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.
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.