Evidence for intra-sexual selection in wild female baboons

Because female reproductive success in social mammals is determined largely by parental rather than mating effort, intra-sexual competition among females has typically been assumed to occur primarily over food. Recently, however, renewed attention has been paid to the importance of other sources of variation in female fitness, with a concomitant focus on a broader definition of intra-sexual selection that encompasses both competition for resources and competition for mates and social partners. We present behavioural and demographic data gathered over 15 years on a group of wild chacma baboons (Papio hamadryas ursinus) which show that females incur several costs when living in groups with many females and comparatively few males: increased intra-sexual aggression, less stable bonds with female partners, and increased mortality. Female-female aggression was higher in years when the adult sex ratio was more highly skewed toward females, with low-ranking female exhibiting particularly high rates of aggression toward other females. Females' social bonds with other females also became less stable in years when the group contained many females. Finally, female mortality rates were highest in years when the group contained comparatively more females and fewer males. The negative correlation between the number of males and female mortality suggests that the increase in mortality was due to increased predation rather than food competition. Results indicate that intra-sexual competition for social partners, mates, paternal investment, and perhaps also male protection occurs even in species where females exhibit low reproductive skew.