Competition has long been thought to be more intense between males than between females in most animals, and consequently more attention has been given to studying within-sex competition in males than in females. Recently, however, this imbalance has begun to be redressed. Researchers have realized that although competition may be less spectacularly violent among females than among males, competition among females may nevertheless have profound effects on the fitness of the individuals involved. Furthermore, female–female competition may not simply be for food, as was traditionally thought, but for an array of other resources as well, and the resulting complications have lent additional interest to the subject. The new generation of studies of intrasexual competition in females is well represented by a paper on female chacma baboons in this issue, written by Dorothy L. Cheney and Robert M. Seyfarth of the University of Pennsylvania and Joan B. Silk of the University of California Los Angeles.
Cheney and colleagues studied a group of wild chacma baboons in the Okavango Delta of Botswana over a 15-year period. As is typical for these baboons, the group was a large one, with a total of between 55 and 76 individuals aged 1 year or older depending on the year. The principal independent variables examined by Cheney et al. were the numbers of adult females, which varied from 24 to 36, and the sex ratio, which varied from a low of 2.5 females per male to a high of 4.5 females per male. These variables were related to potential costs and benefits of competition using linear mixed models.
One result was that the number of overtly aggressive acts given per female increased as the number of females per male increased. This relationship was particularly pronounced among subordinate females. Overt aggression included lunges, chases and bites, behaviours that might well be costly to those attacked. Some at least of the aggression seems to have been motivated by competition for mates. Cheney and colleagues found a positive association between the dominance ranks of males and females associating in consortships; this association could result from dominant females excluding lower-ranking females, although males' preference for dominant females provides another explanation. In years with high ratios of females per males, the researchers observed an increase in mob attacks by females on other females, attacks in which the target was usually an oestrous female in a consortship with a male. Females may compete for long-term male partners because such partners provide some protection against infanticidal attacks on a female's offspring by other males.
Figure 1. A male chacma baboon grooms a female. Females of this species compete with each other for long-term male partners. Photo: Anne Engh.