Social Relationship and Long-term Stress of Captive Chimpanzees (Pan troglodytes): Animal Welfare as a Subject of Scientific Investigation

Both general and academic attention toward animal welfare has been increasing and the importance of scientific investigation into welfare states of captive animals is being recognized. One of the big questions in scientific studies of animal welfare is how we can assess animal welfare in an objective manner, and this is an intensively debated topic. In this paper, I reviewed the studies on captive chimpanzees ( Pan troglodytes ) in order to discuss the methodologies used to assess welfare states and introduce studies that have investigated how social environments affect chimpanzee welfare by combining behavioral and hair cortisol (HC) measurements. Recently, cortisol accumulated in the hair of animals has been considered as an indicator of the long-term hypothalamus-pituitary-adrenal (HPA) axis. From a welfare perspective, long-term stress is more problematic than acute stress as it is challenging for animals to experience distress over a long period and long-term activation of the HPA axis can result in overall health deterioration. A series of studies on captive chimpanzees show that HC is useful for monitoring the long-term stress levels in captive chimpanzees. Furthermore, using the novel measure of long-term stress, I found that the stress level of male chimpanzees is affected by social variables and that male chimpanzees use social play as a means to reduce social tension. Although scientific investigation of animal welfare is still not a prevalent practice in Japan, it is a promising area of study both for improving animal welfare and deepening our understanding about animals.