Abstract
This chapter discusses the hormonal control of larval development and evolution in amphibians. In amphibians, almost all aspects of larval development, including the culminating transformation into adult form (commonly known as metamorphosis), are under the direct and primary control of the thyroid hormone (TH). This combination of a recurring tendency to exploit ontogenetic variation for evolutionary divergence and a singular reliance upon hormones for developmental regulation offers a unique opportunity to investigate the developmental basis of morphological evolution. Much of the amphibian postembryonic development is controlled by the interaction of two parameters: hormone activity, defined here as the profile of hormone plasma concentration over larval and metamorphic stages; and tissue sensitivity, which is a measure of a tissue's responsiveness to a hormone and is the minimum concentration required for its activation. The primary hormones involved (that is, those that directly mediate tissue responses) are the following: the two forms of TH, thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3); the major corticosteroid in larval amphibians, corticosterone (CORT); and prolactin (PRL). Additional hormones appear to modulate the activity of these hormones, as well as regulate larval growth and metabolic activities related to metamorphosis.
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