Copulation Calls in Female Chimpanzees (Pan troglodytes schweinfurthii) Convey Identity but Do Not Accurately Reflect Fertility

Copulation calls are a relatively common feature of female primate behavior thought to function in the advertisement of female receptivity and subsequent incitation of male-male competition. To date, the majority of work on copulation calling behavior has focused on various monkey species, with little empirical evidence from the great apes. Previous research on wild chimpanzees (Pan troglodytes schweinfurthii) has suggested that estrous females produce copulation calls to avoid monopolization by single males and to minimize competition from other females. We here extended these findings by investigating to what degree these social demands were reflected in the calls' acoustic structure. We recorded and acoustically analyzed 71 copulation call bouts from 6 adult female chimpanzees in the Budongo Forest, Uganda. We did not find any acoustic differences in calls given by females in fertile and nonfertile periods, as assessed by their hormonal profiles. However, the calls' acoustic structure did reliably encode identity cues of the calling female. We propose that, in chimpanzees, the use and morphology of copulation calls have jointly been shaped by the selective advantage of concealing fertility. Owing to the low visibility conditions associated with chimpanzees' natural forest habitat and their dispersed social system, providing identity cues may be of particular biological relevance for these nonhuman primates.