Category: Sex & Reproduction
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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.
