Endocrine mechanisms underlying plasticity in metamorphic timing in spadefoot toads

Amphibian larvae respond to heterogeneous environments by varying their rates of growth and development. Several amphibian species are known to accelerate metamorphosis in response to pond drying or resource restriction. Some of the most extensive studies to date on developmental responses to pond drying have been conducted on species of spadefoot toads (family Pelobatidae). We have found that tadpoles of two species of spadefoot toad accelerate metamorphosis when exposed to water volume reduction in the laboratory (to simulate a drying pond). Furthermore, Western spadefoot toad (Spea hammondii) tadpoles accelerated metamorphosis in response to food restriction, which was intended to simulate a decline in resource availability in the larval habitat. Metamorphic acceleration was accompanied by increased whole body 3,5,3'-triiodothyronine and hindbrain corticotropin-releasing hormone content by 24 hr after transfer of tadpoles from high to low water. Food restriction for 4 day accelerated metamorphosis and elevated whole body thyroid hormone content. Although tadpoles accelerated metamorphosis and activated their thyroid axis in response to the two environmental manipulations, the kinetics of the responses were greater for water volume reduction than for resource restriction. The modulation of hormone secretion and action by environmental factors provides a mechanistic basis for plasticity in the timing of amphibian metamorphosis, and the neuroendocrine stress axis may play a central role in developmental plasticity.