Hormonal Correlates of Environmentally Induced Metamorphosis in the Western Spadefoot Toad,

Tadpoles of several amphibian species have been shown to accelerate metamorphosis when their ponds dry. To understand the proximate mechanisms that mediate the developmental response to pond drying, I analyzed changes in endocrine activity in tadpoles of the Western spadefoot toad (Scaphiopus hammondii) exposed to experi-mental water volume reduction in the laboratory. Tad-poles exposed to a declining water level accelerated metamorphosis compared with tadpoles raised in a constant high water environment. The acceleration of development was associated with the precocious eleva-tion of whole-body contents of the hormones that control metamorphosis, the thyroid hormones thyroxine (T4) and triidothyronine (T3), and the interrenal steroid corticosterone (CORT). The precocious activation of the thyroid system preceded external morphological change (i.e., increase in hind limb length, developmental stage) by 3 days. To test if tadpoles are capable of responding rapidly to water volume reduction, mid-prometamorphic tadpoles (Gosner Stage 37–38) were raised in a constant high water environment (10 L) and then transferred to either 1 or 10 L. Tadpoles transferred to 1 L exhibited significant metamorphic changes by 48 h after transfer. In addition, dramatic elevations in whole-body T4, T3, and CORT contents were evident at this time point. Thus, the metamorphic response to pond drying is likely driven by the activation of the thyroid and interrenal axes, the hormones of which control metamorphosis. Furthermore, this response is rapid, occurring within 48 h after exposure to the desiccating environment. r 1998 Academic Press The timing of metamorphosis is a central amphibian life history trait, and amphibians exhibit extreme plasticity in the length of the larval period (Wilbur and Collins, 1973; Werner, 1986; Smith, 1987). This timing is influenced by a combination of both biotic and abiotic factors present in the larval habitat. Probably the most important environmental variable for a tadpole is water availability, and tadpoles of some species, espe-cially those that breed in temporary ponds, have been shown to accelerate metamorphosis when their habita