Evidence for intrasexual selection in wild female baboons

Because female reproductive success in social mammals is determined largely by parental rather than mating effort, intrasexual 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 intrasexual 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 intrasexual 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 towards females, with low-ranking females exhibiting particularly high rates of aggression towards 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 intrasexual competition for social partners, mates, paternal investment, and perhaps also male protection occurs even in species where females exhibit low reproductive skew.

► Female–female competition in baboons increases with increasing sex ratio skew.
► Female baboons’ bonds weaken with increasing female numbers.
► Female mortality increases as female numbers increase and male numbers decrease.
► Females incur costs when living in groups with many females and skewed sex ratios.