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Kahlenberg, Sonya Marie

The extent to which active female mating preferences influence male reproductive success in mammals is unclear, particularly for promiscuously breeding species like chimpanzees (Pan troglodytes). Previous studies from multiple long-term study sites have shown that female chimpanzees mate more restrictively around ovulation, and this has been taken as evidence for female choice. However, none of these studies rigorously evaluated the alternative hypothesis that restrictive mating results not from unconstrained choice, but in response to coercive mate guarding, in which males use punishment and intimidation to reduce female promiscuity and promote their own mating interests. Nor did they consider evidence for the potential genetic or phenotypic benefits that females might be choosing. Using 11 years of data from the Kanyawara community in Kibale National Park, Uganda, we previously demonstrated that males achieve elevated mating success with those females toward whom they direct high levels of aggression. Here we extend those findings to show that even female copulatory approaches, which have previously been attributed to female choice, are correlated with male aggression. Specifically, individual females at our site initiated periovulatory copulations most frequently with the males who were most aggressive toward them throughout their cycles. Those males showed high rates of aggression toward females throughout estrus, despite achieving high copulation rates, demonstrating a continuing conflict of interest over the exclusivity of mating access. Because sexual coercion is potentially widespread in primates and other mammals, our results stress the importance of considering the influence of male aggression in studies of female choice.

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.

In chimpanzees, Pan troglodytes, females transfer from their natal group shortly after sexual maturity to permanently join another group. A conflict of interest exists between female and male residents over the immigration of new females: additional females increase female feeding competition but provide new mating opportunities for males. Resident females express their interests by directing high rates of aggression towards immigrants, and males could protect their interests by intervening in these conflicts. We studied the Kanyawara chimpanzee community in Kibale National Park, Uganda over 10.5 years to assess whether (1) resident female aggression is costly to immigrants, (2) males are effective in protecting immigrants from aggression and (3) immigrants seek out male protective services. Results showed costs of resident aggression on two levels. Immigrants held low dominance ranks and experienced higher physiological stress than natal residents, as indexed by urinary cortisol. Males were found to be effective protectors for immigrants. They intervened aggressively to curtail female conflicts, more often when conflicts involved immigrants and occurred during periods of heightened female competition. When intervening, males nearly always supported immigrants over residents. As a result, females, especially immigrants, experienced less intrasexual aggression in the presence of males than when males were absent. Immigrants took advantage of male protective services. They associated with males more often than resident females and, unlike residents, did not greatly decrease their level of association outside of oestrus. We suggest male protection is an important strategy used by immigrants to integrate themselves into their new group.