Ecological and dietary differences between Ugandan chimpanzee communities with possible implications on tool use

Some East African chimpanzee (Pan troglodytes schweinfurthii) communities, such as the Sonso chimpanzees, display an unusually limited range of tool-use, but it remains unclear whether this is due to ecological and/or cultural factors. Information on ecological conditions and the diet of the Sonso chimpanzees in relation to neighbouring communities is needed. Here, we studied three adjacent communities in Budongo Forest (Sonso, Waibira, and Kamira), and the presumed core area of an undescribed community (Mwera), in the neighbouring Bugoma Forest. Through line-transects, we investigated (i) whether there were differences in food diversity and abundance between the communities’ home ranges; (ii) whether the home ranges differed in abundance of sticks and insect nests; and (iii) whether Sonso and Mwera chimpanzees differed in their diet (using faecal samples). Across communities, Sonso had the richest food availability and the lowest insect nest abundance. However, food availability in Mwera, Bugoma, was richer than Budongo communities that neighbour the Sonso territory, suggesting that there may be variation within Budongo. Data from faecal samples replicated our direct observations of food availability suggesting that Sonso chimpanzees had a broader diet than Mwera chimpanzees. This difference in foods availability may partially explain the Sonso chimpanzees’ lack of stick-tool-use, and low levels of insectivory. The tool repertoire of the other communities is currently unknown; however, we make predictions based on our ecological data. More detailed knowledge of small-scale variation in ecology within and between forest habitats may be important to advancing our understanding of the drivers of tool-use.SIGNIFICANCE STATEMENT – HIGHLIGHTED STUDENT PAPERTo advance our knowledge of the role of ecological factors in the emergence of tool use in chimpanzees, a nuanced understanding of the ecological conditions different chimpanzee communities experience is needed. We studied four Ugandan chimpanzee communities in two forests. One of these communities, Sonso, in the Budongo Forest, is well-known for its restricted range of tool types, including a total absence of stick use. Food diversity and abundance were highest, and stick tool use opportunities (abundance of sticks and insect nests) were lowest for the core-habitat of the Sonso chimpanzees in contrast to the other communities. We argue that ecological factors play a role in their unusual pattern of tool use, and make predictions about the expected types of tool use in the other communities based on their ecology. Thus, our study provides information that may help advance our understanding of how tool use arises under varied socioecological circumstances.