Person:

Wrangham, Richard

Aggression is rare among wild female chimpanzees. However, in the Kanyawara chimpanzee community in Kibale National Park, Uganda, stable use of food-rich core areas is linked to increased reproductive success, suggesting that contest competition might occur over access to the highest-quality ranges. To examine this hypothesis, we studied aggression and dominance relationships among Kanyawara females during a 10-yr period that included the immigration of 5 females into the community. We tested 2 predictions: 1) that female-female aggression should intensify when immigrants enter the community because this is when core area access is determined and 2) that the quality of core areas should reflect relative female dominance relationships. In support of the first prediction, female-female aggression increased 4-fold when new immigrants were in the community, with rates peaking when there were multiple immigrants. This pattern was due primarily to aggression by resident mothers toward immigrants and featured coalitionary aggression, a rare behavior among female chimpanzees. In support of the second prediction, females occupying core areas high in foraging quality ranked high overall and higher than expected for their ages, whereas females occupying low-quality core areas were lower-ranking and ranked lower than expected for their ages. Together, the data indicate that though female aggression does not regularly occur in chimpanzees, contest competition continues to play an important role in determining long-term access to resources, an important correlate of reproductive success.

The ecological pressures shaping chimpanzee anatomy and behavior are the subject of much discussion in primatology and paleoanthropology, yet empirical data on fundamental parameters including body size, morbidity, and mortality are rare for wild chimpanzees. Here, we present skeletal pathology and body size data for 20 (19 crania, 12 postcrania) chimpanzees (Pan troglodytes schweinfurthii) from Kibale National Park, Uganda. We compare these data with other East African populations, especially Gombe National Park. Estimated body size for Kibale chimpanzees was similar to other East African populations and significantly larger than Gombe chimpanzees. The high rates of trauma and other skeletal pathology evident in the Kibale chimpanzee skeletons were similar to those in the Gombe skeletal sample. Much of the major skeletal trauma in the Kibale skeletons was attributable to falls, although other pathologies were noted as well, including apparent injuries from snares, degenerative arthritis, and minor congenital abnormalities.

Female chimpanzees mate promiscuously during a period of extended receptivity marked by prominent sexual swelling. Recent studies of wild chimpanzees indicate that subtle variations in swelling size could act as a reliable cue of female fertilization potential both within and between cycles (Emery and Whitten Behavioral Ecology and Sociobiology, 54, 340–351, 2003; Deschner et al. Hormones and Behavior, 46, 204–215, 2004). Copulation rates increase during the periovulatory period and during conception cycles (Deschner et al. Hormones and Behavior, 46, 204–215, 2004; Emery Thompson American Journal of Primatology, 67, 137–158, 2005a), suggesting that males may be able to assess female fertilization potential. We asked whether facultative timing of copulation in Kanyawara chimpanzees was due to increased male mating interest or to increased female proceptivity during the most fecund days. We assessed multiple measures of male mating effort in cycles aligned relative to the day of detumescence and compared periovulatory days to other days of maximal swelling, and conception cycles to nonconception cycles. The rate and proportion of male initiative in soliciting sexual behavior increased during periods of highest fertilization potential. Males were also more likely to interrupt copulations, associate with estrous females, and compete with other males when females were most likely to conceive. Females initiated copulations more frequently during conception cycles but did not visibly shift mating behavior within cycles. Our results support the hypothesis that male chimpanzees have the ability to assess the profitability of mating attempts, a trait that may act as a counter-adaptation to female strategies to obscure paternity. We discuss potential cues and the implications for female reproductive strategies.