How feeding competition determines female chimpanzee gregariousness and ranging in the Taï National Park, Côte d'Ivoire.

Socioecological theory suggests that feeding competition shapes female social relationships. Chimpanzees (Pan troglodytes) live in fission-fusion societies that allow them to react flexibly to increased feeding competition by forming smaller foraging parties when food is scarce. In chimpanzees at Gombe and Kibale, female dominance rank can crucially influence feeding competition and reproductive success as high-ranking females monopolize core areas of relatively high quality, are more gregarious, and have higher body mass and reproductive success than low-ranking females. Chimpanzee females in Tai National Park do not monopolize core areas; they use the entire territory as do the males of their community and are highly gregarious. Although female chimpanzees in Tai generally exhibit a linear dominance hierarchy benefits of high rank are currently not well understood. We used a multivariate analysis of long-term data from two Tai chimpanzee communities to test whether high-ranking females (1) increase gregariousness and (2) minimize their travel costs. We found that high-ranking females were more gregarious than low-rankers only when food was scarce. During periods of food scarcity, high rank allowed females to enjoy benefits of gregariousness, while low-ranking females strongly decreased their gregariousness. High-ranking females traveled more than low-ranking females, suggesting that low-rankers might follow a strategy to minimize energy expenditure. Our results suggest that, in contrast to other chimpanzee populations and depending on the prevailing ecological conditions, female chimpanzees at Tai respond differently to varying levels of feeding competition. Care needs to be taken before generalizing results found in any one chimpanzee population to the species level. Am. J. Primatol. 73: 305-313, 2011. (C) 2010 Wiley-Liss, Inc.