PROXIMATE MECHANISMS OF PHENOTYPIC PLASTICITY IN AMPHIBIAN METAMORPHOSIS

Amphibians exhibit extreme plasticity in the duration of the larval pe riod, Probably the most important environmental variable for a tadpole is water availability, and some amphibian species have been shown to be capable of accelerating metamorphosis in a desiccating larval habit at, Current data support the view that the response to pond drying is not simply a consequence of unavoidable environmental effects but rath er an adaptive response to the risk of mortality in the drying pond, T he acceleration of development by pond drying results in the precociou s activation of the endocrine systems that control metamorphosis, the thyroid and the interrenal, Furthermore, a single neurohormonal stimul us, corticotropin-releasing hormone (CRH) activates both of these endo crine systems, CRH is the primary vertebrate stress neurohormone; thus , tadpoles mount a classical endocrinological stress response to accel erate development and avoid mortality in the desiccating habitat, Comp arison of recent data in mammals with amphibians suggests that the neu rohormonal stress response is a phylogenetically ancient developmental trigger that allows the developing organism to assess habitat quality and respond accordingly.