No abstract

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.

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 <i>Pan</i>species.

No abstract

The spatial organization of a set of individuals may reflect the underlying relationships between them. This study investigated whether inter-individual distance, or the proximity between a pair of individuals, predicts the patterns of grooming interactions. The subjects were twelve chimpanzees (Pan troglodytes; M = 2, F = 10; age: mean = 34.8, range = 25-45) and six bonobos (Pan paniscus; M = 2, F = 4; age: mean = 27.3, range = 13-44) studied since September 2015 and living at Kumamoto Sanctuary (Japan). Proximity, time in contact, and time at less than one meter of all group members were recorded using focal animal sampling. The full temporal organization of grooming patterns was analyzed after ad libitum video records of the interactions. A pair of individuals that spends more time in close proximity was predicted to (i) show a shorter latency to approach before the onset of grooming, (ii) groom sensitive body parts (e.g. face and genitals) more often, and (iii) take turns in grooming more frequently than two individuals that stay far from each other. The results may suggest species-specific or relationship-dependent social tolerance, reflected in both inter-individual distance and patterns of grooming.PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.1849v1 | CC-BY 4.0 Open Access |