Corticosterone levels predict survival probabilities of Galapagos marine iguanas during El Nino events

Plasma levels of corticosterone are often used as a measure of stress in wild animal populations. However, we lack conclusive evidence that different stress levels reflect different survival probabilities between populations. Galapagos marine iguanas offer an ideal test case because island populations are affected differently by recurring El Nino famine events, and population-level survival can be quantified by counting iguanas locally. We surveyed corticosterone levels in six populations during the 1998 El Nino famine and the 1999 La Nina feast period. Iguanas had higher baseline and handling stress-induced corticosterone concentrations during famine than feast conditions. Corticosterone levels differed between islands and predicted survival through an El Nino period. However. among individuals, baseline corticosterone was only elevated when body condition dropped below a critical threshold. Thus. the population-level corticosterone response was variable but nevertheless predicted overall population health. Our results lend support to the use of corticosterone as a rapid quantitative predictor of survival in wild animal populations.