Variability in Population Density Is Paralleled by Large Differences in Foraging Efficiency in Chimpanzees (Pan troglodytes)

Abstract

The assumption that population density restricts the foraging efficiency of individuals in the population via increased competition for resources underpins socioecological models of female social relationships in primates. We examined this assumption by comparing quantitative measures of foraging efficiency in two communities of chimpanzees (Pan troglodytes schweinfurthii) that inhabit the same contiguous forest in Kibale National Park, Uganda, but differ substantially in size and density. To calculate net caloric gain rates (our measure of foraging efficiency) we obtained data directly from focal follows of individual chimpanzees on 1) residency time in a feeding patch, 2) feeding rate in the patch, 3) the nutrient content of the items fed upon, and 4) the distance walked between feeding patches. We collected foraging efficiency data over 1059 h at Ngogo and 961 h at Kanyawara. We found that individuals in the high-density community (Ngogo) had higher mean foraging efficiency values than those in the low-density community (Kanyawara), and that foraging efficiency varied less over time at Ngogo when assessed over the time scales of individual feeding/traveling bouts and of daily net caloric gain rates. Fluctuation in net caloric gain rates on a monthly time scale was greater at Ngogo than at Kanyawara, but this was likely due to the nutritional effects of a mast fruiting event by one of the most important species at the site. These findings suggest that high population density at Ngogo has not precluded high foraging efficiency. The classic view of increased population density inevitably increasing feeding competition and reducing foraging efficiency likely underemphasizes the ability of primates, especially those in rich habitats, to maximize caloric intake.