Most of what is currently known about western gorilla (Gorilla gorilla) diet is based on indirect studies using fecal samples and trail signs rather than measures based on direct observations. Here we report results on adult male and female western gorilla foraging behavior, based on systematic focal observations and nutritional analyses of foods. We found that western gorillas, like other apes, are highly selective ripe fruit specialists, seeking fruit high in energy, low in antifeedants, and rare in the environment. During seasonal fruiting peaks, fruit accounted for up to 70% of feeding time. When ripe fruit was scarce, gorillas increased time spent feeding on leaves and nonpreferred fruits and herbs. Leaves were the major fallback food, accounting for up to 70% of feeding time in males and 50% in females during periods of fruit scarcity. In spite of large differences in body size, the sexes were remarkably similar in their overall diet, not differing in time spent feeding on fruit or preferred herbs. However, the male consistently fed more often and on a greater variety of leaves than did females, whereas females fed more often on fallback herbs and termites. Our findings, when considered in light of previous findings on sympatric mangabeys, indicate that the foraging strategy of western gorillas is broadly similar to that of chimpanzees and orangutans, and distinct from that of old world monkeys.

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.

Habituation of western gorillas to human presence is generally an expensive, lengthy and difficult process. Here we describe the habituation process for two groups of western gorillas at the Mondika Research Center, with the hope that the lessons we learned will facilitate future gorilla studies. We expand upon earlier studies by describing the process through complete habituation for both males and females, and for more than one group. The major obstacle to habituation was developing sufficient tracking skills to follow gorilla trail on a daily basis. Once this was achieved, the silverback became semi-habituated (i.e. ignoring human presence during half of contacts) within a year, although the majority of group females continued to avoid humans. As female presence at contacts increased, a period of male recidivism followed, requiring an additional year before his complete habituation was reached. Habituating the females took longer than the male, but we found, contrary to earlier studies, that it consisted of the same stages, including avoidance, aggression, and curiosity before habituation. We compare results between groups and across sites and discuss how factors such as tracking abilities, group size and cohesion, population density and home range overlap, and the manner of approaching gorillas during contacts influence the habituation process.