Chapman et al.’s (Behav Ecol Sociobiol 36:59–70, 1995) ecological constraints model posits that the size and distribution of food patches place restrictions upon foraging group size. Larger groups incur increased travel costs for any given array of patches, and thus, to fulfil individual energetic and nutritional requirements, foragers should adjust group sizes to balance energy obtained against that spent on travelling. Support for this model comes from both comparative and species-specific studies but findings are contradictory, and the utility of the model has been questioned. This study provides a rigorous test, analysing measurements from distinct food patches and individual inter-patch movements, on an appropriately shorter temporal scale. Using data drawn from two social groups of a species characterised by a high degree of fission–fusion dynamics, the chimpanzee (Pan troglodytes), we show that larger parties foraged in larger food patches and for longer durations, and that larger parties were associated with further travel between patches. Overt contest competition over food increased with party size. We found no evidence of distinct sex differences in either party size or travel distances: the predictive power of forager sex was low compared to that of ecological variables. We propose that analysis at the patch level is more appropriate than a daily averaging approach that may smooth out the very variation being investigated. Our findings suggest that, despite certain limitations, Chapman et al.’s (Behav Ecol Sociobiol 36:59–70, 1995) model of ecological constraints remains a useful tool. Ecology does indeed constrain grouping patterns, and the impact of this is not necessarily differentiated by sex. Significance statement Foraging animals face the ‘more mouths to feed’ problem: as the numbers in a group increase, the group must travel further to find enough food, using up energy. Hence, foragers should adjust numbers to minimise these costs, but tests of this idea have proved inconclusive. We investigated the foraging behaviour of chimpanzees, a species with highly flexible grouping, considering their travel between specific patches of food. We found clear support for this proposition, with larger patches of food hosting larger numbers of foragers, and such groups having to travel further to find food. Although it is often thought that female animals should respond more strongly to foraging costs, we found little evidence of sex differences. Our results show that ecology does indeed constrain grouping patterns, and that the impact is felt equally by males and females.

of the DissertationChimpanzees (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 iii 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 siz...

Associating with kin provides individual benefits but requires that these relationships be detectable. In humans, facial phenotype matching might help assess paternity; however, evidence for it is mixed. In chimpanzees, concealing visual cues of paternity may be beneficial due to their promiscuous mating system and the considerable risk of infanticide by males. On the other hand, detecting kin can also aid chimpanzees in avoiding inbreeding and in forming alliances that improve kin-mediated fitness.

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