This study investigates the social relationships in captive groups of Bornean orangutans (Pongo pygmaeus) concerning their social system and sociality. Wild orangutans are supposed to live in fission-fusion societies with a high flexibility in spatial and social structures. Zoo-living orangutans however are usually housed in stable groups. It is unclear whether a permanent group-living under the spatial restrictions of captivity over longer periods of time corresponds to the orangutans` social potential. The orangutans’ social system is still not well understood. Proximate data referring to mechanisms regulating the social relationships, including a sufficient sample size of individuals, are largely missing so far. \ud This study examines the structure of social relationships, the persistence and form of fission-fusion tendencies, and how the individuals deal with the group-living conditions. Three sets of orangutans were comparatively studied. Two groups were housed under constant group-keeping conditions; one group was kept under more flexible conditions. The analyses focused on the patterns of the interactive and spatial behaviour. Additionally, long-term observations were carried out on one of the groups, and a second sampling method was implemented allowing a sequence analysis of behaviours. \ud The results of this study support the assumption that orangutans have the potential to engage in a higher amount of social activities compared to the wild without many conflicts. To evaluate this social potential under natural conditions is hardly possible. The structure of social relationships between the studied animals was largely compatible to what is described for free-ranging orangutans, though the relationships tended to be generally friendlier. The different keeping conditions seem to have little influence on the social patterns. The socio-spatial patterns were characterized by frequent encounters and leavings; interactions were mainly sociopositive but brief. On the structural level, this pattern is in accordance with the fission-fusion sociality described for wild orangutans. Orangutans may possibly have an intrinsic motivation to merge and split regularly and to establish only weak social bonds. Future work should refer to this, both under the conditions in the wild and under human care. The findings of this study can contribute to the further development of new concepts on fission-fusion dynamics in primates. This work points out that a detailed analysis of the interactive and spatial-temporal patterns under the conditions in captivity provides important and supplemental clues about the sociality of orangutans. This may account also for the management of this endangered species in zoos and fragmented areas in the wild.\u
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