce a few selfishly fertile offspring, like most mammals do, and, at the same time, a huge number of disposable offspring, like fish do?”

Cassill wasn’t always this interested in ants. Her first degrees were a BA in psychology and an MPA in public administration. She worked in mental health for 20 years until one day she had an epiphany while watching a nature documentary. She decided she wanted to make nature documentaries, not watch them. She went back to school at age 40 and by 49 earned a B.S., M.S., and Ph.D. in biology from Florida State University.

In subsequent papers, Cassill will present a theory of sexual reproduction that suggests Earth was an all-female planet for billions of years, followed by a theory of evolution, which she presented at the Conference of the Animal Behavior Society in July. Finally, she will present a unifying theory of behavioral economics later this fall.

Exponential growth

In his theory of natural selection, Charles Darwin was greatly influenced by the English clergyman Thomas Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very rapidly, after which population growth decreases as resources become depleted. This accelerating pattern of increasing population size is called exponential growth.

The best example of exponential growth is seen in bacteria. Bacteria are prokaryotes that reproduce by prokaryotic fission. This division takes about an hour for many bacterial species. If 1000 bacteria are placed in a large flask with an unlimited supply of nutrients (so the nutrients will not become depleted), after an hour there will be one round of division (with each organism dividing once), resulting in 2000 organisms. In another hour, each of the 2000 organisms will double, producing 4000 after the third hour, there should be 8000 bacteria in the flask and so on. The important concept of exponential growth is that the population growth rate, the number of organisms added in each reproductive generation, is accelerating that is, it is increasing at a greater and greater rate. After 1 day and 24 of these cycles, the population would have increased from 1000 to more than 16 billion. When the population size, N, is plotted over time, a J-shaped growth curve is produced.

Figure (PageIndex<1>): Exponential population growth: When resources are unlimited, populations exhibit exponential growth, resulting in a J-shaped curve. When resources are limited, populations exhibit logistic growth. In logistic growth, population expansion decreases as resources become scarce. It levels off when the carrying capacity of the environment is reached, resulting in an S-shaped curve.

The bacteria example is not representative of the real world where resources are limited. Furthermore, some bacteria will die during the experiment and, thus, not reproduce, lowering the growth rate. Therefore, when calculating the growth rate of a population, the death rate (D the number organisms that die during a particular time interval) is subtracted from the birth rate (B the number organisms that are born during that interval). This is shown in the following formula:

where &DeltaN&DeltaN = change in number, &DeltaT&DeltaT = change in time, BB = birth rate, and DD = death rate. The birth rate is usually expressed on a per capita (for each individual) basis. Thus, B (birth rate) = bN (the per capita birth rate &ldquob&rdquo multiplied by the number of individuals &ldquoN&rdquo) and D (death rate) = dN (the per capita death rate &ldquod&rdquo multiplied by the number of individuals &ldquoN&rdquo). Additionally, ecologists are interested in the population at a particular point in time: an infinitely small time interval. For this reason, the terminology of differential calculus is used to obtain the &ldquoinstantaneous&rdquo growth rate, replacing the change in number and time with an instant-specific measurement of number and time.

Notice that the &ldquod&rdquo associated with the first term refers to the derivative (as the term is used in calculus) and is different from the death rate, also called &ldquod.&rdquo The difference between birth and death rates is further simplified by substituting the term &ldquor&rdquo (intrinsic rate of increase) for the relationship between birth and death rates:

The value &ldquor&rdquo can be positive, meaning the population is increasing in size negative, meaning the population is decreasing in size or zero, where the population&rsquos size is unchanging, a condition known as zero population growth. A further refinement of the formula recognizes that different species have inherent differences in their intrinsic rate of increase (often thought of as the potential for reproduction), even under ideal conditions. Obviously, a bacterium can reproduce more rapidly and have a higher intrinsic rate of growth than a human. The maximal growth rate for a species is its biotic potential, or rmax, thus changing the equation to:

Plant & Animal

Guess what guys, this standard is about animals and plants. If only it was that easy. It is about all the major responses of living organisms to their external environment. You will need to first recap what an ecosystem is and a habitat is, learn the abiotic and biotic factors and then go from there. You will get 3 essay style questions at the end of the year that will be directed by bullet points. Below I have started to add some links that will help but it is only the beginnings. For this topic there seems to be a larger focus on understanding the niche than previous years so make sure you have a good grasp of what that is. Also Practice old exam questions, yes it has changed but not to a huge extent.

Remember to not apply anthropomorphism to you answers – giving organisms human qualities. Don’t say ‘the plants think that …’ because plants do not think but rather respond to the abiotic and biotic factors in the environment.

Stimuli to know (learn these)

f) Chemicals chemo (Many organisms including plant growth)

g) Water current Rheo (Salmon and some spawning fish)

A habitat is made of physical (abiotic) and living (biotic) factors. A habitat is not necessarily a geographic area — for a parasite it is the body of its host.

The ecological niche describes how an organism or population responds to the distribution of resources and competitors (for example, by growing when resources are abundant, and when predators, parasites and pathogens are scarce) and how it in turn alters those same factors (for example, limiting access to resources by other organisms, acting as a food source for predators and a consumer of prey). "The type and number of variables comprising the dimensions of an environmental niche vary from one species to another and the relative importance of particular environmental variables for a species may vary according to the geographic and biotic contexts".

This is super important to learn as this may be a major focus in questions this year!

  • Its habitat (abiotic and biotic factors, including any if it's requirements)
  • Its adaptations (any structural, physiological or behavioural traits that enable to survive in its habitat)
  • Its role (e.g. producer, how it fits into a food web or interacts with other species

•Niche The limits, for all important environmental features, within which individuals of a species can survive, grow and reproduce.

The 'occupation' or 'profession' of an organism or species.

The ecological niche of the maggot is moist and dark, with a food supply for growth and development into an adult. For the maggot an adaptive advantage is that it can avoid light and possible desiccation or predation, but also it can locate dark places, which is likely to be where its food supply is (inside moist bodies).

The ecological niche of the barnacle is exposed, tidal, subject to wave action and often densely populated, with a fixed / sessile way of life. For the barnacle an adaptive advantage is being able to locate a suitable environment with other successful barnacles, reducing chances of predation within a dense population and being able to attach firmly to a substrate without being washed away.

For each response you will need to be able to describe:

  • the process(es) within each response
  • how the responses occur
  • the adaptive advantage provided for the organism in relation to its ecological niche
  • why the responses provide an adaptive advantage for the organism in relation to its ecological niche

(plants only) – growth towards or away from a stimulus coming from one direction.

towards light a positive phototropism, away a negative phototropism

Responses of different parts of the plant

Describe the role of auxin in the control of plant growth

Interpret historical experiments, e.g Darwin and Went Interactive workshop

Describe the effects of other plant hormones (ethylene, abscissic acid, cytokinins) and know some applications of plant hormones in industry. Flash cards to help you learn

Explain how the growth response contributes to the plant’s survival

Analyse and interpret information to explain examples of responses

Auxin is sent from the growing tip. It collects on the lower surface of the shoot and promotes cell elongation on that side, so the shoot bends up. In the root, auxin has the opposite effect – cell elongation is inhibited. This results in the root growing down, instead of up. These responses suit the requirements of the different plant parts the shoot must photosynthesise so must grow away from gravity, whereas the roots must stabilise the plant so grow down. In orbit there is little gravity to cause the differential in auxin concentration therefore there is little differential growth so plants struggle with the gravitropic response-roots can’t anchor and shoots can’t get max sunlight / achieve max photosynthesis.

Root: positive gravitropism as it is growing towards gravity. • Shoot: negative gravitropism as it is growing away from gravity . Mechanism: • Auxin / IAA / plant hormone / phytohormone. Purpose: • Shoot: photosynthesis. • Root: stability/ more likely to find water Effect of microgravity: • Roots and shoots unable to detect stimulus / direction of gravity.


Gibberellins promote stem elongation. They are not produced in stem tip. Gibberellic acid was the first of this class of hormone to be discovered.

Cytokinins promote cell division. They are produced in growing areas, such as meristems at tip of the shoot. Zeatin is a hormone in this class, and occurs in corn (Zea ).

Abscisic Acid

promotes seed dormancy by inhibiting cell growth. It is also involved in opening and closing of stomata as leaves wilt.

Ethylene is a gas produced by ripe fruits. Why does one bad apple spoil the whole bunch? Ethylene is used to ripen crops at the same time. Sprayed on a field it will cause all fruits to ripen at the same time so they can be harvested.

From Wiki - Phototropism in plants such as Arabidopsis thaliana is directed by blue light receptors called phototropins. Other photosensitive receptors in plants include phytochromes that sense red light and cryptochromes that sense blue light. Different organs of the plant may exhibit different phototropic reactions to different wavelengths of light. Stem tips exhibit positive phototropic reactions to blue light, while root tips exhibit negative phototropic reactions to blue light. Both root tips and most stem tips exhibit positive phototropism to red light. Cryptochromes are photoreceptors that absorb blue/ UV-A light, and they help control the circadian rhythm in plants and timing of flowering. Phytochromes are photoreceptors that sense red/far-red light, but they also absorb blue light. The combination of responses from phytochromes and cryptochromes allow the plant to respond to various kinds of light. Together phytochromes and cryptochromes inhibit gravitropism in hypocotyls and contribute to phototropism .

Apical growth in plants

(plants) – this is a response to a stimulus which is independent of the direction of the stimulus, e.g. Photonastic - the opening and closing of flowers in response to changes in light intensity.

In animals – a movement towards or away from a stimulus.

(animals and a few mobile plants) – movement of the whole organism towards or away from a stimulus coming from one direction. May be positive or negative.

Towards light – positive phototaxis, away – negative phototaxis

A taxis is where an animal moves away or towards a directional stimulus. The maggot’s behaviour is a negative phototaxis because it is a movement away from the light stimulus detected by different receptors. The barnacle behaviour is a positive chemotaxis. In this case the larva detects and moves towards the chemical, showing a positive response. On detecting the rocky environment its cement glands attach it to the rock. This is a thigmokinesis, as the attachment is as soon as the rock is touched (there is no change in rock intensity)

2 or more receptors which can simultaneously judge the intensity of the stimulus, the animal can find a balance between them. This allows the animal to move directly towards or away from the stimulus

Rate of movement in an animal is affected by the intensity of a stimuli.


Rate of turning is determined by the intensity of a stimulus. Increased chance of an animal finding favorable conditions.


Speed of an animals movement is determined by the intensity of a stimulus. Increased chance of an animal finding favorable conditions

Ability of an animal to find its way home over unfamiliar terrain. Daylight cues are Daylength – from year to year this does not change. Unreliable is the Weather/temperature – this changes from day to day, year to year (e.g. a cold winter vs. a warm winter).

Migration is the movement of individuals from one geographic location to another.

including sun compass, visual cues, magnetic field, chemical trails

Movement from a familiar landmark to another until an animal

Used over short distances using visual cues.

Compass orientation

Animal can detect a compass direction and travel in a straight line

path until it reaches its destination.

Accomplished using magnetic field lines, chemical cues, sound

True navigation

Determining one’s position relative to other locations.

Requires a map sense and a sense of timing

The ability to orient towards a target area without the use of landmarks and regardless of its direction

Map sense – ability to be aware of the latitude and longitude of an area.

Sense of timing – an internal clock that can compensate for the movement of the sun or stars.

Both are required for solar and stellar navigation.

Methods used for Navigation

Visual cues – animal learns its surroundings. Memorise the shape of coastlines, other topography of the area e.g. trees, streams, hills

Solar Navigation

Requires a precise internal clock

Use of the sun to navigate home/migrate. Sun always moves East to west.

Some animals can detect polarised (due to differential absorption, some of the light becomes polarized plane of polarization tells position of sun)

Enables them to detect where the sun is even with the smallest patch of blue sky.

e.g. honeybees – keep the sun on one ommatidium of their compound eye during the outward journey and on a corresponding opposite ommatidium on the return journey.

Bee indicates where a food source is to the hive, by doing one of two dances.

Round dance – points directly to a food source within 50m

Waggle dance – bee traces a figure 8. The axis of the waggle (in regards to the vertical nature of the comb in the hive) indicates the direction of the food in relation to the direction of the sun.

The number of waggles indicates the distance.

Fewer, slower waggles the further away the food is.

Birds – fly mainly during the day

Compensate for the changing direction of the sun related to time of day.

e.g. northern hemisphere bird flying south in autumn

Q9am flies 45º left of the sun

Q3pm flies 45º right of the sun

Retard internal clock with artificial light-dark cycles and the bird will fly in direction based on perceived time. Reset internal clock.

e.g. retard by 6 hours bird released at 3pm sees sun at 9am and will fly west.

Star Compass

Star compass orientation similar to sun compass.

• Groups and geometric patterns of stars are important.

• Generally based on the brightest (northern stars) stars as they move the least during the night.

• Rotational axis of star field (around North and South celestial poles) is important

• Requires precise internal clock

• Animal has a magnetic compass

• Able to follow the magnetic field lines of the earth

• Direction and position derived from direction and inclination of field components

• Some animals have small amounts of magnetite in their brains these respond to magnetic fields and information is transferred to nerve endings and processed by brain

• Used by pigeons, whales, dolphins, turtles, salamanders, some bacteria, some bees

• Magnetic storms, over magnetic anomalies and orientation in room with manipulated magnetic field animals navigation disrupted

First make sure you understand circadian rhythms, a good link is here. If the rhythm were exogenous it would happen at any time of the year and not at the same time each year.

Crepuscular, nocturnal, and diurnal are all terms used to describe the time of day that certain animals are active. When an animal is said to be "crepuscular" it means that that animal is active during the twilight hours at dawn and dusk. Nocturnal animals are only active at night. Many of these animals have specially adapted vision to help them see in the dark, and they often have excellent hearing as well. Animals that are only active during the day are diurnal. This can have great advantages for animals that have poor visibility like humans.

Astronomical cycles creates environmental cues

This is a rhythm that is controlled by the external environmental stimuli detected by the organisms.

Controlled by an internal Biological clock.

Circadian – daily activity period (24hrs)

Circatidal – Tidal period (12.4hrs)

Circasemilunar – spring/neap tide (14.7 days)

Circalunar – monthly activity (29 days)

Circannual – yearly activity (365 days)

This is a rhythm that is controlled by the external environmental stimuli detected by the organisms.

These are internal timing system which continue without external time cues.

They control the timing activities of plants and animals.

What they do?

Control of daily body rhythms such as sleep, blood pressure, temperature, blood cell count, alertness, urine composition, metabolic rate and sex drive.

Reproductive timing i.e. animals in heat, courtship rituals, simultaneous release of sperm and eggs into water.

Animals may migrate to and from breeding grounds twice a year, and have many annual cycles of reproduction and hibernation.

This is the way that animals survive over winter, usually by slowing down their metabolism. Small animals are particularly susceptible to the cold as they have a large surface area to their volume and can lose heat rapidly.

This is a form of summer hibernation. When the soil gets too dry, earthworms will dig down deep and curl into a ball, secrete mucus and will aestivate until the soil becomes moist again.

Humans - Sleep and Wake cycles Circadian Rhythms

  • Vary from person to person
  • Children sleep about 12 hrs per night
  • Teenagers sleep about 9 hrs per night
  • Adults on average sleep about 7-8 hrs
  • Elderly people make do with 6hrs (and nap during the day.)


This rises during the day and drops at night, the lowest point being at 3 am.

This keeps in step with temperature.

This varies during the day. We are more sensitive to the pain of a needle at noon, but are more sensitive to the pain of cold at night.

Birth and Death

You are most likely to be born or to die in the early morning

Some animals synchronise their behaviour with the phases of the moon.

The positions of the sun and moon generate our tidal patterns, so the response to these tidal changes during a 24hr period is considered to be a lunar cycle.

The ovulatory cycles of primate females is about 4 weeks long but there is no firm evidence that these are synchronised with the lunar month.

The spawning behaviour of certain marine worms is synchronised by the moon, which ensures that eggs and sperms will be released at the same time.

The spawning of the palolo worm is governed by a combination of tidal, lunar and annual rhythms.

  • The Grunion is a fish that spawns on land. From April to June, on the 3 or 4 nights that the spring tide occurs at precisely high tide, the fish squirm onto the beach. The female buries her tail in the sand. The male wraps around her to release sperm. Then they catch the outgoing tide. By the time the tide next reaches that part of the beach, 15 days later, the young grunions have hatched and catch a ride out to sea.
  • Arrhythmic ( No regular pattern) - These organisms are usually found in areas where changes in the micro-climate are negligible. E.g. in caves, deep under the ocean or soil.

An actogram is a diagram showing the periods of activity and rest in an organism over a number of 24 hour periods so that trends in activity can be identified.

A good short video by NZ teacher, Carmen Kenton.

The following video is by NZ teacher Chris Clay, excellent videos.

Understand entrainment, free running, period and phase shift

Biological clocks are self sustaining oscillators which will continue a period of free-running cycling even in the absence of external cues. However, clocks are usually linked to and can be reset by the environment via cues (i.e., Zeitgeber). Such entrainment keeps an organisms clock synched to its surrounding conditions.

  • Understand the control of flowering in short day, long day and day neutral plants in terms of the phytochrome system and the critical day length.

An excellent powerpoint can be found here

In plants the phytochrome system controls flowering. Pfr is converted from Pr during the day. Pfr formed during the day breaks down to Pr overnight. The longer the night, the lower the Pfr concentration. When the night length gets to a critical level the fuchsia flowers. In animals the SCN in the brain controls the biorhythm , detecting changes in daylength. When it gets to a critical length the tui will court and breed. / In diurnal animals the pineal gland produces the hormone melatonin to promote sleep / Biological clock produces an endogenous rhythm that controls internal timing and can be entrained by a zeitgeber such as daylight, temp or similar. By ensuring activities happen at a certain time of year the tui can maximise the resources available, eg young appear when fuchsia is flowering (providing food). Similarly the fuchsia guarantees pollination by flowering when many pollinators, such as tūi, are present.

Powerpoint covering biological clocks

Vernalization is the acquisition of a plant's ability to flower in the spring by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower. Vernalization can also refer to herbal (non-woody) plants requiring a cold dormancy to produce new shoots and leaves. [1]

Many plants grown in temperate climates require vernalization and must experience a period of low winter temperature to initiate or accelerate the flowering process. This ensures that reproductive development and seed production occurs in spring and summer, rather than in autumn. [2] The needed cold is often expressed in chill hours. Typical vernalization temperatures are between 5 and 10 degrees Celsius.

18.1 How Animals Reproduce

Some animals produce offspring through asexual reproduction while other animals produce offspring through sexual reproduction. Both methods have advantages and disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly these are two advantages that asexually reproducing organisms have over sexually reproducing organisms. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment, species that reproduce asexually may be at a disadvantage because all the offspring are genetically identical and may not be adapted to different conditions.

During sexual reproduction , the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give sexually reproducing individuals greater fitness because more of their offspring may survive and reproduce in an unpredictable or changing environment. Species that reproduce sexually (and have separate sexes) must maintain two different types of individuals, males and females. Only half the population (females) can produce the offspring, so fewer offspring will be produced when compared to asexual reproduction. This is a disadvantage of sexual reproduction compared to asexual reproduction.

Asexual Reproduction

Asexual reproduction occurs in prokaryotic microorganisms (bacteria and archaea) and in many eukaryotic, single-celled and multi-celled organisms. There are several ways that animals reproduce asexually, the details of which vary among individual species.


Fission , also called binary fission, occurs in some invertebrate, multi-celled organisms. It is in some ways analogous to the process of binary fission of single-celled prokaryotic organisms. The term fission is applied to instances in which an organism appears to split itself into two parts and, if necessary, regenerate the missing parts of each new organism. For example, species of turbellarian flatworms commonly called the planarians, such as Dugesia dorotocephala, are able to separate their bodies into head and tail regions and then regenerate the missing half in each of the two new organisms. Sea anemones (Cnidaria), such as species of the genus Anthopleura (Figure 18.2), will divide along the oral-aboral axis, and sea cucumbers (Echinodermata) of the genus Holothuria, will divide into two halves across the oral-aboral axis and regenerate the other half in each of the resulting individuals.


Budding is a form of asexual reproduction that results from the outgrowth of a part of the body leading to a separation of the “bud” from the original organism and the formation of two individuals, one smaller than the other. Budding occurs commonly in some invertebrate animals such as hydras and corals. In hydras, a bud forms that develops into an adult and breaks away from the main body (Figure 18.3).

Concepts in Action

View this video to see a hydra budding.


Fragmentation is the breaking of an individual into parts followed by regeneration. If the animal is capable of fragmentation, and the parts are big enough, a separate individual will regrow from each part. Fragmentation may occur through accidental damage, damage from predators, or as a natural form of reproduction. Reproduction through fragmentation is observed in sponges, some cnidarians, turbellarians, echinoderms, and annelids. In some sea stars, a new individual can be regenerated from a broken arm and a piece of the central disc. This sea star (Figure 18.4) is in the process of growing a complete sea star from an arm that has been cut off. Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams.


Parthenogenesis is a form of asexual reproduction in which an egg develops into an individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process in the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, and ants, wasps, and bees. Ants, bees, and wasps use parthenogenesis to produce haploid males (drones). The diploid females (workers and queens) are the result of a fertilized egg.

Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis. Parthenogenesis has been observed in species in which the sexes were separated in terrestrial or marine zoos. Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males. It is possible that the asexual reproduction observed occurred in response to unusual circumstances and would normally not occur.

Sexual Reproduction

Sexual reproduction is the combination of reproductive cells from two individuals to form genetically unique offspring. The nature of the individuals that produce the two kinds of gametes can vary, having for example separate sexes or both sexes in each individual. Sex determination, the mechanism that determines which sex an individual develops into, also can vary.


Hermaphroditism occurs in animals in which one individual has both male and female reproductive systems. Invertebrates such as earthworms, slugs, tapeworms, and snails (Figure 18.5) are often hermaphroditic. Hermaphrodites may self-fertilize, but typically they will mate with another of their species, fertilizing each other and both producing offspring. Self-fertilization is more common in animals that have limited mobility or are not motile, such as barnacles and clams. Many species have specific mechanisms in place to prevent self-fertilization, because it is an extreme form of inbreeding and usually produces less fit offspring.

Sex Determination

Mammalian sex is determined genetically by the combination of X and Y chromosomes. Individuals homozygous for X (XX) are female and heterozygous individuals (XY) are male. In mammals, the presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants.

Bird sex determination is dependent on the combination of Z and W chromosomes. Homozygous for Z (ZZ) results in a male and heterozygous (ZW) results in a female. Notice that this system is the opposite of the mammalian system because in birds the female is the sex with the different sex chromosomes. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use the ZW system.

More complicated chromosomal sex determining systems also exist. For example, some swordtail fish have three sex chromosomes in a population.

The sex of some other species is not determined by chromosomes, but by some aspect of the environment. Sex determination in alligators, some turtles, and tuataras, for example, is dependent on the temperature during the middle third of egg development. This is referred to as environmental sex determination, or more specifically, as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females, while in many other species of turtles, the reverse is true. In some crocodiles and some turtles, moderate temperatures produce males and both warm and cool temperatures produce females.

Individuals of some species change their sex during their lives, switching from one to the other. If the individual is female first, it is termed protogyny or “first female,” if it is male first, it is termed protandry or “first male.” Oysters are born male, grow in size, and become female and lay eggs. The wrasses, a family of reef fishes, are all sequential hermaphrodites. Some of these species live in closely coordinated schools with a dominant male and a large number of smaller females. If the male dies, a female increases in size, changes sex, and becomes the new dominant male.


The fusion of a sperm and an egg is a process called fertilization. This can occur either inside ( internal fertilization ) or outside ( external fertilization ) the body of the female. Humans provide an example of the former, whereas frog reproduction is an example of the latter.

External Fertilization

External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization takes place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The spawning may be triggered by environmental signals, such as water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). Revise to "Frogs, corals, squid, and octopuses also spawn (Figure 18.6).

Internal Fertilization

Internal fertilization occurs most often in terrestrial animals, although some aquatic animals also use this method. Internal fertilization may occur by the male directly depositing sperm in the female during mating. It may also occur by the male depositing sperm in the environment, usually in a protective structure, which a female picks up to deposit the sperm in her reproductive tract. There are three ways that offspring are produced following internal fertilization. In oviparity , fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg (Figure 18.7a). This occurs in some bony fish, some reptiles, a few cartilaginous fish, some amphibians, a few mammals, and all birds. Most non-avian reptiles and insects produce leathery eggs, while birds and some turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. Chicken eggs are an example of a hard shell. The eggs of the egg-laying mammals such as the platypus and echidna are leathery.

In ovoviparity , fertilized eggs are retained in the female, and the embryo obtains its nourishment from the egg’s yolk. The eggs are retained in the female’s body until they hatch inside of her, or she lays the eggs right before they hatch. This process helps protect the eggs until hatching. This occurs in some bony fish (like the platyfish Xiphophorus maculatus, Figure 18.7b), some sharks, lizards, some snakes (garter snake Thamnophis sirtalis), some vipers, and some invertebrate animals (Madagascar hissing cockroach Gromphadorhina portentosa).

In viviparity the young are born alive. They obtain their nourishment from the female and are born in varying states of maturity. This occurs in most mammals (Figure 18.7c), some cartilaginous fish, and a few reptiles.

Sex Determination

Mammalian sex determination is determined genetically by the presence of X and Y chromosomes. Individuals homozygous for X (XX) are female and heterozygous individuals (XY) are male. The presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants.

Avian sex determination is dependent on the presence of Z and W chromosomes. Homozygous for Z (ZZ) results in a male and heterozygous (ZW) results in a female. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use this system.

The sex of some species is not determined by genetics but by some aspect of the environment. Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development. This is referred to as environmental sex determination, or more specifically as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females. In some crocodiles, moderate temperatures produce males and both warm and cool temperatures produce females. In some species, sex is both genetic- and temperature-dependent.

Individuals of some species change their sex during their lives, alternating between male and female. If the individual is female first, it is termed protogyny or “first female,” if it is male first, its termed protandry or “first male.” Oysters, for example, are born male, grow, and become female and lay eggs some oyster species change sex multiple times.

Watch the video: What Is Asexual Reproduction. Genetics. Biology. FuseSchool (May 2022).


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