A pigheaded compromise: do competition and predation explain variation in warthog group size?

The reproductive payoff to an individual for participating in a group will often be affected by the size of the group. Competition for resources and predation pressure are 2 primary factors that influence sociality and group size in a variety of species. In this correlative study, I investigated how resource competition and predation influenced group size and female reproduction in the common warthog, Phacochoerus africanus. The distribution of group sizes indicated that group size depended on the age and sex of the participants. Yearlings formed larger groups than adults, whereas adult males and adult females formed similar sized groups, although males were more likely to be solitary. The demand for preferred habitat, clan density, predation pressure, and the time of year explained significant variation in the size of adult female groups but not in the size of adult male or yearling groups. Clan areas with more warthogs per area of preferred habitat were associated with smaller adult female groups, whereas higher densities of warthogs were associated with larger group sizes. Increasing indication of predator presence in a clan area had little influence on grouping behavior in general but resulted in larger female group sizes late in the dry season and with increased sightings of females with juveniles. The relationship between group size and predation pressure may be a consequence of group stability and higher mortality in larger groups. Copyright 2010, Oxford University Press.