Changes in salivary cortisol and testosterone levels in male chimpanzees during the process of reunion with group members

The impact of re-introduction of a temporally separated individual to a group on social integration and hormone levels is an important topic in terms of the management and welfare of captive animals. Using two cases in which a temporally separated male re-joined an all-male group of captive chimpanzees, Pan troglodytes, we investigated how social reunion affected social interactions and salivary steroid hormone levels (C, cortisol; T, testosterone). In both cases, social reunion was conducted over three stages, separation, acclimation, and reunion. In both cases, behavioral observations revealed increased agonism and tension reduction after the reunion, suggesting that the reunion induced reformation of social relationships. In both cases, the separated male showed an increased C level at the acclimation stage, in which the separated male was moved to its original indoor room and released solely into an outdoor compound. The T level did not vary among males or stages, but increased for the re-joined male after reunion in one of the two cases. This study suggests that the relationships between hormones and behavior during a reunion could be affected by multiple factors and that special attention should be paid to the acclimation rather than characteristics of social reunion, to ensure the success of member reunion.