Social relationship and hair cortisol level in captive male chimpanzees (Pan troglodytes)

Understanding how social relationships affect long-term stress is important because stress has a profound impact on the welfare of animals and social relationships often exert a strong influence on their stress responses. The purpose of this study was to investigate the relationship between social behaviors and long-term stress levels as assessed by hair cortisol (HC) concentration. The subjects were 11 chimpanzees living in an all-male group (divided into two sub-groups) in Kumamoto Sanctuary, Kyoto University, Japan. Behavioral data were collected between December 2014 and March 2015. The total observation time was 129 h. Hair samples were collected in late March and early April 2015, and cortisol was extracted from the hair and measured with enzyme immunoassay. The hair growth rate was estimated to be 1.33 ± 0.06 cm/month. The results revealed that there was a positive correlation between the rate of receiving aggression and HC levels. We also found a significant negative correlation between the balance between giving and receiving grooming (grooming balance index: GBI), which was calculated by subtracting the rate with which grooming is given from that with which it is received, and the rate of receiving aggression and between the GBI and HC levels. Thus, individuals receiving higher levels of aggression also tended to give grooming for relatively long periods compared to the time they were being groomed. In contrast, the rate of initiating aggression did not have a relationship with either HC levels or any measure of social grooming. We did not find social buffering effects, as there was no correlation between mutual social grooming and HC levels. These results show that not only aggressive interactions but also overall social situations in which animals do not have balanced relationships with others might result in the long-term elevation of cortisol levels in captive male chimpanzees.