Behavioural and hormonal changes during group formation by male chimpanzees

Group housing of socially-deprived individuals facilitates welfare and socialisation of primates. Here, we studied behavioural and hormonal changes in the course of group formation among nine male chimpanzees, Pan troglodytes. Aggression, reassurance, and grooming were observed in various dyads during group formation. The pattern of fluctuations in salivary cortisol level changed through the process of group formation, with particularly high cortisol levels immediately after group interactions relative to other sampling timings during group formation. Salivary testosterone levels were unaffected by the process of group formation or sampling time. These results suggested that a combination of behavioural observation and hormonal analyses is a powerful approach to assess the impact of group formation.