Developmental and Sex‐Based Variation in Nest Building Among Wild Immature Chimpanzees

All great apes build nests. Nests in trees or on the ground provide apes with a safe and comfortable place to sleep and rest at night and during the day. Nest building is a necessary skill and form of tool use that individuals learn and practice early in life, but little is known about its development and about the factors affecting the expression of nest building in infancy. We studied the development of daytime nest building of 72 wild immature chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda. We evaluated the effects of age, sex, and maternal parity on the likelihood of infant nest building (presence or absence), as well as on the rates (frequency/hour) and durations (time from start to finish of each nest) with which infants built nests. Compared to the youngest infants (≤ 1 year), older infants were more likely to build nests, and built them at higher rates, consistent with hypotheses that nest building requires learning in early infancy and a threshold of physical development to manipulate tree branches. Female infants were more likely to build nests and built them more often than male infants, corroborating other developmental markers suggesting females attain functional independence at younger ages than males in some chimpanzee populations. Among infants who were seen to build nests at least once, rates and durations were similar, regardless of age, sex, or maternal parity, which indicated that there was little interindividual variation in nest building development once infants started practicing. The observed patterns of prior and subsequent behaviors to nest building suggested that infants > 4 years old built nests more functionally, for resting, than younger infants. This study underscores the value of nest building as a developmental measure in wild great apes. Studying factors that affect the expression of nest building in infancy allows for a better understanding of the ontogeny and evolution of learning and tool use in hominids.