HEIDINGER , B.J.; NISBET, I.C.T.; KETTERSON, E.D.; LEPIRE, J.A.; Indiana University: Age and the glucocorticoid stress response in the common tern, Sterna hirundo

Investment in reproduction is predicted to increase with age. Although empirical studies generally support this expectation, the physiological mechanisms underlying variation in reproductive investment are poorly understood. In response to stressors, vertebrates release glucocorticoids, which stimulate a physiological cascade that inhibits reproduction and enhances self-maintenance behaviors. Therefore, the stress response may be an endocrine mechanism mediating the trade-off between investment in current reproduction versus future reproduction and survival. When the value of current reproduction is high, the stress response is often suppressed to ensure critical resources are not diverted away from reproduction. Consequently, adults would be expected to modulate their response to stressors with age. We tested this prediction in a long-lived seabird, the common tern Sterna hirundo. To measure the stress response, free-living adults were captured during incubation, and blood samples were collected repeatedly at 0-3, 10, 30, and 50 minutes using a standard handling protocol. Corticosterone levels were then measured using radioimmunoassay techniques. We collected stress response data for 70 known-aged individuals, ages ranging from 3-28 years old. The relationship between age and the stress response will be discussed in relation to life-history evolution.