Mellowing with age: older parents are less responsive to a stressor in a long-lived seabird
Résumé
1. Reproductive success often increases with age; however the mechanisms underlying this commonly observed pattern are poorly understood. One mechanism that may be important is a set of physiological responses (the stress response) that allows organisms to evade and cope with stressors, but often inhibits reproduction. 2. If older parents respond less strongly to stressors than younger parents, this age-related difference in the stress response may contribute to the higher reproductive success that often characterizes older parents. 3. Typically the stress response is measured as an increase in plasma glucocorticoid (CORT) concentration, and we have previously reported that stress-induced CORT levels decline with age in the common tern (Sterna hirundo). Another hormone, prolactin (PRL), has been reported to decrease in response to stressors in breeding birds and is often positively associated with parental behaviour. We predicted that like the CORT stress response, the PRL stress response would also be suppressed with age. 4. To test this prediction, we captured known-age, incubating common terns ranging in age from 3 to 29 years and measured stress-induced changes in PRL and CORT levels within the same individuals. 5. We found that PRL levels decreased less rapidly in response to capture and restraint stress in older than in younger parents. In these same birds, we also found that stress-induced maximum CORT levels decreased with age, which is consistent with what we have previously reported for this species. Measures of PRL and CORT were not, however, correlated within individuals. 6. Taken together, these results support the hypothesis that modulations of both the PRL and CORT stress response are flexible hormonal mechanisms that help to account for the increase in reproductive success that occurs with age.