Female chimpanzees use copulation calls flexibly to prevent social competition

Abstract

The adaptive function of copulation calls in female primates has been debated for years. One influential idea is that copulation calls are a sexually selected trait, which enables females to advertise their receptive state to males. Male-male competition ensues and females benefit by getting better mating partners and higher quality offspring. We analysed the copulation calling behaviour of wild female chimpanzees (Pan troglodytes schweinfurthii) at Budongo Forest, Uganda, but found no support for the male-male competition hypothesis. Hormone analysis showed that the calling behaviour of copulating females was unrelated to their fertile period and likelihood of conception. Instead, females called significantly more while with high-ranking males, but suppressed their calls if high-ranking females were nearby. Copulation calling may therefore be one potential strategy employed by female chimpanzees to advertise receptivity to high-ranked males, confuse paternity and secure future support from these socially important individuals. Competition between females can be dangerously high in wild chimpanzees, and our results indicate that females use their copulation calls strategically to minimise the risks associated with such competition.

Figure 1. Copulation calls and the effect of male rank.

Bar graphs showing the percentage of copulations accompanied by calls, N = 75, given by seven females when copulating with high (N = 5) and low (N = 3) ranking males.

Figure 2. Individual variation in copulation calling behaviour.

Line graphs showing the proportion of copulations accompanied by a call when copulating with high (N = 5) and low (N = 3) ranking males for each of the seven females.

Figure 3. Audience effects.

Mean number of individuals in the audience in the presence/absence of a copulation call. Female audience (N_females = 5, N_copulations = 90): the number of adult females present. Male audience (N_females = 7, N_copulations = 126): the number of adult males present. Error bars represent Mean+−1 SE.

Figure 4. Male rank and female audience composition interaction.

Line graphs showing the mean number of high and equal ranked females in the audience when copulating with a) high ranked males and b) low ranked males. Error bars represent Mean+−1SE.

Figure 5. Copulation call spectrogram.

Time-frequency spectrogram of a female copulation call from Budongo Forest during maximum tumescence. Filter bandwidth: 159 Hz, Frequency resolution: 86.1 Hz. Depicted is (A) the total copulation calling bout of approximately 6.5 s and (B) a single copulation call of approximately 0.6 s by the female JL. (C) The lowest visible band is the fundamental frequency from which acoustic measurements were taken with three visible harmonic bands. Copulation calls have a frequency range of 700–1000 Hz.

Figure 6. Chimpanzee ovulation profile.

Profiles of urinary pregnandiol in ng/mg creatinine and perineal swelling for the adult female WL during April 2007. POP: periovulatory or fertile period, defined as the day of ovulation plus the three preceding days; PrePOP: period of maximum tumescence prior to POP PostPOP: period of maximum tumescence following POP