Copulation Call in Female Hamadryas Baboons (Papio hamadryas): Adaptive Function and Information Content

Introduction: In primates, copulation calls are vocalizations emitted by females at the end of copulation, being common in promiscuous species organized in multi-male, multi-female groups. These calls are thought to increase male competition for access to females, enhancing either male guarding or sperm competition around ovulation, which could respectively increase or confuse paternity certainty and reduce the risk of infanticide. Female hamadryas baboons also produce these calls. However, this species is organized in one-male units, where leader males sexually monopolize the females in their units, resulting in high paternity certainty. Therefore, these vocalizations appear to provide no obvious benefit in hamadryas baboons regarding risk of infanticide, raising the question of their role in this species. Objective: This study aims to investigate the adaptive function of copulation calls in hamadryas baboons by examining their use (chapter 2) and the information they convey (chapter 3). We propose that if these vocalizations increase male competition in other primate species, in hamadryas baboons, they might be related to the coercive takeovers of females, as this is the primary consequence of male competition. Methods: This study was based on an observational design using behavioral sampling to collect data on copulations and their acoustic behavior in the Hamadryas baboon colony at Madrid Zoo during July and September 1999. Additionally, previous databases from the colony’s longitudinal study and grooming interaction records were included. Using these data, we analyzed the presence of copulation calls in each event, their acoustic structure, the male’s and female’s social status, the occurrence of ejaculation, the female’s reproductive status, the presence of related females in the unit, and female transfers to other units. Results: We observed that copulation calls are more likely in peripheral females within large units and central females in small units, occurring more frequently when the male ejaculates and during female’s maximal sexual swelling. These vocalizations were also associated with an increased likelihood of females transferring to other units, although such transfers occurred only in females belonging to small units. On the other hand, no evidence was found that copulation calls convey nformation about the male’s or female’s social status, but they did convey information about male ejaculation. Furthermore, significant acoustic variability was observed among females. Conclusion: We conclude that copulation calls in hamadryas baboons are used and contain information different from that observed in other primates with promiscuous mating systems, suggesting a potentially distinct adaptive role. The evidence indicates that these vocalizations might be primarily related to promoting female transfers to other units, where their reproductive success could be increased. However, due to the limitations of our study, definitive conclusions cannot be drawn, and we highlight the need for longitudinal studies with longer observation periods to better understand their function.