Fission–fusion species under restricted living conditions: a comparative study of dyadic interactions and physical proximity in captive bonobos and Bornean orangutans

The present study investigates how the 'fission–fusion-adapted' bonobos and Bornean orangutans manage social relationships when kept under permanent group-living conditions. Our results showed that the bonobos and orangutans did not differ in the overall frequency of dyadic interactions. The orangutans evidently realized a potential to interact with partners, which on a surface did not differ from what was found in the bonobos. However, the bonobos spent more time on sociopositive interactions, especially on grooming and sit in contact, whereas the orangutans agonistically interacted with each other more often. Though frequencies of approaching were similar between the two species, orangutans actively left the proximity to a partner more often than the bonobos, which in turn were more often in spatial proximity. The three groups of bonobos housed under different conditions differed from each other for sociopositive and agonistic interactions. The orangutans differed for agonistic but not for sociopositive interactions. As a striking difference between the species, it appeared that between subadult/adult orangutans, behaviours which required prolonged body contact occurred only rarely and briefly. Differences in bonding patterns have been discussed as a possible explanatory factor. The 'short and distant nature' of interactions between adult orangutans suggests the existence of social relationships, the management of which requires less 'servicing behaviours'. Subadult/adult orangutans may be less attracted by each other than individuals in more gregarious species: they may have the cognitive skills to interact, but may not be motivated to stay together for long.