There is a tribal sentient species living in a forest (somewhat similar to Avatar one, with giant trees). I'm not gonna go into too much detail about them (if it will be required, I'll edit this question to add details). The peculiar thing about them is the distribution of probabilities of birth of individuals of certain genders genders:
- 50% probability of birth of a fertile female (capable of giving birth);
- 25% probability of birth of a sterile female (incapable of giving birth);
- 25% probability of birth of a male.
And the distribution of genders in their society is approximately the same.
My question is, can such a civilization exist and survive (and maybe even develop itself), or is it biologically or even mathematically impossible?
EDIT1: Speaking of their reproduction rate: they are mammals with pregnancy period of about 8 Earth months, a female gets fertile at ~15y/o and generally has 1-3 children at once. As for lifespan, those who die from old age generally live up to 70 Earth years. The actual average lifespan is lower due to dangers like predators.