Patch choice decisions by a fission-fusion forager as a test of the ecological constraints model

Chapman et al.’s (1995, Behav Ecol Sociobiol 36:59-70) 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 characterized 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 (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.