Quadratic relationships between group size and foraging efficiency in a herbivorous primate

The effect of feeding competition on foragingefficiency is an important link between ecological factors and the social organization of gregarious species. We examined the effects of group size on daily travel distances, activity budgets, and energy intake of mountain gorillasin Rwanda. We measured daily travel distances of five groups, activity budgets of 79 gorillasin nine groups, and energy intake data for 23 adult females in three groups over a 16-month period. Travel distances and the proportion of time spent traveling increased with size for most groups, which would be expected if their foragingefficiency is limited by intragroup feeding competition. However, travel distances and times decreased for the largest group, which also had higher energy intake rates than intermediate sized groups. The improved foragingefficiency of the largest group may be explained by advantages in intergroup contest competition. The largest group had much lower home rangeoverlap than the other study groups which may be due to groups avoiding one another as a result of male mating competition. Collectively, our results indicate that intermediate sized groups had the lowest foragingefficiency and provide a new twist on the growing evidence of non-linear relationships between group size and foragingefficiency in primates.