In search of relationships between the acute adrenocortical response and fitness.

The assumption that the acute response to stress is adaptive is pervasive in the literature, but there is little direct evidence regarding potential positive fitness consequences of an acute stress response. If acute glucocorticoid (GC) elevation increases lifetime reproductive success (fitness), in what contexts does this occur, and through what combination of effects on annual reproductive output and interannual survival? Here we examine the framework under which most comparative acute GC studies fall, evaluate the commonalities of those studies in the light of expected fitness effects, and suggest methods to better examine the potentially beneficial effects of the acute GC response for free living animals. An overwhelming majority of papers from this area examine environmental-physiological-social effects on GC reactivity. Fewer evaluate intermediate performance measures (fitness proxies). We could only find 11 that directly examine GC effects on reproductive output and survival. The environment-GC-performance papers suggest that greater GC reactivity favors self-maintenance behavior (survival) at the expense of current reproduction. However, the two studies that directly address GC reactivity and fitness (2 of the 11) find the opposite effect (greater GC reactivity predicts lower annual survival). We suggest that it is time to move past simple evaluation of factors regulating GC secretion. These studies will be much richer and informative if researchers include performance and fitness measures. We especially support incorporating and testing ideas of context dependency, coping strategies, and possible fluctuating selection pressures when considering the fitness benefits of the acute GC response.