Nest-building is an elementary behaviour in all of the great apes, but also a fundamental mis-nomer. Nests are more accurately described as beds, which are constructed nightly by each weaned individual for sleep and often during the day for rest or play. Little is known about the techniques or skills involved in nest construction. These nightly structures survive in the landscape providing long-lasting signs of chimpanzees’ use of their range, and are frequently studied as a means to assess chimpanzee presence, absence, or numbers in new or existing areas where chimpanzees live. Several projects are ongoing at Ugalla, investigating nest characteristics and distribution in the landscape.
Nest architecture and functions
Nest building is known, from captive studies, to involve learning (although interestingly also an innate tendency towards construction), yet how nests are built has not yet been examined in nature. Nest architecture was examined at Issa through detailed step-by-step deconstruction of each component of nests, and correlated with ecological factors such as tree species, or weather conditions.
Why nests are built may seem an obvious question considering their nightly construction for sleep. Yet when set in the evolutionary context whereby no other higher primate is known to construct nests, including large bodied primates such as baboons, the question of why they are built is significant. In a lifetime, an individual chimpanzee may build over 19,000 structures. Several functions of nest building have been investigated: protection from predators, parasites, or biting-insects, or thermoregulation (see below). For example, chimpanzees in Issa nest higher within the trees than those at other sites, perhaps due to the presence of four possible predators – lions, leopards, hyenas, and wild dogs. These chimpanzees also build more insulating, thicker nests in colder conditions. Future work will investigate individual variation in nest building through indirect identification of builders through genetic analyses of faecal samples.
Nest Site Selection
Chimpanzee nest site selection in Ugalla shows preference for woodland vegetation on slopes over other combinations of vegetation and topographic level not explained by differences in tree species or morphology. If nests serve specific functions hypothesised above then site preference is expected to correlate with microclimatic differences (temperature, humidity, wind-speed, vector or biting-insect density) in preferred versus non-preferred locations. Alternatively, nesting-site selection could be unrelated to nest function and correlated with acoustic properties of the locations, for example, nesting on slopes may provide long-calls to travel farther down valleys saving energy expended on essential communication in this species. By combining different research techniques and monitoring chimpanzee habitat use for nesting and calling we can investigate chimpanzee landscape use and nesting-site selection.
Nest Site Re-use
Previous research in Issa has shown that specific areas of the landscape are repeatedly selected for nesting and that tree-morphology plays a role in selection when comparing used versus not-used locations (see Hernandez-Aguilar 2009). Such comparisons are possible in a savanna site due to the long-life of nests in Ugalla.
Long-term monitoring of re-use of these areas is underway and has already demonstrated the repeated use of specific nesting locations within trees (see Stewart et al. 2011). Continued monitoring will continue to investigate how chimpanzees modify nest-building sites and how this may influence patterns of re-use, both specifically within trees and across the landscape.
Further tree and nest morphology observations are being made at re-used versus non-re-used nesting-sites to investigate the impact nest-building may have on the trees and tree morphology.