The foraging ecology of two neighbouring chimpanzee communities from Budongo Forest

Chimpanzees (Pan troglodytes) often serve as a model species to test socio-ecological theories of foraging behaviour. Due to a high degree of fission-fusion dynamics, essential foraging variables, such as group size, patch size and travel distance, are expected to be more closely linked in chimpanzees than in animals that forage in cohesive groups. While it has been clearly established that the relationship between party size and patch size follows theoretical predictions, the importance of other foraging variables, such as travel distance, and sex differences in foraging strategies are less well understood. Also, the picture of chimpanzee feeding ecology is informed by a large number of individual chimpanzee communities from all across Africa, but foraging behaviour in chimpanzees of the same population has rarely been studied in detail. Here I present the feeding ecology of two neighbouring, interbreeding chimpanzee communities from the Budongo Forest Reserve in Uganda, Sonso and Waibira, that occupy home ranges of different vegetation composition and differ in overall size. From October 2015 to June 2017 I followed adult male and female chimpanzees in each study community for a period of 8 months, collecting data on individual food patches and inter-patch distances by specific focals. These were combined with measurements of food availability and data on forest composition within the home ranges of each community. A first assessment of forest composition across home ranges showed that two important non-fig food species might be more abundant within the Waibira home range. During this study, the diet of the Sonso community was characterized by a low diversity and considerable variation in quality. Food availability for this community varied and chimpanzees, especially males, supplemented their diet with field-crops. Food availability and diet quality for the Waibira community was more stable, resulting in overall more balanced activity budgets. Waibira chimpanzees do not forage on field crops and, possibly as a result of this, diet composition of this community was more variable and diet diversity was higher. Young leaves were an integral part of the diet in both communities, a result which underlines the importance of protein in chimpanzee diet and the need to reassess chimpanzee foraging strategies with regard to a balanced intake of macronutrients. The Waibira community foraged on average in smaller parties and smaller food patches, travelling shorter inter-patch distances when all travel was considered. However, the general foraging strategy of using fission-fusion dynamics to minimize feeding competition appeared to be very similar in both communities: Larger parties foraged in larger food patches and party size increased with travel distance and feeding bout length. Chimpanzees in both communities chose food patches in a similar way: Across sexes and communities, chimpanzees exhibited a clear preference for closer as well as novel food patches, whereas the predictive power of patch size was generally low. Overall, sex differences in activity budgets and foraging behaviour were insignificant, questioning the general assumption that female chimpanzees need to forage in a fundamentally different way compared to male chimpanzees. Findings of this study demonstrate that, in order to judge the utility of socio-ecological models and advance our understanding of factors that shape foraging strategies, comprehensive models of foraging behaviour are needed, that incorporate several important variables simultaneously. The extent to which chimpanzees were able to adjust activity patterns and dietary composition as well as diversity to different forest environments, while maintaining a general strategy to maximize foraging success, suggest that they are more generalist foragers than currently acknowledged.