1Department of Anthropology, University of New Mexico, 2Department of Human Evolutionary Biology, Harvard University, 3Department of Biology, Bates College
Thursday 3:15-3:30, Ballroom B
Females of many primate species produce distinctive calls in association with copulation. Numerous functional hypotheses have been advanced, many proposing coevolution of copulation calls with promiscuous mating, infanticide avoidance, and sperm competition. In predictive models for primates, chimpanzees produce calls less frequently than expected. However, the dynamic social environment of chimpanzees offers an excellent opportunity to study contextual variation in call production. We studied the incidence of copulation calls in over 10,000 copulations occurring during 14 years of study in wild, unprovisioned chimpanzees at Kanyawara, Kibale National Park, Uganda. We conducted goodness of fit tests and multivariate logistical regressions with repeated measures to control for individual variation in call production. Nulliparous females called most frequently. Calls were elicited most commonly from adult males, particularly middle-ranking males, and during morning copulations. Contrary to the hypothesis that calls serve to promote male aggressive or sperm competition, females were most likely to give calls when few males were present and when pregnant rather than cycling. Calls did not appear to promote mate guarding or in female competition because call rates were independent of the presence of the alpha male or of other estrus females. Call production peaked at the median copulation length and was strongly associated with female darting behavior, suggesting that it may be a signal of ejaculation. Whereas models of copulation calls have focused on positive outcomes of call production, data from chimpanzees suggest that it is important to consider factors, such as sexual coercion, that may promote call suppression.
Funding support from The Leakey Foundation.