Female gregariousness in Western Chimpanzees (Pan troglodytes verus) is influenced by resource aggregation and the number of females in estrus

Abstract

Among social animals, group size is constrained by competition over resources. Because female reproductive success is limited by access to food resources, and that of males by access to fertile females, chimpanzee females are proposed to be less social than males and to maintain weak intrasexual relations. Findings from Tai National Park, Cote d’Ivoire, challenged this view, as chimpanzee females were described as generally gregarious, and close intrasexual bonds were common. Here, in a new analysis that focuses on the South Group of chimpanzees in Tai forest, we reevaluate the proposed differences in female association patterns between the Tai and East African populations. We find that mean party size and dyadic association index between females has decreased in Tai, although the level of dyadic associations remains high compared with East African chimpanzees. We attribute the decrease in female gregariousness to the decline in community size over the last 10 years. In addition, we use a multivariate approach to analyze social and ecological factors influencing party size in females. We show that female gregariousness increased when the fruit resources were more clumped and with increased number of females in estrus present. Party size of mothers with sons, however, was smaller with increasing number of sexually receptive females. The results of our model and the reviewed findings of other studies support the socioecological model because food distribution affects female gregariousness, but social and demographic aspects are equally influencing female grouping tendencies.