Is there an example of a species wherein the female lays eggs in the male?

Is there an example of a species wherein the female lays eggs in the male?

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There are of course many species which lay eggs in the bodies of another species and also many species where either the male is eaten after mating or the male dies soon thereafter. So it seems possible that a species might exist where the female lays fertilized eggs in the body of the male with whom she mated or even a different male. I could see however the problem being that there is a significant time gap between mating and the eggs being ready to be laid, so perhaps there are no cases of the former but perhaps there are cases of the latter?

Note: Looking for a form of egg-laying that is destructive to the "host" -- where the developing larvae eat the host, like in the cross-species case.

Is there an example of a species wherein the female lays eggs in the male?


Pipefish & Seahorses to name two.

Both widely mentioned in many wildlife programs as being an example of what you've asked.

Watch a video of flatworms penis fencing.

Nature never disappoints, it created a real kicker. Some species of flat worms are hermaphrodites, they have penises but also produce and carry eggs. No flatworm wants to be the mom as it uses a lot of energy and food, so the flat worms fight to inseminate each other.

They fight with their penises. the loser gets their eggs fertilized and is the designated mother of the relationship.

all information I used was drawn from Matt Simon's "the wasp that brain washed the caterpillar". If you are interested in weird abilities of strange creatures, I would really recommend checking it out.

Common green bottle fly

The common green bottle fly (Lucilia sericata) is a blowfly found in most areas of the world and is the most well-known of the numerous green bottle fly species. Its body is 10–14 mm (0.39–0.55 in) in length – slightly larger than a house fly – and has brilliant, metallic, blue-green or golden coloration with black markings. It has short, sparse, black bristles (setae) and three cross-grooves on the thorax. The wings are clear with light brown veins, and the legs and antennae are black. The larvae of the fly may be used for maggot therapy, are commonly used in forensic entomology, and can be the cause of myiasis in livestock and pets. The common green bottle fly emerges in the spring for mating.

  • Phaenicia sericata(Meigen, 1826)
  • Lucilia nobilis(Meigen, 1826)[2]
  • Musca nobilisMeigen, 1826[2]
  • Musca sericataMeigen, 1826[2]


The cowpea weevil lacks the "snout" of a true weevil. It is more elongated in shape than other members of the leaf beetle family. It is reddish-brown overall, with black and gray elytra marked with two central black spots. The last segment of the abdomen extends out from under the short elytra, and also has two black spots. [4]

The beetle is sexually dimorphic and males are easily distinguished from females. The females are sometimes larger than males, but this is not true of all strains. Females are darker overall, while males are brown. The plate covering the end of the abdomen is large and dark in color along the sides in females, and smaller without the dark areas in males. [5]

There are two morphs of C. maculatus, a flightless form and a flying form. The flying form is more common in beetles that developed in conditions of high larval density and high temperatures. The flying form has a longer lifespan and lower fecundity, and the sexes are less dimorphic and can be more difficult to tell apart. [5]

The egg is clear, shiny, oval to spindle-shaped, and about 0.75 millimeters long. [5] The larva is whitish in color. [4]

A female adult can lay over a hundred eggs, and most of them will hatch. She lays an egg on the surface of a bean, and when the larva emerges about 4 to 8 days later, it burrows into the bean. [6] During development, the larva feeds on the interior of the bean, eating the tissue just under the surface, leaving a very thin layer through which it will exit when it matures. [4] It emerges after a larval period of 3 to 7 weeks, depending on conditions. [5] In colder climates the gestation period is typically longer taking anywhere from 4–13 weeks to emerge.

Larval crowding can occur when up to 8 or 10 larvae feed and grow within one bean. Crowding limits resources for each individual, leading to longer development time, higher mortality, smaller adult size, and lower fecundity. [5]

Once the beetle emerges as an adult, it may take 24 to 36 hours to mature completely. The lifespan is 10 to 14 days. However, in colder climates lifespans typically range from three to four weeks. The adult requires neither food nor water, but if offered water, sugared water, or yeast, it may consume it. A female given nutrients may lay more eggs. [6]

The beetle tolerates a range of humidity and temperature, making it adaptable in climates worldwide. Its developmental time varies with factors such as humidity, temperature, legume type, crowding, and inbreeding levels in the population. [5] [7] A bean that is too dry will be impossible for the larva to bore into, and wet beans may have fungal growth. In experiments, a humidity range of 25% to 80% was acceptable, with different optimal levels at each life stage. The most eggs hatched between 44% and 63% humidity, and 44% produced the highest survival. The adult lives longer at 81% to 90%. [8] In another experiment, temperatures of 17 °C (63 °F) and 37 °C (99 °F) with a constant humidity stressed the beetle, and the ideal temperature range was 24 °C (75 °F) to 28 °C (82 °F). [7]

The age of the female at oviposition affects the development and survival of the offspring. The eggs of older females are less likely to hatch, the larvae take longer to develop, and fewer larvae survive to adulthood. [9]

Copulation is injurious for the female beetle. The male possesses penile spines which damage the female reproductive tract. The female may forcefully kick the male during copulation, ending the mating. It is possible that male may benefit from harming the female because the injury could reduce matings or mating success with other males, or increase her egg production. When the female is experimentally prevented from kicking the male (by removal of the hind legs), matings continued for longer than usual, and injury increased. However, the amount of time before she became receptive again and the rate of oviposition were not affected. This suggests that penile spines do not increase the reproductive success of males, and it is suspected that the spines do not increase reproductive success for either sex, and may have no adaptive value. [10]

After mating, the female glues single eggs to a bean. [5] The female generally lays fewer eggs when there are fewer hosts. In an experiment, females presented with three large beans laid more eggs than females presented with three small beans. Occasionally, females deposit many eggs onto nonviable surfaces, especially if there are few or no hosts available. This leads to a higher mortality rate in eggs and potential larvae, but it may also lead to host expansion in the long term. [11]

Temperature and humidity in legume storage areas are relatively constant and the food density is high. [11] The female lays eggs on legumes in the field or in storage. [4] Inbreeding is more common in laboratory situations where the beetle is allowed to breed continuously breeding in the field is more limited. [9]

The beetle is known for attacking the cowpea (Vigna unguiculata), but it readily attacks other beans and peas such as the mung bean (Vigna radiata) and adzuki bean (Vigna angularis). [4] [12] The adult is more likely to seek the legume in which it developed as a larva, but if it is not available or less common, the beetle will utilize another type. [13]

Females are more likely than males to take advantage of sugar water or other resources. In an experiment to test the hypothesis that access to nutritional resources would affect the frequency of second matings, females with access to sugar were less likely to mate more than once. Available food makes the female less receptive to advances from males, which present a nuptial gift as part of courtship. [14] The gift is a spermatophore, nutritional content mixed into his ejaculate, a package which can be up to 20% of his body weight. [5] Females with other nutritional resources can afford to refuse a mating. [14]

A female without an additional nutrient source is less choosy in the mating process. She does not even refuse matings with close relatives, such as brothers. [15] The species does suffer from inbreeding depression, but it does not seem to take behavioral action to avoid it. [7] [9]

In addition, bean beetles are known to exhibit homosexual behavior. Males will mount both females and other males. This could potentially have fitness benefits for the male, as the male does not waste time determining whether his partner is male or female. Sometimes it is faster to test physically than to attempt to determine via other methods.

The female usually oviposits on the smooth side of a bean rather than the rough top, and it avoids legumes without smooth surfaces. [16] It also has a way of distributing the eggs among small and large legumes so that each larva has access to roughly the same amount of nutrients its assessment of legumes is based on mass rather than surface area, and on the number of eggs already present. [11]

When preparing to pupate, the larva digs a cell in the bean and lines it with feces. If it encounters another larva in the bean, both retreat and create walls of feces. If the wall is removed, the two larvae fight to the death. This behavior is not well understood. [17]

The bean beetle, Callosobruchus maculatus, oviposit their eggs on the cowpea bean. The species that are most common for the beetle to lay their eggs on are black eyed peas, mung beans, and adzuki beans. [12] If more than one host is available, the beetle will choose its host depending on the variety and size of the bean as well as the texture of the seed coat. [18] One study showed that the beetles will choose their host depending on the geographic region in which they live. [19] It has also been found that the beetle will often switch hosts if a new host becomes available to them. [20] Over time the beetles will start specializing on the new host and will lose preference for the ancestral host. [20]

The beetle larvae grow inside the bean until they emerge as an adult. [12] The time it takes the larvae to develop varies across hosts, with longer development times on less suitable hosts. [12] It has been found that beetles that choose to oviposit their eggs on the black eyed pea have a shorter development time, suggesting that the black eyed pea is a more suitable host. [19] The temperature and relative humidity have an effect on the developmental time as well higher temperatures and a relative humidity range of 40%-60% shorten developmental time. [12]

The emerged adult beetles mate assortatively, meaning they mate with others that developed on the same host bean. [20] If the hybrids of the population are less fit, assortative mating can lead to speciation. [20] One study looked at this and found speciation beginning to occur in early generations but because there was no selection against hybrids, recombination destroyed any linkage that was formed between host and mating preference which did not allow speciation to be completed. [20]

The predators of C. maculatus include several parasitoid wasps. Anisopteromalus calandrae, Uscana mukerjii, and Dinarmus wasps specifically target Callosobruchus species. [21] [22] [23] Dinarmus basalis parasitizes small larvae and halts their development. This limits the damage they can do to beans, but their presence still makes the beans unfit for human consumption and usually makes them unfit for sowing, as well. [21] Uscana mukerjii is an egg parasite which prevents the egg from hatching, thereby preventing damage to the legume. [23]

The beetle is considered "medically harmless" to humans. [4] It is a damaging agricultural pest.

In developing countries, small-scale farmers mix the crushed leaves of Cassia occidentalis into bean stores to deter the beetle. [24] Other Cassia are useful, as well. The powdered leaves are effective, and a warm-water extract and the essential oil from the seeds are better. [24] [25] The seed oil does not stop oviposition, but it increases the mortality of the eggs and the first-instar larvae. [24] The warm-water extract deters the adult female from ovipositing. [25]

Other botanical biological pest control agents tested include nishinda (Vitex negundo), Tasmanian blue gum (Eucalyptus globulus), bankalmi (Ipomoea sepiaria), neem (Azadirachta indica), safflower (Carthamus tinctorius), sesame (Sesamum indicum), and gum arabic (Acacia nilotica syn. Acacia arabica). [26]

Hermetic storage technologies like the Purdue Improved Cowpea Storage bags have also proven successful in controlling C. maculatus. These technologies work by separating the container environment from the surrounding air and forcing the insects inside to deplete the available oxygen inside the container [27] Not only does this ultimately kill the insects, but it also reduces the level of damage they inflict as active feeding ceases below a certain threshold of oxygen [28]

Animal agents of biological control include the parasitoid wasps that target the beetle. In laboratory trials D. basalis has totally eliminated the beetle. [23] A. calandrae and U. mukerjii may also prove useful. [22] [23]

Freezing the whole storage area will also control C. maculatus. A period of six to 24 hours at -18 °C kills all the adults and larvae. If the cooling is slow, the beetle can acclimatize, so longer freezing is required. [29]

4.9: Sexual dimorphism

  • Contributed by Michael W. Klymkowsky and Melanie M. Cooper
  • Professors (MSCD and Chemistry) at University of Colorado Boulder and Michigan State University

What, biologically, defines whether an organism is female or male, and why does it matter? The question is largely irrelevant in unicellular organisms with multiple mating types. For example, the microbe Tetrahymena has seven different mating types, all of which appear morphologically identical. An individual Tetrahymena cell can mate with another individual of a different mating type but not with an individual of the same mating type as itself. Mating involves fusion and so the identity of the parents is lost the four cells that result are of one or the other of the original mating types.

In multicellular organisms, the parents do not themselves fuse with one another. Rather they produce cells, known as gametes, which do. Also, instead of two or more mating types, there are usually only two sexes, male and female. This, of course, leads to the question, how do we define male and female? The answer is superficially simple but its implications are profound. Which sex is which is defined by the relative size of the fusing cells the organisms produce. The larger fusing cell is termed the egg and an organism that produces eggs it is termed a female. The smaller fusing cell, which is often motile (while eggs are generally immotile), is termed a sperm and organisms that produce sperm are termed a male. At this point, we should note the limits of these definitions. There are organisms that can change their sex, which is known generically as sequential hermaphroditism. For example, in a number of fish it is common for all individuals to originally develop as males based on environmental cues, the largest of these males changes its sex to become female. Alternatively, one organism can produce both eggs and sperm such an organism is known as a hermaphrodite.

The size difference between male and female gametes changes the reproductive stakes for the two sexes. Simply because of the larger size of the egg, the female invests more energy in its production (per egg) than a male invests in the production of a sperm cell. It is therefore relatively more important, from the perspective of reproductive success, that each egg produce a viable and fertile offspring. As the cost to the female of generating an egg increases, the more important the egg&rsquos reproductive success becomes. Because sperm are relatively cheap to produce, the selection pressure associated with their production is significantly less than that associated with producing an egg. The end result is that there emerges a conflict of interest between females and males. This conflict of interest increases as the disparity in the relative investment per gamete or offspring increases.

This is the beginning of an evolutionary economics, cost-benefit analysis. First there is what is known as the two-fold cost of sex, which is associated with the fact that each asexual organism can produce offspring but that two sexually reproducing individuals must cooperate to produce offspring. Other, more specific factors influence an individual&rsquos reproductive costs. For example, the cost to a large female laying a small number of small eggs that develop independently is less than that of a small female laying a large number of large eggs. Similarly, the cost to an organism that feeds and defends its young for some period of time after they are born (that is, leave the body of the female) is larger than the cost to an organism that lays eggs and leaves them to fend for themselves. Similarly, the investment of a female that raises its young on its own is different from that of the male that simply supplies sperm and leaves. As you can imagine, there are many different reproductive strategies (many more than we can consider here), and they all have distinct implications. For example, a contributing factor in social evolution is that where raising offspring is particularly biologically expensive, cooperation between the sexes or within groups of organisms in child rearing can improve reproductive success and increase the return on the investment of the organisms involved. It is important to remember (and be able to apply in specific situations) that the reproductive investments, and so evolutionary interests, of the two sexes can diverge dramatically from one another, and that such divergence has evolutionary and behavioral implications .

Consider, for example, the situation in placental mammals, in which fertilization occurs within the female and relatively few new organisms are born from any one female. The female must commit resources to supporting the new organisms from the period from fertilization to birth. In addition, female mammals both protect their young and feed them with milk, using specialized mammary glands. Depending on the species, the young are born at various stages of development, from the active and frisky (such as goats) to the relatively helpless (humans). During the period when the female feeds and protects its offspring, the female is more stressed and vulnerable than other times. Under specific conditions, cooperation with other females can occur (as often happens in pack animals) or with a specific male (typically the father) can greatly increase the rate of survival of both mother and offspring, as well as the reproductive success of the male. But consider this: how does a cooperating male know that the offspring he is helping to protect and nurture are his? Spending time protecting and gathering food for unrelated offspring is time and energy diverted from the male&rsquos search for a new mate it will reduce the male&rsquo s overall reproductive success, and so is a behavior likely to be selected against. Carrying this logic out to its conclusion can lead to behaviors such as males guarding of females from interactions with other males.

As we look at the natural world, we see a wide range of sexual behaviors, from males who sexually monopolize multiple females (polygyny) to polyandry, where the female has multiple male &ldquopartners.&rdquo In some situations, no pair bond forms between male and female, whereas in others male and female pairs are stable and (largely) exclusive. In some cases these pairs last for extremely long times in others there is what has been called serial monogamy, pairs form for a while, break up, and new pairs form (this seems relatively common among performing arts celebrities). Sometimes females will mate with multiple males, a behavior that is thought to confuse males (they cannot know which offspring are theirs) and so reduces infanticide by males 133 .

It is common that while caring for their young, females are reproductively inactive. Where a male monopolizes a female, the arrival of a new male who displaces the previous male can lead to behaviors such as infanticide. By killing the young, the female becomes reproductively active and able to produce offspring related to the new male. There are situations, for example in some spiders, in which the male will allow itself to be eaten during the course of sexual intercourse as a type of nuptial gift, which both blocks other males from mating with a female (who is busy eating) and increases the number of offspring that result from the mating. This is an effective reproductive strategy for the male if its odds of mating with a female are low: better (evolutionarily) to mate and die than never to have mated at all. An interesting variation on this behavior is described in a paper by Albo et al 134 . Male Pisaura mirablis spiders offer females nuptial gifts, in part perhaps to avoid being eaten during intercourse. Of course, where there is a strategy, there are counter strategies. In some cases, instead of an insect wrapped in silk, the males offer a worthless gift, an inedible object wrapped in silk. Females cannot initially tell that the gift is worthless but quickly terminate mating if they discover that it is. This reduces the odds of a male&rsquos reproductive success. As deceptive male strategies become common, females are likely to display counter strategies. For example, a number of female organisms store sperm from a mating and can eject that sperm and replace it with that of another male (or multiple males) obtained from subsequent mating events 135 . There is even evidence that in some organisms, such as the wild fowl Gallus gallus, females can bias against fertilization by certain males, a situation known as cryptic female choice, cryptic since it is not overtly visible in terms of who the female does or does not mate with 136 . And so it goes, each reproductive strategy leads, over time, to counter measures 137 . For example, in species in which a male guards a set of females (its harem), groups of males can work together to distract the guarding male, allowing members of their group to mate with the females. These are only a few of the mating and reproductive strategies that exist in the living world 138 . Molecular studies that can distinguish an offspring&rsquos parents suggest that cheating by both males and females is not unknown even among highly monogamous species. The extent of cheating will, of course, depend on the stakes. The more negative the effects on reproductive success, the more evolutionary processes will select against it.

In humans, a female can have at most one pregnancy a year, while a totally irresponsible male could, in theory at least, make a rather large number of females pregnant during a similar time period. Moreover, the biological cost of generating offspring is substantially greater for the female, compared to the male 139 . There is a low but real danger of the death of the mother during pregnancy, whereas males are not so vulnerable, at least in this context. So, if the female is going to have offspring, it would be in her evolutionary interest that those offspring be as robust as possible, meaning that they are likely to survive and reproduce. How can the female influence that outcome? One approach is to control fertility, that is, the probability that a &ldquoreproductive encounter&rdquo results in pregnancy. This is accomplished physiologically, so that the odds of pregnancy increase when the female has enough resources to successfully carry the pregnancy to term. It should be noted that these are not conscious decisions on the part of the female but physiological responses to various cues. There are a number of examples within the biological world where females can control whether a particular mating is successful, i.e., produces offspring. For example, female wild fowl are able to bias the success of a mating event in favor of dominant males by actively ejecting the sperm of subdominant males following mating with a more dominant male, a mating event likely to result in more robust offspring, that is, off-spring more likely to survive and reproduce 140 . One might argue that the development of various forms of contraception are yet another facet of this type of behavior, but one in which females (and males) consciously control reproductive outcomes.

Is there an example of a species wherein the female lays eggs in the male? - Biology

Reptiles are increasing in popularity as pets, and with this interest comes a demand for healthy, domestically bred reptiles and amphibians. Many hobbyists are becoming curious about the possibility of breeding their pets. Other reptile owners are doing such a good job of caring for their pets that the herps are growing, reaching a point that they are becoming sexually mature. For example, we are seeing an unprecedented number of adult female green iguanas presented to veterinary clinics for problems related to egg laying.

Most reptiles lay eggs. The act of laying eggs is called oviposition. Reptiles that lay eggs are called oviparous. Some reptiles bear live young, and the term for this is viviparous. Technically, a female that lays eggs is said to be gravidwhen she is holding eggs inside of her. A female that gives birth to live young may correctly be called pregnant.

Below is a list of some of the more common species of reptiles and the method of reproduction they employ.

Egg Layers All turtles All tortoises All crocodilians Some lizards Iguanas Water dragons Geckos Veiled chameleons Panther chameleons Monitors Snakes All pythons Kingsnakes Milksnakes Rat snakes Corn snakes Livebearers Some lizards Solomon Island skink Blue-tongue skink Shingle-backed skink Some chameleons Jackson's chameleon Some snakes All boas All vipers Garter snakes

Male and female reptiles do not have external genitalia to help owners determine the sex of a herp. Males and females do possess different reproductive organs, however. The male possesses two testicles, housed inside the body. The male also has a copulatory organ, either a single penis (turtles and tortoises, crocodilians) or a pair of hemipenes (lizards, snakes) that can often be seen as two bulges behind the cloaca at the base of the tail. The penis or hemipenis is not connected to the urinary tract, and is strictly an organ of reproduction. Lizards and snakes can be sexed by the use of a probe that is inserted into the cloaca, directed towards the tail, off of the midline. The probe will travel farther in the male than in the female.

There are also secondary sexual characteristics that can help differentiate males from females. Often, in male chelonians (turtles and tortoises), the plastron (bottom shell) is somewhat concave, and the tail is proportionally longer. Often, the head and general body size are proportionally larger in males of reptile species. The male Jackson's chameleon has three prominent horns on the head that are lacking in the female. Many male iguanids and geckos possess femoral or preanal pores that secrete a waxy substance making them more prominent than those found in females. Many boas and pythons possess spurs located on either side of the vent, and in many males, these spurs are larger. In general, the tail of male reptiles is proportionally longer than the female's.

There are other ways to differentiate the sexes of reptiles, including ultrasound, surgical sexing and radiographs. For specific information, please consult your herp vet if you are unsure about the sex of your reptile.

While it would seem that reproduction is a natural event, without correct circumstances, such as a balanced diet and a suitable environment for egg-laying, eggs may not develop normally or be laid in a timely manner. Owners are often surprised to find that their single pet female lizard has developed eggs. A healthy adult female does NOT need the presence of a male to become gravid.

For fertilization, a male reptile inserts either one of his two hemipenes into the female's cloaca, or the single penis is inserted. Before actual copulation, the pair usually engages in some type of ritualized courtship. After copulation, sperm can be stored for up to six years, and this stored sperm can fertilize subsequent clutches without additional contact by a male.

Using the green iguana as an example, even without a male present to fertilize eggs, a healthy adult female may begin developing eggs. The process begins with the ovaries, where eggs are stored. The ovaries are located inside the body. Most green iguana females become mature when they are between two and four years of age. At that time, follicles begin developing in the ovaries. Each follicle is composed of a tiny egg and a sac filled with yolk. The follicles then detach and move into the oviducts where the egg white is added, and then a shell is placed around the yolk and white. The gravid female usually will not eat for three to six weeks prior to laying her eggs. It makes sense since her abdomen will be full of eggs in the oviducts, and the stomach is quite compressed, and there is little space for food in the stomach.

9.1: The Evolution of Frog Life History Strategies

  • Contributed by Luke J. Harmon
  • Professor (Biological Sciences) at University of Idaho

Frog reproduction is one of the most bizarrely interesting topics in all of biology. Across the nearly 6,000 species of living frogs, one can observe a bewildering variety of reproductive strategies and modes (Zamudio et al. 2016) . As children, we learn of the &ldquoclassic&rdquo frog life history strategy: the female lays jellied eggs in water, which hatch into tadpoles, then later metamorphose into their adult form [e.g. Rey (2007) Figure 9.1A]. But this is really just the tip of the frog reproduction iceberg. Many species have direct development, where the tadpole stage is skipped and tiny froglets hatch from eggs. There are foam-nesting frogs, which hang their eggs from leaves in foamy sacs over streams when the eggs hatch, they drop into the water [e.g. Fukuyama (1991) Figure 9.1B]. Male midwife toads carry fertilized eggs on their backs until they are ready to hatch, at which point they wade into water and their tadpoles wriggle free [ Marquez and Verrell (1991) Figure 9.1C]. Perhaps most bizarre of all are the gastric-brooding frogs, now thought to be extinct. In this species, female frogs swallow their fertilized eggs, which hatch and undergo early development in their mother&rsquos stomach (Tyler and Carter 1981) . The young were then regurgitated to start their independent lives.

Figure 9.1. Examples of frog reproductive modes. (A) European common frogs lay jellied eggs in water, which hatch as tadpoles and metamorphose (B) Malabar gliding frogs make nests that, supported by foam created during amplexus, hang from leaves and branches (C) Male midwife toads carry fertilized eggs on their back. Photo credits: A: Thomas Brown / Wikimedia Commons / CC-BY-2.0, B: Vikram Gupchup / Wikimedia Commons / CC-BY-SA-4.0 C: Christian Fischer / Wikimedia Commons / CC-BY-SA-3.0

The great diversity of frog reproductive modes brings up several key questions that can potentially be addressed via comparative methods. How rapidly do these different types of reproductive modes evolve? Do they evolve more than once on the tree? Were &ldquoancient&rdquo frogs more flexible in their reproductive mode than more recent species? Do some clades of frog show more flexibility in reproductive mode than others?

Many of the key questions stated above do not fall neatly into the Mk or extended-Mk framework presented in the previous characters. In this chapter, I will review approaches that elaborate on this framework and allow scientists to address a broader range of questions about the evolution of discrete traits.

To explore these questions, I will refer to a dataset of frog reproductive modes from Gomez-Mestre et al. (2012) , specifically data classifying species as those that lay eggs in water, lay eggs on land without direct development (terrestrial), and species with direct development (Figure 9.2).

Figure 9.2. Ancestral state reconstruction of frog reproductive modes. Data from Gomez-Mestre et al. (2012) . Image by the author, can be reused under a CC-BY-4.0 license.


Female seahorses produce eggs for reproduction that are then fertilized by the male. Unlike almost all other animals, the male seahorse then gestates the young until birth. (credit: modification of work by "cliff1066"/Flickr)

Animal reproduction is necessary for the survival of a species. In the animal kingdom, there are innumerable ways that species reproduce. Asexual reproduction produces genetically identical organisms (clones), whereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from their parents. During sexual reproduction the male gamete (sperm) may be placed inside the female’s body for internal fertilization, or the sperm and eggs may be released into the environment for external fertilization. Seahorses, like the one shown in Figure, provide an example of the latter. Following a mating dance, the female lays eggs in the male seahorse’s abdominal brood pouch where they are fertilized. The eggs hatch and the offspring develop in the pouch for several weeks.

Chapter 24. Animal Reproduction and Development

Figure 24.1. Female seahorses produce eggs for reproduction that are then fertilized by the male. Unlike almost all other animals, the male seahorse then gestates the young until birth. (credit: modification of work by “cliff1066″/Flickr)


Animal reproduction is necessary for the survival of a species. In the animal kingdom, there are innumerable ways that species reproduce. Asexual reproduction produces genetically identical organisms (clones), whereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from their parents. During sexual reproduction the male gamete (sperm) may be placed inside the female’s body for internal fertilization, or the sperm and eggs may be released into the environment for external fertilization. Seahorses, like the one shown in Figure 24.1, provide an example of the latter. Following a mating dance, the female lays eggs in the male seahorse’s abdominal brood pouch where they are fertilized. The eggs hatch and the offspring develop in the pouch for several weeks.

Too Much Estrogen

Estrogen-like chemicals are the suspected culprits. A higher-than-expected level of estrogen activity was detected in water collected from 79 percent of the sites. However, no tests have occurred yet to identify specific chemicals.

An unusual signature in the blood of fish from the Missisquoi River also points toward environmental estrogens. Researchers found high levels of vitellogenin—a protein involved in producing egg yolk—in many smallmouth bass. In male fish, the gene that tells the body to produce vitellogenin is usually “turned off,” explains Iwanowicz. That gene only “switches on” in the presence of estrogen, a female sex hormone.

“When we find vitellogenin in the blood it’s a pretty clear indication that those male fish were exposed to extra estrogens of some kind,” says Iwanowicz.

One environmental estrogen is ethinyl estradiol—a chemical found in birth control pills. In laboratory studies, scientists have been able to induce intersex in some fish by exposing males to the compound.

Yet the answers in nature—where fish are exposed to a variety of chemicals and other stressors—are never as clear-cut as in the tightly controlled laboratory environment.

Other environmental factors might contribute to intersex in fish, including low levels of dissolved oxygen and warming water temperatures. Scientists don’t yet understand how some of these other factors may be influencing feminization, explains Tillitt.

Researchers are also probing the consequences of intersex for fish health.

Experiments with minnows suggest that exposure to environmental estrogens may cause problems for fish populations. Very severe levels of feminization—having a lot of egg cells in the testes—can impair sperm quality, impeding a fish’s ability to reproduce. But feminization is a continuum: Males with only a few eggs in their testes may have no trouble at all reproducing.

The intersex findings in bass at the refuges don’t appear to be linked to any population-scale reproductive problems for the popular sport fish.

Nevertheless, Sturm worries about what the findings mean for some of the refuge’s more vulnerable species, such as the spiny softshell turtle. About 200 of the leathery-skinned turtles reside in Lake Champlain, most of them clustering around the mouth of the river. They’re vulnerable to water pollution and other threats, including predation, boating, and fishing hooks. No other population of spiny softshell turtle exists in New England or Quebec, though it is found elsewhere in North America.

“Bass are a sentinel species, but it’s possible that other animals could be affected too,” says Sturm.

Reproductive impairment isn’t the only concern, says Blazer. In the Potomac and Susquehanna Rivers, she’s seen an increase in diseases, die offs, and infections in some fish species. Their immune systems are weak. These health problems seem to correlate with levels of intersex.

“It’s possible that the environmental chemicals inducing intersex may also be causing immune system problems,” she says.

Exposures to endocrine disrupting chemicals in drinking water, food, and household products have been linked to health problems in people too, including reduced fertility, developmental delays in children, and some cancers. But it’s too soon to say whether feminized fish are indicative of health effects for humans too.

“Knowing that environmental chemicals which disrupt endocrine function are out in the environment at concentrations above thresholds for effect should lead us to try to evaluate the risk in a more comprehensive fashion,” says Tillitt.


The rhabditid nematode (roundworm) Strongyloides stercoralis is the major causative agent of strongyloidiasis in humans. Rarer human-infecting species of Strongyloides are the zoonotic S. fuelleborni (fülleborni) subsp. fuelleborni and S. fuelleborni subsp. kellyi, for which the only currently known host is humans. Strongyloides spp. are sometimes called &ldquothreadworms&rdquo (although in some countries this common name refers to Enterobius vermicularis).

Other animal-associated Strongyloides spp., including S. myopotami (nutria), S. procyonis (raccoons), and possibly others, may produce mild short-lived cutaneous infections in human hosts (larva currens, &ldquonutria itch&rdquo), but do not cause true strongyloidiasis.

Life Cycle

Strongyloides stercoralis

The Strongyloides stercoralis life cycle is complex, alternating between free-living and parasitic cycles and involving autoinfection. In the free-living cycle: Rhabditiform larvae are passed in the stool of an infected definitive host , develop into either infective filariform larvae (direct development) or free-living adult males and females that mate and produce eggs , from which rhabditiform larvae hatch and eventually become infective filariform (L3) slarvae . The filariform larvae penetrate the human host skin to initiate the parasitic cycle (see below) . This second generation of filariform larvae cannot mature into free-living adults and must find a new host to continue the life cycle.

Parasitic cycle: Filariform larvae in contaminated soil penetrate human skin when skin contacts soil , and migrate to the small intestine . It has been thought that the L3 larvae migrate via the bloodstream and lymphatics to the lungs, where they are eventually coughed up and swallowed. However, L3 larvae appear capable of migrating to the intestine via alternate routes (e.g. through abdominal viscera or connective tissue). In the small intestine, the larvae molt twice and become adult female worms . The females live embedded in the submucosa of the small intestine and produce eggs via parthenogenesis (parasitic males do not exist) , which yield rhabditiform larvae. The rhabditiform larvae can either be passed in the stool (see &ldquoFree-living cycle&rdquo above), or can cause autoinfection .

Rhabditiform larvae in the gut become infective filariform larvae that can penetrate either the intestinal mucosa or the skin of the perianal area, resulting in autoinfection. Once the filariform larvae reinfect the host, they are carried to the lungs, pharynx and small intestine as described above, or disseminate throughout the body. The significance of autoinfection in Strongyloides is that untreated cases can result in persistent infection, even after many decades of residence in a non-endemic area, and may contribute to the development of hyperinfection syndrome.

Life Cycle

Strongyloides fuelleborni

Strongyloides fuelleborni follows the same life cycle as S. stercoralis, with the important distinction that eggs (rather than larvae) are passed in the stool . Eggs hatch shortly after passage into the environment, releasing rhabditiform larvae , that develop to either infective filariform larvae (direct development) or free-living adult males and females . The free-living adults mate and produce eggs, from which more rhabditiform larvae hatch and eventually become infective filariform larvae . The filariform larvae penetrate the human host skin to initiate the parasitic cycle . These larvae migrate via the bloodstream to the lungs, where they are eventually coughed up and swallowed, or reach the intestine via migration through connective tissue or abdominal viscera . In the small intestine, larvae molt twice and become adult female worms. Parasitic females embedded in the submucosa of the small intestine and produce eggs via parthenogenesis (parasitic males do not exist) .

Since eggs do not hatch within the host as with S. stercoralis, autoinfection is believed to be impossible. Transmission of S. fuelleborni subsp. kellyi to infants as a result of breastfeeding has been reported.


Strongyloides spp. are generally host-specific, and S. stercoralis is primarily a human parasite. However, patent infections with parasitic females have been detected in other primates (chimpanzees, monkeys, etc.) and domestic dogs. Two genetic populations have been found in domestic dogs, one that appears to only infect dogs and one that may infect both dogs and humans all human infections have been attributed to this second genetic population. Domestic cats are experimentally susceptible to S. stercoralis infections although it is unknown if they have a role as a natural reservoir.

Strongyloides fuelleborni subsp. fuelleborni is a parasite of Old World apes and monkeys. The only identified host of S. fuelleborni subsp. kellyi is humans.

Geographic Distribution

Strongyloides stercoralis is broadly distributed in tropical and subtropical areas across the globe. Transmission has been reported during summer months in temperate areas. Infections are most common in areas with poor sanitation, rural and remote communities, institutional settings, and among socially marginalized groups.

S. fuelleborni subsp. fuelleborni occurs in non-human primates throughout the Old World. The vast majority of human infections are reported from sub-Saharan Africa. Sporadic cases have been reported from Southeast Asia. S. fuelleborni subsp. kellyi is found in Papua New Guinea, and has not been reported elsewhere thus far.

Clinical Presentation

The initial sign of acute strongyloidiasis, if noticed at all, is a localized pruritic, erythematous rash at the site of skin penetration. Patients may then develop tracheal irritation and a dry cough as the larvae migrate from the lungs up through the trachea. After the larvae are swallowed into the gastrointestinal tract, patients may experience diarrhea, constipation, abdominal pain, and anorexia. Chronic strongyloidiasis is generally asymptomatic, but a variety of gastrointestinal and cutaneous manifestations may occur. Rarely, patients with chronic strongyloidiasis may develop other complications (e.g. arthritis, cardiac arrhythmias, chronic malabsorption, duodenal obstruction, nephrotic syndrome, recurrent asthma). Up to 75% of people with chronic strongyloidiasis have mild peripheral eosinophilia or elevated IgE levels.

Hyperinfection syndrome and disseminated strongyloidiasis are most frequently associated with subclinical infection in patients receiving high-dose corticosteroids. Subsequent impaired host immunity leads to accelerated autoinfection and an overwhelming number of migrating larvae. In chronic strongyloidiasis and in hyperinfection syndrome, the larvae are limited to the GI tract and the lungs, whereas in disseminated strongyloidiasis the larvae invade numerous organs. A variety of systemic, gastrointestinal, pulmonary, and neurologic signs/symptoms have been documented complications can be severe. Left untreated, the mortality rates of hyperinfection syndrome and disseminated strongyloidiasis can approach 90%.

The subcutaneous migration of filariform larvae in the autoinfective cycle, or &ldquolarva currens&rdquo, presents as a recurrent serpiginous maculopapular or urticarial rash along the buttocks, perineum, and thighs due to repeated autoinfection. This rash usually advances very rapidly (up to 10 cm/hr).

In infants infected with S. fuelleborni subsp. kellyi, a severe, often fatal, systemic illness involving protein-losing enteropathy has been described, which sometimes presents with peritoneal ascites (&ldquoswollen belly syndrome&rdquo).

Is there an example of a species wherein the female lays eggs in the male? - Biology

T he mating of one female with more than one male while each male mates with only one female is known as polyandry (literally, "many males"). It is a rare mating system, occurring in less than one percent of all bird species, and is found mostly in shorebirds. Polyandry is often accompanied by a reversal of sexual roles in which males perform all or most parental duties and females compete for mates. The common pattern of sexual dimorphism is often reversed in polyandrous birds: the female is often larger and more colorful than the male. This reversal confused early biologists and led Audubon to mislabel males and females in all of his phalarope plates.

Two types of polyandry have been documented: simultaneous polyandry and sequential polyandry. In simultaneous polyandry, each female holds a large territory containing the smaller nesting territories of two or more males who care for the eggs and tend the young. In our region, only Northern jacanas characteristically practice this form of polyandry. Females may mate with all of their consorts in one day and provide each male with help in defending his territory. A female will not copulate with a mate while their eggs are being incubated or during the first six weeks of the fife of the chicks. If a clutch is lost, she will quickly copulate with the broodless male and lay a new batch of eggs within a few days.

A very rare variation on the preceding theme is "cooperative simultaneous polyandry," in which more than one male mates with a single female and the single clutch of mixed parentage is reared cooperatively by the female and her several mates. This arrangement occurs in some populations of Harris' Hawks and occasionally in Acorn Woodpecker groups.

In sequential polyandry (the most typical form of this mating system), a female mates with a male, lays eggs, and then terminates the relationship with that male, leaving him to incubate the eggs while she goes off to repeat this sequence with another male. Spotted Sandpipers, Red-necked and Red Phalaropes are examples of sequentially polyandrous species that breed in North America. A possible evolutionary precursor of sequential polyandry is found in Temminck's Stint, Little Stint, Mountain Plover, and Sanderling. In these species, each female lays a clutch of eggs that is incubated by the male, followed by a second clutch that she incubates herself. These two-clutch systems can be envisioned as a step toward the sort of sequential polyandry seen in the Spotted Sandpiper, but females of that species never incubate a clutch alone unless their mate is killed -- even when resources are abundant.

There is an interesting sidelight to the story of polyandry in birds. In polygynous mammals (one male mating with several females) such as lions and gorillas, infanticide can occur when a new male takes over a harem. By killing the young of the previous harem ruler, the new male presumably brings females back into heat. This gives him a chance to increase his own reproductive contributions and, perhaps, to reduce use of resources by unrelated offspring. In Northern jacanas it has been reported that females taking over the territories of other females occasionally practice infanticide, destroying the offspring of previous females. The males attempt to defend their broods (which represent their genes, but not those of the new female), just as lionesses attempt to defend their cubs from infanticidal male lions taking over a pride. However, the actual killing of young has not been observed -- only empty nests. If substantiated, this behavior in jacanas is the first known example of infanticide being used as a reproductive strategy by females.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.

Watch the video: Ο αρσενικός γκέκο πάχυνε, η θηλυκή αδυνάτισε (May 2022).


  1. Mikagal

    In gonivo

  2. Finnbar

    cool pictures

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