Glucocorticoids: mediators of vertebrate ontogenetic transitions.

In adult vertebrates, glucocorticoids are thought to trigger transitions between life history stages within breeding cycles. This review explores possible roles of glucocorticoids as mediators for vertebrate ontogenetic transitions. Overall, glucocorticoids prepare organisms and trigger transitions into the subsequent life history stage. Across taxa, ability to secrete glucocorticoids appears to depend on functional maturity at birth. Slow strategist or precocial species tend to have larger, fewer, and more mature young that can secrete glucocorticoids earlier than fast strategist or altricial species. Across life history transitions, glucocorticoids have direct and permissive effects on various ontogenetic transitions in vertebrates. Glucocorticoids directly (1) promote maturation of critical organs before birth/hatch in mammals and birds, (2) initiate parturition events in mammals and possibly controls hatching in birds and reptiles (but not in "small"-egg fish), (3) facilitate acquisition of osmoregulatory ability in fish during smoltification, and (4) affect dispersal behavior in mammals, birds, and reptiles and are potential candidates for the timing of fledging in birds, although further studies are needed to determine the causal relationship. Glucocorticoids also have a permissive action on thyroid hormones in amphibian and fish metamorphosis.