Sleeping tree choice by Bwindi chimpanzees.

Unlike nearly all other nonhuman primates, great apes build sleeping nests. In Bwindi Impenetrable National Park, Uganda, chimpanzees build nests nightly and also build day nests. We investigated patterns of nest tree use by Bwindi chimpanzees to understand ecological influences on nest tree selection. We analyzed data on 3,414 chimpanzee nests located from 2000 to 2004. Chimpanzees at Bwindi were selective in their use of nest trees. Of at least 163 tree species known to occur in Bwindi [Butynski, Ecological survey of the Impenetrable (Bwindi) Forest, Uganda, and recommendations for its conservation and management. Report to the Government of Uganda, 1984], chimpanzees utilized only 38 species for nesting. Of these, four tree species (Cassipourea sp., Chrysophyllum gorungosanum, Drypetes gerrardii, and Teclea nobilis) accounted for 72.1% of all nest trees. There was considerable variation in nesting frequencies among the top four species between and within years. However, these species were used significantly more often for nesting than other species in 70.9% (39 of 55) of the months of this study. A Spearman rank correlation found no significant relationship between tree abundance and tree species preference. Ninety-three percent of all nests were constructed in food tree species, although not necessarily at the same time the trees bore food items used by chimpanzees. The results indicate that nesting tree species preferences exist. Bwindi chimpanzees' choice of nesting tree species does not appear to be dependent on tree species density or use of the tree for food. We discuss possible reasons for the selectivity in nest trees by the Bwindi population.