Copulation calls of female chimpanzees (Pan troglodytes) signal fertility

In many species, females produce vocalizations during or soon after mating. In chimpanzees, these calls accompany only some matings while visual signals of sexual receptivity are always visible. Proposed explanations for chimpanzee copulation calls include (1) increasing paternity confusion and thus reducing infanticide risk, and (2) competing more effectively with other females for male mating interest. We used information theoretic model selection to test competing hypotheses using a record of 1,168 copulation events recorded by the mother-infant project team at Gombe National Park, Tanzania (1976-2015). We found that females produced copulation calls more frequently in the last week of their reproductive cycle, when they are most fertile. This timing is consistent with the hypothesis that copulation calls function as an honest signal of fertility. While females also signal fertility visually (with sexual swellings), copulation calls may signal to a broader audience, encouraging males to invest in mating effort. Male primates are often portrayed as always being ready to mate, but in reality, mating entails potential costs for males, including the risk of aggression from other males, and opportunity costs that must be weighed against the risk of mating with a non-ovulating female.