Strategies of habitat use by mountain gorillas.

Patterns of home range use by a mountain gorilla group are examined here in relation to variation in food abundance and quality, diet quality and rainfall, to interactions between different social units and to the distribution of hazards (poaching risk). Variation in habitat quality influenced both long-term area occupation densities and shorter-term movement parameters (day journey length and the distance moved between consecutive feeding sites). Observed home range use patterns increased the gorillas' foraging efficiency. Rainfall influenced the evenness of home range use slightly, but this was the only seasonal variability. Interactions with other gorillas and poaching risk had proximate effects superimposed on the fundamental influence of food distribution patterns. Data from a larger sample of groups show a positive influence of group size on day journey length, but this was apparent only at very small and relatively large group sizes. The absolute distance travelled per day tends to be short even for large groups. The data support arguments that the costs of social foraging are low for mountain gorillas.