Category: Sex & Reproduction
[Skip to list of Topics for this Category →]
Reproduction is the most crucial aspect of an organism's life as it ensures that its genes are passed on to the next generation. While single-celled organisms (e.g. bacteria) and some groups of fungi and plants reproduce asexually, the majority of animals show sexual reproduction, which involves the fusion of male and female gametes. However, some animals can alternate between sexual and asexual reproduction, depending on the environmental conditions (e.g. aphids and some bees), or are exclusively asexual (e.g. rotifers). A number of amphibians and reptiles show parthenogenetic development, where offspring hatch from unfertilised eggs, and there is also anecdotal evidence of parthenogenesis in sharks and turkeys. Interestingly, strong selection for asexual reproduction is assumed to have driven convergent evolution of parthenogenesis in response to environmental change within the 'Australian arid zone', resulting in an astonishingly high frequency of parthenogenetic species, including lizards, insects and plants.
In most animals, a male and a female (or two hermaphrodites) of the same species mate, with the male transferring sperm to fertilise the female's egg(s). This is aided by intromittent organs in some groups, such as mammals and turtles, which superbly demonstrate convergent evolution of an erectile penis. While a male can easily produce millions of tiny spermatozoa and father a large number of offspring, a female invests far more resources in fewer and substantially larger eggs. This asymmetric investment leads to sexual selection - normally, a female chooses a male (female choice) to ensure that her few offspring are of high quality and thus more likely to survive and reproduce themselves. As females are the limiting resource, males compete for females (male-male competition).
Courtship behaviour, where males display to females, is rampantly convergent. Here, males can use vocalisations, visual signals, such as plumage colouration in birds (which shows convergent evolution across taxa) or olfactory attractants, i.e. pheromones (the perception of which depends on closely analogous systems in vertebrates and insects). A familiar example of courtship behaviour is lekking. Typically, males display within an arena to visiting females, which then choose a mate. Classical cases of lekking occur in birds (particularly grouse), but it can also be found in various amphibians, some mammals, fish and even a number of invertebrates, including some insects, squid and fiddler crabs. In several animals, the male presents a nuptial gift (e.g. food or silk) to the female to increase his chances of fertilisation. This has evolved several times in insects and, more unusually, in spiders.
Fascinatingly, competition between males does not only occur before mating but also afterwards. When a female mates with more than one male, then the males' spermatozoa will compete in the female's reproductive tract for fertilisation of the egg. This is known as sperm competition and is particularly pronounced if the female possesses organs for sperm storage (which have evolved multiple times independently in different animal groups). Males have come up with many different (and convergent) strategies to increase their success in sperm competition, which can be behavioural (e.g. mate guarding) or anatomical (e.g. large testes to produce a vast amount of sperm or specially shaped penises to scrape out rival sperm). A particularly curious example are the love darts of slugs, snails and earthworms, which inject allohormones that probably increase sperm competition success.
In animals, there are three different reproductive "modes" - oviparity (egg laying), viviparity (live birth) and an intermediate form termed ovoviviparity, where the eggs are retained in the body and laid at the time of hatching. Viviparity does not only occur in marsupial and placental mammals, but is rampantly convergent, having evolved more than 120 times in vertebrates alone. Although almost all of these transitions have occurred in reptiles (mainly in snakes and lizards, but also in several disparate lineages of extinct marine reptiles such as ichthyosaurs and mosasaurs), some amphibians and fish (e.g. sharks) are viviparous as well. Examples, however, spread far beyond the vertebrates and include molluscs and a number of insects, such as cockroaches, where the female even produces "milk" for the nymphs, in a striking analogy to the viviparous mammals (that even implicates the same proteins, lipocalins). Viviparity may involve the formation of complex placental structures for foetal nutrition, again not only in mammals but remarkably also in some lizards (most famously Mabuya skinks) and, to a certain degree in some fish.
Many parents show some form of care for their offspring. Elaborate parental care is considered the hallmark of birds and mammals (where it has evidently arisen independently, possibly linked to the evolution of endothermy), although it can also be found in some groups of insects (particularly eusocial insects and burying beetles). A number of reptiles, amphibians and fish provide some form of care to their young as well. Protective care of offspring, typically in a recessed cavity or pouch, has evolved numerous times - well-known instances include snails and brachiopods.
| Topic title | Teaser text | Availability |
|---|---|---|
| Males who build complex nests to entice females | Japanese pufferfish males expend gargantuan amounts of energy building complex sand nests to attract females, who lay their eggs there. Reminds remind one of bower-birds among others... | Unavailable |
| Hermaphroditism in nematodes | n/a | Unavailable |
| Nest-building in birds | n/a | Unavailable |
| Brood parasitism in cuckoos and other birds | Obligate brood parasitism has evolved several times independently in birds. Apart from the cuckoos, it can be found in four other, only distantly related families. | Available |
| Sperm chemotaxis in tunicates | n/a | Unavailable |
| Asexuality in ascidians (sea squirts) | n/a | Unavailable |
| Evolution of oviparity from viviparity in sponges (Desmospongeae) | n/a | Unavailable |
| Asexuality in rotifers | n/a | Unavailable |
| Cichlids | Cichlids are one of the cause celebr� of evolution, and rightly so because these freshwater fish show a remarkable diversity and exemplify many key aspects of adaptive radiation. But why so successful? The answer lies in the versatility of the jaws (and yes, they are convergent). | Available |
| Foam nests in animals | Nests crop up everywhere, but one made out of foam? Might not sound like a great idea, but it is. And no surprise, it has evolved several times... | Available |
| Muts proteins in plants and corals | n/a | Unavailable |
| Mimicry in fungi | Insects pollinating flowers are a familiar sight. But what happens when the "flower" is actually a fungus? Still "pollination", but now it is fungal spores. Read on to learn more about the fungi that mimic flowers... | Available |
| Explosive discharge in fungi and plants | The very rapid release of reproductive bodies is perhaps most famous in the fungi, where several methods of flinging spores at high velocity have evolved independently. | Available |
| Sociality in mole-rats and meerkats | n/a | Unavailable |
| Crabs: insights into convergence | You might think of crabs mainly as food, but this group is also highly instructive in terms of convergence… | Available |
| Nuptial gifts in insects and spiders | Male dance flies lure females with a dead insect. Not very romantic, you might think, but it certainly does the trick. Hence, such nuptial gifts have evolved in numerous other arthropods... | Available |
| Zygomycetes: convergent forms and symbioses | Zygomycete fungi show convergent evolution, and one of the most striking examples is the reproductive morphologies associated with asexual reproduction in the mucoraleans. | Unavailable |
| Parthenogenesis in Australian lizards and insects | “Evidence on the origin and spread of the two best-studied cases of parthenogenesis from the Australian arid zone, the grasshopper Warramaba virgo and the gecko Heteronotia binoei, suggests that they evolved in parallel.” – Kearney et al. (2006) Molecular Ecology vol. 15, p.1743 | Available |
| Mammal-like placentation in skinks (and fish) | “Only two types of vertebrates [have] evolved a reproductive pattern in which the chorioallantoic placenta provides the nutrients for fetal development. One is [...] the eutherian mammals […], and the other, a few lineages of the family Scincidae.” A.F. Flemming (2003) J Exp Zool 299A 33-47 | Available |
| Viviparity in sauropterygians | “The [fossilised] embryos are mostly in articulation and their distribution on each side indicates that female Keichousaurus hui had a pair of oviducts as in ichthyosaurs and many extant lizards.” Y. Cheng et al. (2003) Nature vol. 432, p.383 | Available |
| Viviparity in mosasaurs | An exceptionally preserved gravid female of the aigalosaur Carsosaurus contains at least at least four advanced embryos […] Their orientation suggests that they were born tail-first […] to reduce the possibility of drowning, an adaptation shared with other other highly aquatic amniotes” M.W. Caldwell & M.S.Y. Lee (2001) Proceedings of the Royal Society of London B, vol. 268, p.2397 | Available |
| Viviparity in ichthyosaurs | “For me, the fossil is a transporting piece of evidence. It shows a female ichthyosaur that died late in pregnancy or perhaps while giving birth; the baby was entombed with its mother in the mud.” J. Rennie (2000) Scientific American, vol. 283(6), p.8 | Available |
| Viviparity in lizards, snakes and mammals | “In over 100 lineages of […] squamates, the oviduct has been recruited for viviparous gestation of the embryos, representing a degree of evolutionary convergence that is unparalleled in vertebrate history.” D. G. Blackburn (1998) Journal of Experimental Zoology, vol.282, p.560 | Available |
| Lipocalins for milk and pheromone transport | Lipocalins are proteins that bind to and transport small hydrophobic molecules such as lipids and steroids, and have been associated with biological processes such as milk production, pheromone transport and immune responses. | Available |
| Biological uses of silk: from webs to ballooning | What material is so versatile that it can be used for capturing prey, building nests, communication and even cleaning? The answer: that most remarkable of biomaterials - silk. | Available |
| Viviparity in insects | n/a | Unavailable |
| Sexual dimorphism | n/a | Unavailable |
| Courtship behaviours | n/a | Unavailable |
| Baculum (penile bone) in mammals | Ouch!! Gentlemen, fancy a bone in your penis? Seems a bit risky, given it could fracture during copulation. Even our near ancestors had such a bone. It has probably evolved several times, but what is its function? | Available |
| Raptorial appendages in mantids and other arthropods | The praying mantises exercise a peculiar fascination, not only because of their lunging predatory habits, but also because on occasion the process of copulation ends with a decapitated male being chewed to pieces by the female while the reproductive movements continue. | Unavailable |
| Sharks and rays (elasmobranchs): insights into convergence | In terms of sensory evolution the elasmobranchs are of particular interest, because independently of other fish and even some mammals (e.g. duck-billed platypus) they have evolved electrosensory systems. | Unavailable |
| Asexuality in insects | Viviparity has emerged in a number of insects, including the thrips and cockroaches. In the latter case the female also produces a “milk” for the nymphs. | Unavailable |
| Pheromones in arthropods | Not surprisingly this is a rich area of insights into evolutionary convergence because if an animal, such as a spider, can independently evolve the pheromone then a sexual lure is turned into a metaphorical honey trap. | Unavailable |
| Parental care in vertebrates, echinoids, molluscs and brachiopods | The independent evolution of parental care is far more widespread than birds and mammals, extending as far as molluscs and echinoderms! | Unavailable |
| Love darts in slugs, snails and annelid worms | The curious habit of stabbing their partners with sharp calcareous (or chitinous) darts during courtship and prior to actual copulation has understandably attracted considerable attention. | Available |
| Penis form in mammals, turtles, birds and octopus | The specific case of a penis with a hydrostatic structure, as well as an array of collagen fibres that allows both expansion and guards against aneurysms, has evolved in a strikingly convergent fashion in mammals and turtles. | Available |
| Viviparity (live birth) in animals | Viviparity is rampantly convergent, with famous examples in the reptiles, notably the lizards and snakes. | Unavailable |
| Amphibian life cycles and terrestrialisation | Certain groups show a dramatic transition between a juvenile stage and the adult. This is perhaps most familiar in the frogs which in particular, are well-known for their aquatic tadpoles. | Unavailable |
| Lekking in birds, fish, mammals and cephalopods | Complex interactions between males and females prior to mating have evolved independently many times. Amongst the most familiar examples are leks. | Unavailable |
| Annelids: insights into convergence | Notable instances of convergence involving annelids include luminescence, moulting and the independent evolution of both compound eyes (e.g. in sabellids) and camera eyes (in alciopids). | Unavailable |
| Sexual mimicry in mammals and cephalopods | Sexual mimicry is widespread, and the most famous example is probably the male-like genitalia in the females of the hyaena. | Unavailable |
| Octopus and other cephalopods: convergence with vertebrates | What could be more different from us than the alien-like octopus? Hold on. Look it in the eye and think again. | Available |
