Female-female aggression around mating: an extra cost of sociality in a multimale primate society

Multimale-multifemale groups, where both sexes mate promiscuously and the operational sex ratio is male biased, represent a classical mammalian society. Theory predicts low mating competition between females in such societies, but this is inconsistent with the frequent occurrence of female sexual signals. This study explores the determinants of female competition under such conditions by testing 3 hypotheses relating to patterns of aggression over the reproductive cycle in wild chacma baboons (Papio ursinus). Primarily, we expect the frequency of aggression to be highest among 1) lactating and pregnant females, who experience the greatest energetic demands, if females compete mainly over food, 2) lactating females, if females compete mainly over paternal care of infants, or 3) sexually active (swollen) females, if females compete mainly over mates. Data were collected from 27 females in 2 groups over 18 months and analyzed using mixed models. Our results provide most support for the mating competition hypothesis: aggression increases with the number of swollen females in a group, swollen females receive the most aggression, and mate-guarded swollen females receive more aggression than when unguarded. However, our analyses further indicate that such aggression rather than arising from direct mating competition, most likely reflects reproductive suppression and/or an increased exposure of swollen females to incidental aggression. These findings reveal the importance of sex in shaping social relationships among females in large primate groups where they were traditionally considered to be determined primarily by access to resources. Aggression associated with access to mates represents an extra cost of sociality to females.