Rj. Denver, HORMONAL CORRELATES OF ENVIRONMENTALLY-INDUCED METAMORPHOSIS IN THE WESTERN SPADEFOOT TOAD, SCAPHIOPUS-HAMMONDII, General and comparative endocrinology, 110(3), 1998, pp. 326-336
Tadpoles of several amphibian species have been shown to accelerate me
tamorphosis when their ponds dry. To understand the proximate mechanis
ms that mediate the developmental response to pond drying, I analyzed
changes in endocrine activity in tadpoles of the Western spadefoot toa
d (Scaphiopus hammondii) exposed to experimental water volume reductio
n in the laboratory. Tadpoles exposed to a declining water level accel
erated metamorphosis compared with tadpoles raised in a constant high
water environment. The acceleration of development was associated with
the precocious elevation of whole-body contents of the hormones that
control metamorphosis, the thyroid hormones thyroxine (T-4) and triido
thyronine (T-3), and the interrenal steroid corticosterone (CORT). The
precocious activation of the thyroid system preceded external morphol
ogical 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 37-3
8) were raised in a constant high water environment (10 L) and then tr
ansferred to either 1 or PO L. Tadpoles transferred to 1 L, exhibited
significant metamorphic changes by 48 h after transfer. In addition, d
ramatic elevations in whole-body T-4, T-3, and CORT contents were evid
ent 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 respon
se is rapid, occurring within 48 h after exposure to the desiccating e
nvironment. (C) 1998 Academic Press.