Heart rate and endocrine responses to stress in adolescent male rhesus monkeys on Cayo Santiago.

Patterns of stress-responsiveness were studied in 19 adolescent male rhesus monkeys (Macaca mulatta) who are part of the free-ranging colony on Cayo Santiago, PR. Morphometric and blood samples were obtained as each male was captured and after holding overnight. Subject heart rate was recorded on Day 2, using surface EKG telemetry. Males showed marked individual differences in cardiac and endocrine profiles which were generally unrelated to their age, size or maternal rank. Heart rate patterns were correlated with several endocrine measures: males with low and variable heart rates showed lower cortisol, higher-prolactin and higher growth hormone levels on Day 2 relative to males with higher and less variable heart rates, and their testosterone levels increased rather than decreased. Males with low and variable heart rates appear to have an endocrine response profile that is less adversely affected by acute stress, and which may potentially give them a competitive advantage in social interactions.