Prolonged maximal sexual swelling in wild bonobos facilitates affiliative interactions between females

Abstract

Perineal sexual skin swelling in relation to menstrual cycle occurs in a variety of primate taxa. However, sexual swelling with exaggerated size and colour is found only in some Old World monkeys and the two Pan species. Although several hypotheses have been proposed (e.g., reliable indicator hypothesis and graded signal hypothesis), it seems unlikely that a single explanation can account for the significance of the sexual swelling in all of these species. Bonobos (Pan paniscus) provide an excellent opportunity for studying sexual swelling since they have the most prolonged maximal swelling periods among primates. In this study we propose a new hypothesis that sexual swelling in female bonobos increases their attractiveness to other females and thereby facilitates affiliative social interaction with them. We found that free-ranging female bonobos with maximal sexual swelling engaged in affiliative social interactions with other females, including genito-genital rubbing, staying in close proximity and grooming, more frequently than females without maximal swelling. These tendencies suggest that females with maximal swelling were attractive to other females. The results also suggest that the benefits of maximal swelling might vary among females depending on their life-history stage. In particular, young females may get more benefits from prolonged maximal swelling through increased grooming reciprocity and staying in close proximity to other females. Thus our study supported the hypothesis that one function of prolonged maximal swelling in bonobos is to increase attractiveness to other females, thereby enhancing affiliative relationships between females in a male-philopatric social system.

A Comparison Between Bonobos and Chimpanzees: A Review and Update

“…At the same time, shifts in female bonobo sexual physiology, especially prolonged periods of maximum female swelling, are also considered to play a key role in the shift toward more female-centric societies with lower aggression. 42,72 More stable access to resources may also have driven between-and within-species differences in tool use, another major species difference. In sum, it will be necessary to investigate the ecological factors, particularly food availability, as well as other potential environmental factors such as predation, 20 that led to the behavioral convergence of bonobos with either western or eastern chimpanzees.…”

“…6 Female bonobos exhibit exceptionally prolonged periods of maximal swelling (sometimes up to 20 days), as well as variable timing in ovulation and the onset of the peak swelling periods. 72 A promiscuous mating strategy combined with concealed ovulation appears to enhance and extend female attractiveness and female mate choice, which, in turn, may promote female social status and reduce the value of aggressive male competition to dominate them. 42 Among chimpanzees, dominant males tend to monopolize copulations in order to sire the most offspring; however, female chimpanzees in Ta€ ı 20 or Mahale 34 can be selective in choosing a potential sire, suggesting that they are more in control of their sexuality than had been previously thought.…”

Intergroup Transfer of Females and Social Relationships Between Immigrants and Residents in Bonobo (Pan paniscus) Societies

“…This reduces their tension, and they then eat together. GG rubbing enables female bonobos to tolerate and peacefully coexist with one another in potentially conflictual situations to a greater extent than males are able to do (Hohmann and Fruth 2000;Ryu et al 2014). …”

Dispersing Primate Females

“…GG rubbing enables female bonobos to tolerate and peacefully coexist with one another in potentially conflictual situations to a greater extent than males are able to do (Hohmann and Fruth 2000;Ryu et al 2014). Social association with resident individuals may be essential for immigrant females to establish affinitive bonds in a new group.…”