Environment, glucocorticoids, and the timing of reproduction.

Glucocorticoids mediate glucose availability under stressful and non-stressful conditions and, therefore, are essential for life. However, data across taxa demonstrate that chronic or elevated secretion of corticosterone or cortisol (CORT) can have negative effects at many levels and can trigger physiological or behavioral responses that may delay or, even halt reproduction. We present a brief overview of the effects that glucocorticoids, primarily the avian form, corticosterone, can have on the reproductive axis. Considerable data have demonstrated that environmental perturbations can result in elevated CORT levels that alter a bird's investment in current reproduction. Studies in our laboratory have shown a link between CORT and timing of reproduction in Florida scrub-jays: in "bad" years, clutch initiation dates are positively correlated with baseline CORT levels of female breeders. Also, population-level differences in CORT levels may explain timing of reproduction as lower CORT levels in suburban-dwelling jays are coupled with early breeding. Most research on stress and CORT concentrates on transient effects of CORT secretion. However, developmental CORT exposure, either from the yolk or embryo, may have long-term effects upon adult phenotype. For example, CORT levels in nestling scrub-jays predicts later 'personality,' as levels were highly correlated (r(2)=0.84) with fearfulness at 7 months of age. One can imagine that such 'personality' traits might also translate into differential success in gaining a territory or a mate. While speculative, it may be that early CORT exposure effectively programs adult behaviors that have wide ranging effects, including upon reproduction.