Body Site and Body Orientation Preferences during Social Grooming: A Comparison between Wild and Captive Chimpanzees and Bonobos.

Grooming site preferences have been relatively well studied in monkey species in order to investigate the function of social grooming. They are not only influenced by the amount of ectoparasites, but also by different social variables such as the dominance rank between individuals or their levels of affiliation. However, studies on this topic mainly come from monkey species, with almost no report on great apes. This study aimed to explore whether body site and body orientation preferences during social grooming show species-specific differences (bonobos vs. chimpanzees) and environment-specific differences (captivity vs. wild). Results showed that bonobos groomed the head, the front and faced each other more often than chimpanzees, while chimpanzees groomed the back, anogenitals and more frequently in face-to-back positions. Moreover, captive individuals were found to groom facing one another more often than wild ones, whereas wild individuals groomed the back and in face-to-back positions more. While future studies should expand their scope to include more populations per condition, our preliminary 2 by 2 comparison study highlights the influence of (i) species-specific social differences such as social tolerance, social attention and facial communication, and (ii) socioenvironmental constraints such as risk of predation, spatial crowding and levels of hygiene, that might be the two important factors determining the grooming patterns in two Panspecies.

Investigating the Function of Mutual Grooming in Captive Bonobos (Pan paniscus) and Chimpanzees (Pan troglodytes).

Social grooming is often exchanged between individuals in many primate species. Rates of bidirectional (or simultaneous mutual) grooming vary across primate species, and its function is not yet fully understood. For example, mutual grooming is frequent in chimpanzees but rare in most primate species including wild bonobos. There are, however, no quantitative data available in captive bonobos. Therefore, through the direct comparison between captive bonobos and chimpanzees, this study aimed to (i) compare the frequency of mutual grooming between Pan species, (ii) explore and compare the function of mutual grooming, and (iii) discuss the rarity of this behavior in wild bonobo populations. We tested three hypotheses following the previous literature in wild chimpanzees. The social bonding hypothesis states that mutual grooming facilitates the maintenance of strong dyadic bonds. The immediate investment hypothesis states that it serves to signal willingness to invest in the interaction. The switching hypothesis states that mutual grooming serves no function but only occurs as an overlap to change the direction of unidirectional grooming. Our findings strongly supported the immediate investment hypothesis, but not the others. Grooming bouts that included mutual grooming were longer and more equitable than bouts without, illustrating that captive Pan species use mutual grooming to maximize their short-term benefits and increase the social value of their interaction. Captive bonobos performed mutual grooming in similar proportions and for similar functions as captive and wild chimpanzees do. This contrasts with wild bonobos who engage in this behavior only rarely. We suggest that the differences in patterns of mutual grooming between captive and wild bonobos might be explained by different degrees of kinship or by a potential intraspecies variation.

Social influences on grooming site preferences in wild bonobos (Pan paniscus) at Wamba, DRC.

Social grooming is assumed to have both social and hygienic functions. In monkey species, body site preferences during social grooming are related to both the dominance ranks of participating individuals and strength of social bonds. It is suggested that low-ranking groomees and individuals with weak social bonds expose less vulnerable sites of their body because doing so allows them to avoid eye contact, thus reducing the risk of aggression. This study aimed to investigate the influence of (1) the levels of affiliation (through analyses of sex, grooming frequency, and group membership during intergroup encounters), (2) dominance, (3) kinship, and (4) age on grooming site preferences (head, front, back, anogenital region, and limbs) in wild bonobos. Subjects were two groups of wild bonobos living in Wamba, DR Congo. Body site preferences were documented from video clips. Our results illustrate that the levels of affiliation between individuals and their dominance status do not seem to influence body site preferences, contrasting with results in monkey species. However, the selection of body sites was mostly influenced by age and kinship. Mothers were found to groom less accessible sites on their immature offspring's body more than when grooming a non-related immature, suggesting that mothers invest in their offspring more to prevent diseases linked to ectoparasites. Finally, mature individuals groomed the inaccessible back more often than immature individuals, while the latter focused their grooming on the head and front. Overall, this study suggests hygienic functions of social grooming in wild bonobos.