Nocturnal activity in wild chimpanzees (Pan troglodytes): evidence for flexible sleeping patterns and insights into human evolution

Abstract Objectives We investigated occurrences and patterns of terrestrial nocturnalactivity in wild chimpanzees (Pan troglodytes) and modelled the influence of various ecological predictors on nocturnalactivity. Methods Data were extracted from terrestrial camera‐trapfootage and ecological surveys from 22 chimpanzee study sites participating in the Pan African Programme: The Cultured Chimpanzee. We described videos demonstrating nocturnalactivity, and we tested the effects of the percentage of forest, abundance of predators (lions, leopardsand hyenas), abundance of large mammals (buffalos and elephants), average daily temperature, rainfall, human activity, and percent illumination on the probability of nocturnalactivity. Results We found terrestrial nocturnalactivity to occur at 18 of the 22 study sites, at an overall average proportion of 1.80% of total chimpanzee activity, and to occur during all hours of the night, but more frequently during twilighthours. We found a higher probability of nocturnalactivity with lower levels of human activity, higher average daily temperature, and at sites with a larger percentage of forest. We found no effect of the abundance of predators and large mammals, rainfall, or moon illumination. Discussion Chimpanzee terrestrial nocturnalactivity appears widespread yet infrequent, which suggests a consolidated sleeping pattern. Nocturnalactivity may be driven by the stress of high daily temperatures and may be enabled at low levels of human activity. Human activity may exert a relatively greater influence on chimpanzee nocturnalbehavior than predator presence. We suggest that chimpanzee nocturnalactivity is flexible, enabling them to respond to changing environmental factors.