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Stable Social Structures for Species with Three sexes

As a continuation of this question, I'm curious about the viability of long-term social groups with the following characteristics:

There are three sexes, identified as A, B and C. Sex A is a sperm donor. Sex B and C are egg donors and carry the pregnancy to term.

  • When A and B mate, the child will always be a C.
  • When A and C mate, the child will always be a B.
  • When B and C mate, the child will always be an A.

Assume that physiology is approximately mammalian with breast milk for the children. Gestation periods are less than one year. Sexual maturity is reached in approximately two years. Fertility periods are biannually. Total lifespan for all members of this species is approximately 20 years.

Social Organization

Bs and Cs form stable herds with only a one or two As in the herd. All child rearing is done collectively by the Bs and Cs of the herd. Extra A children are raised to maturity then ejected from the herd to form small groups with other ejected As from other herds.

Producing children of sex A is restricted to the alpha B and alpha C members of the herd. The herd's A may mate with as many Bs and Cs as they please as this always produces more Bs and Cs.

Is this reproductive and social arrangement stable on the scale of millions of years? If it is not stable, why not? I'm aware that this arrangement is very similar to existing social structures on Earth so I'm hopeful that it is stable over the long term.

Out of Scope

  • This is a pre-technology species. Any signalling that needs to be done happens without human-like language.
  • How a species with three sexes appeared in the first place. This species just is.
  • Inter-herd warfare is out of scope.
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  • $\begingroup$ Are B and C hermaphroditic? $\endgroup$ – Alexander May 7 '18 at 18:42
  • $\begingroup$ @Alexander Haven't gotten to that level of detail. Also, I don't know the mechanism that determines which B or C gets pregnant from a mating. $\endgroup$ – Green May 7 '18 at 19:19
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    $\begingroup$ I'd put up a framing answer with heaps of detail but the crux of it is that you're FAR more likely to have 2x sperm donors than you are 2x egg donors. Also, having all 3 sexes able to mate with either remaining sex isn't viable either because of the plumbing and energy requirements involved in bringing children into the world. Better to leave women the way they are, and introduce 2 different types of male; it's simpler, meaning that if evolution requires 3 genders, this is the model that would likely develop (although 2 would be preferred by evolution every time but that's another issue). $\endgroup$ – Tim B II May 7 '18 at 22:36
  • $\begingroup$ I agree, the acres dies bit make much sense, the closest you get is alternating generations found in many types of insects sheet one generation reproduces aesexually, and the next sexually, essentially giving three sexes the male, female, and aesexually reproducing female. Though the female and the asexual female Halen to be one and the same, just no males were present from the sexual births. $\endgroup$ – Garret Gang May 7 '18 at 23:15
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It should be, what you're describing is essentially the same social arrangement as most gregarious animals on Earth from Lions and Wolves to Deer and Bison. The introduction of a second gender to the breeding-group structure shouldn't be too disruptive, the only thing I would point out is that there will be a strange balancing act between the sex-drives of each gender towards the others and the number of individuals of a given gender needed for population stability, this will largely depend on the degree to which the A's mortality rate is affected by being outside the pack in what in wolves are termed lobo packs. I don't know about wolves or herbivores but I seem to recall that male Lions without a pride life about half as long as those with one.

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