Determinants and evolutionary consequences of dominance among female gelada baboons

Summary

Dominance relations among free-living female gelada baboons (Theropithecus gelada) are shown to depend on the individual's aggressiveness, modified by coalitionary support from female relatives. Relatives rank adjacent to each other more of often than expected by chance.

Females are more willing both to give coalitionary support and to do so on an asymmetric basis to females with whom they interact socially (usually relatives) than to those with whom they rarely interact.

The number of offspring that a female has is shown to be a function of her dominance rank. This phenomenon is probably due to the fact that females harass individuals subordinate to them when the latter are in oestrus. It is suggested that the stress caused by this harassment appears to disrupt the female's reproductive physiology, thereby inducing a high rate of anovulatory cycles and amenorrhea. In consequence, subordinate females take longer to conceive than do dominants.

Simulation is used to show that females who form coalitions gain a life-time reproductive advantage over those who do not because coalitions with younger females help to prevent the decline in rank that would otherwise occur in old age.

It is argued that females prefer to form coalitions with caughters rather than unrelated females because the mother-daughter relationship is the only bond of sufficient strength to provide the basis for an investment which is asymmetric in the short-term and reciprocal only over the length of a life-time. Any benefits that accrue from kin selection are considered to be secondary.