Male Mating Interest Varies with Female Fecundity in Pan troglodytes schweinfurthii of Kanyawara, Kibale National Park
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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.