Determinants of group size and composition for Colobus vellerosus at Boabeng-Fiema, Ghana: Ecological versus social factors

Intense within-group food competition is not thought to influence social organization for folivorous primates. Thus, they are expected by the socioecological model to live in large groups, exploiting benefits such as predation avoidance. However, many folivores form small groups, well below the threshold to avoid within-group scramble competition for food (the "folivore paradox", Janson & Goldsmith, 1995). Social factors, rather than ecological factors, are thought to determine social organization for folivores, though this has rarely been systematically tested. In this study, the relative contribution of ecological versus social factors on the group size and composition of ursine colobus monkeys (Colobus vellerosus) at the Boabeng-Fiema Monkey Sanctuary, Ghana was explored. Four groups were followed for 22-months (from 2002-2005) and behavioural observations were done using focal-animal sampling and ranging scans. Phenology, a large-tree survey, and a quadrat survey were used to determine home-range quality. It was found that within-group scramble competition for food constrained group size, despite a highly folivorous diet. Social factors also constrained overall group size and appeared to have more influence on group composition. High-ranking males used infanticide and other aggressive behaviours to influence female mate choice. Female dispersal, forced female emigration, and female resistance to female immigration indicated female competition for group membership. Therefore, females actively sought to maintain small group sizes. Within groups, females mated promiscuously, despite differing male agonistic display outputs, which appeared to indicate fighting ability. Promiscuous mating was likely a female counter-strategy to the high rate of infanticide. However, females may have attempted to immigrate into groups where the male(s) won in previous encounters with the male(s) in the females' original group. This suggests a female preference for residing with strong males and may be linked to protection against infanticide. Theoretical examination of the folivore paradox has suggested that infanticide risk should affect females at smaller group sizes than increased food competition, which has a detrimental effect at larger group sizes. Colobus vellerosus seems to conform to this idea, in that social factors appear to constrain group size at an earlier point than ecological factors while also showing more effect on group composition.