Some species which reproduce sexually will replace one sex with (or add) hermaphrodites. The coexistence of males and hermaphrodites is androdioecious, females and hermaphrodites gynodioecious, and all three trioecious. In the absence of a male or female to mate with, a hermaphrodite is capable of self-fertilization. On Earth these strategies are found in plants, nematodes, and rarely fish (the same terminology is applied to both plants and animals in papers).

In all these cases the species only ever has two sexes. A sex is defined by the types of gametes produced: males produce sperm, females ovum. Hermaphrodites do not constitute a distinct sex because 1) they produce ovum and sperm and 2) they reproduce by fusing ovum and sperm. To constitute a third sex, a species must produce three structurally distinct types of gametes and all three must be required to reproduce.

  • $\begingroup$ Many questions similar to this one have been asked before, I'd recommend looking at the answers to this question as well as those of the questions linked to it. $\endgroup$ – DaaaahWhoosh Mar 30 '17 at 16:32
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    $\begingroup$ In many animal species individuals belong to only one sex, which would make them kind-of dioecious in botanical terminology ("kind-of" because plant reproduction is fundamentally different from animal reproduction). I think that the question should avoid using inappropriate botanical terminology for animals, and ask what selective pressures would favor hermaphroditism in some individuals of an animal species which reproduces sexually. $\endgroup$ – AlexP Mar 30 '17 at 16:50
  • $\begingroup$ It's worth to note that some higher animals, like a few reptiles, replace self-fertilization for parthenogenesis in those situations. $\endgroup$ – T. Sar Mar 31 '17 at 12:48

In the absence of a male or female to mate with, a hermaphrodite is capable of self-fertilization

This would suggest that the pressure is a frequent absence of males or females to mate with. This could occur in a species with very low population densities - species living in deserts, high mountains and other places with low levels of plant life to support a food chain. High-level predators (think snow leopards, tigers) need large territories to find enough food (each level of the food chain biomass pyramid is only 10% the mass of the one before - 200 kg of predator needs 2 tons of prey), so they are more likely to experience this difficulty in finding mates than other kinds of animals.

Population densities don't have to be permanently low - a situation (like the famous owls and lemmings in the tundra) with cycles of population spikes and crashes in prey causing similar cycles in predators might be enough to cause sufficiently frequent absences of mating partners.

Population explosion cycles caused by desert rainfall could also provide a situation where it would be beneficial. There's a boom in plant life after the rain, followed by a proliferation of herbivores, which march/fly off to find more food/territory. After the plant life dies off, herbivores start to die off, leaving behind possibly isolated individuals who can't find a mate.

Such species are perhaps unlikely to be social (due to low population density), or they could be social at times of peak population.

They are also unlikely to 'spawn' huge numbers of offspring; instead rearing a small number carefully to adulthood (as most birds and mammals do). If they were capable of spawning thousands of eggs at each mating, the rareness of the matings would be less of a problem. Producing only a few offspring at a time would make it more advantageous to increase 'matings' by allowing self-fertilization.


A strong hedge against sex linked problems.

If the animals have serious behavior or physical dimorphism (trimorphisim?) and events occasionally largely wipe out a single sex occasionally hermaphrodites allow rapid recovery.

Say you have females better or more inclined to eating from trees and sometimes earthquakes wipe out them, but the males more at home on plains sometimes get wiped out by floods and the hermaphrodites are adept at scrub land which is vulnerable to fire. If they are all fairly common events being able to recover very fast from a single even would be very valuable. Self fertilization only becomes interesting in the case where a double event not hurting the hermaphrodites happens.

  • $\begingroup$ This seems to just shift the question to "what selection pressures would favor male, female, and hermaphrodite individuals of one species inhabiting three different habitats and only coming together for mating?" This kind of situation doesn't seem very likely to occur - especially if events sometimes wipe out all of one gender in one of the three habitats. In this situation it seems more adaptive to just live in mixed groups. $\endgroup$ – Tharaib Mar 31 '17 at 2:03

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