Choice of Sleeping Sites by Chimpanzees in Savanna Woodlands, Western Tanzania

We studied sleeping sites used by chimpanzees (Pan troglodytes) in savanna woodland areas of Tanzania outside national parks in Tanzania from 1994 to 2011. We walked 1,043 km of census lines and recorded 479 beds in 193 clusters within 30 m from the census line. We prepared vegetation and topological maps using satellite and global information system data. We divided the census lines into 60 m square quadrates. One or more chimpanzee beds occurred in 154 quadrates, and no beds occurred in the remaining 16,597 quadrates. Evergreen forests occupied more area (mean 25.4%) in the quadrates with beds than in those without beds (5.5%). The ground slope was steeper in the quadrates with beds (10.5°) than those with no beds (6.0°). A GLM showed that evergreen forests and slope affected the choice of sleeping sites. Chimpanzees made few beds in woodlands during the dry season simply because most woodland trees were defoliated. Bed distribution during the rainy season also showed that evergreen forests were included in more areas (mean 28.2%) in the 41 quadrates with beds than in the other 2,297 quadrates with no beds (5.1%). Lions (Panthera leo) and leopards (Panthera pardus) inhabit chimpanzee habitats in Tanzania. Chimpanzees might sleep in forests to reduce the predation risk by such carnivores, because distances among trees was shorter and tree height higher than those in woodlands, and there was less grass in forests for carnivores to hide and hunt. Chimpanzees might sleep in hilly areas, because water is close to the bottom of the slope, and the carnivores might have difficulty chasing chimpanzees up the slope. Predation risk as well as the distribution of food trees and availability of sleeping trees might affect the choice of sleeping sites by chimpanzees in the savanna woodlands.