Impact of ecological and social factors on ranging in western gorillas

We examined the influence of ecological (diet, swamp use, and rainfall) and social (intergroup interaction rate) factors on ranging behavior in one group of western gorillas (Gorilla gorilla gorilla) during a 16-month study. Relative to mountain gorillas, western gorillas live in habitats with reduced herb densities, more readily available fruit (from seasonal and rare fruit trees), and, at some sites, localized large open clearings (swamps and "bais"). Ranging behavior reflects these ecological differences. The daily path length (DPL) of western gorillas was longer (mean=2,014 m) than that of mountain gorillas, and was largely related to fruit acquisition. Swamp use occurred frequently (27% of days) and incurred a 50% increase in DPL, and 77% of the variation in monthly frequency of swamp use was explained by ripe fruit availability within the swamp, and not by the absence of resources outside the swamp. The annual home-range size was 15.4 km2. The western gorilla group foraged in larger areas each month, and reused them more frequently and consistently through time compared to mountain gorillas. In contrast to mountain gorillas, intergroup encounters occurred at least four times more frequently, were usually calm rather than aggressive, and had no consistent effect on DPL or monthly range size for one group of western gorillas. High genetic relatedness among at least some neighboring males [Bradley et al., Current Biology, in press] may help to explain these results, and raises intriguing questions about western gorilla social relationships.