Hunting and Prey Switching by Chimpanzees (Pan troglodytes schweinfurthii) at Ngogo

Abstract

Predation can have major effects on vertebrate population dynamics, including predator–prey cycles that occur on multiple time scales when predators switch between preferred and alternative prey and changes in the abundance of preferred prey when the availability of alternative prey changes. Predation has been an important selective force in primate social evolution and may regulate some primate populations, but assessing its impact is often difficult. Chimpanzees (Pan troglodytes) provide opportunities for direct collection of data on predation intensity by following habituated predators because they regularly prey on a variety of vertebrates, including other primates. Chimpanzees mostly hunt red colobus monkeys (Procolobus spp.) wherever they live in sympatry. Previous reports have documented heavy predation on red colobus by chimpanzees in the unusually large Ngogo community, in Kibale National Park, Uganda, and have shown an apparent decline in the local red colobus population. Here, we update information on predation at Ngogo, give further evidence of a red colobus population decline, and ask whether the chimpanzees have made more effort to hunt alternative prey in response. Data collected between 1995 and 2014 show that the rate of alternative prey hunts has increased significantly, while the rate of red colobus hunts has declined. Predation on red colobus peaked in 2002; other prey have since accounted for significantly higher proportions of hunts and prey captures. The overall hunting rate has not changed significantly, but per capita meat availability has decreased because hunts of alternative prey lead to fewer captures than hunts of red colobus. The likelihood that the chimpanzees hunt red colobus on encounter has not changed significantly. In contrast, the likelihood of hunting on encounter has increased (redtail monkeys, Cercopithecus ascanius; gray-cheeked mangabeys, Lophocebus albigena) or remained stable (guerezas, Colobus guereza) for other primates that are major alternative prey species. Encounter rates have not changed for redtails and mangabeys, but the guereza encounter rate has decreased. These results imply that the red colobus population is not recovering and raise the possibility that the chimpanzees are having a negative impact on the guereza population. Chimpanzees are not obligate carnivores, but this may make them prone to cause such impacts because their own populations are probably not limited by those of their prey.