Do chimpanzees build comfortable nests?

Nests built by wild chimpanzees (Pan troglodytes verus) were studied at the Fongoli research site in southeastern Senegal from January 2004-May 2004 to investigate the role of comfort in nest building behavior by relating measures of nest comfort and building effort. Nest comfort across zones of the nest surface were compared with construction effort for 25 nests. Several variables of nest comfort were assessed: (1) physical discomfort, (2) visible discomfort, and (3) softness. Physical discomfort was used as a representative measure of nest discomfort. Building effort was measured by (1) construction force, (2) complexity, and (3) added material. Spearman rank correlations compared Effort and Comfort measures for both whole nests and central versus edge zones. The results show that construction force and complexity do not influence comfort of the nest as a whole. Greater Construction force correlates with more nest edge discomfort, yet the central area shows no difference. More complex nests do result in a more comfortable central area in the nest. Nests built with greater force may result in more discomfort, whereas complexity may allow chimpanzees to maintain comfort in a central area for sleep. Chimpanzees may place additional leaves or twigs over hard branches, protruding from the nest surface after construction, to increase comfort of the central nest area. Functions of chimpanzee nest building are likely to be several, but these results suggest comfort is a factor in nest building behavior.