BODY-SIZE EFFECTS ON VERTICAL CLIMBING AMONG CHIMPANZEES

I analyzed observations from a yearlong study of the positional behavi or of Pan troglodytes at the Mahale Mountains National Park to determi ne whether there are detectable differences in behavior between large and small individuals. Analysis was complicated by a weak correlation between body size and social rank. To factor out rank effects, I perfo rmed two types of analyses, depending on the type of data: (1) multipl e regressions or (2) comparisons of similarly ranked animals of differ ent body size. With social rank effects accounted for, larger males fe d lower in the canopy, fed on the ground more often, fed preferentiall y among food tree species with smaller adult heights, and climbed sign ificantly less often than smaller males did. Contrary to expectation, large males utilized smaller weight-bearing structures than small male s did. These results suggest that large males minimized climbing versu s optimizing support diameters, perhaps because vertical climbing is d isproportionally expensive for larger animals. The large body wieght o f chimpanzees compared with other primates suggests that minimizing al titude changes, and therefore vertical climbing, is an important consi deration in budgeting daily energy expenditures.