Cold Tolerance and Thyroid Activity in Developing Tadpoles of Xenopus laevis

Abstract

Cold temperature tolerance was measured in laboratory reared tadpoles of Xenopus laevis and was found to vary with metamorphic stage. Cold tolerance rises slowly during prometamorphosis, when size is also increasing, but much more rapidly at the metamorphic climax when size is decreasing. Experiments with tadpoles reared in goitrogens, in which they grow but do not develop beyond the early hind limb stage (stage 55), indicate that larger size may increase initial resistance to cold but does not prevent early mortality. Transfer from the goitrogen to tap water, allowing thyroid activity to slowly resume, decreased mortality. The addition of thyroxine (T4) to the water results in a more marked reduction in mortality. These data suggest that circulating T4 may confer resistance to cold in Xenopus tadpoles. Histological studies of the amphibian thyroid and direct measures of plasma thy- roxine level indicate that, although thy- roid activity peaks in the spring just before the breeding season, the level of thyroid activity is generally higher in the winter than in the summer (Morgan and Fales, 1942; Miller and Robins, 1955; Norris et