Stress, reproduction, and adrenocortical modulation in amphibians and reptiles

While the hypothalamo-pituitary-adrenocortical (HPA) response to stress appears to be conserved in vertebrates, the manner in which it is activated and its actions vary. We examine two trends in the stress biology literature that have been addressed in amphibian and reptilian species: (1). variable interactions among stress, corticosterone, and reproduction and (2). adrenocortical modulation. In the first topic we examine context-dependent interactions among stress, corticosterone, and reproduction. An increasing number of studies report positive associations between reproduction and corticosterone that contradict the generalization that stress inhibits reproduction. Moderately elevated levels of stress hormones appear to facilitate reproduction by mobilizing energy stores. In contrast, pronounced activation of the HPA axis and extremely elevated levels of stress hormones appear to inhibit reproduction. Much of these contrasting effects of stress and reproduction can be explained by expanding the Energetics-Hormone Vocalization Model, proposed for anuran calling behavior, to other taxa. In the second topic, a number of amphibians and reptiles modulate their HPA stress response. Adrenocortical modulation can occur at multiple levels and due to a variety of factors. However, we have little information as to the physiological basis for the variability. We suggest that several ecologically based ideas, such as variability in the length of the breeding season and lifetime reproductive opportunities, can be used to explain the utility of adrenocortical modulation in these taxa.