Effects of Growth History and Exogenous Thyroxine on Size and Age at Metamorphosis in the Toad Bufo americanus
By using food rations and thyroxine supplements, I manipulated the growth and differentiation of tadpoles of the toad Bufo americanus to test the hypothesis that the plasticity in metamorphic timing can be lost. A significant food effect indicated that tadpoles that grew rapidly during the middle period of the experiment metamorphosed earlier than slow-growing tadpoles. The changes in growth induced early and late in this experiment did not influence metamorphic timing. There was a significant thyroxine effect: all tadpoles treated with thyroxine metamorphosed early. All thyroxine-treated tadpoles metamorphosed at the same time, indicating that differentiation at the time of thyroxine supplementation was independent of growth rate. A food-by-thyroxine interaction provided evidence that the growth rate/differentiation antagonism may have been active at least during the middle of the experiment. This suggests that the growth rate/differentiation antagonism is decoupled or overridden during later stages of larval development. This result implies limits to metamorphic plasticity and is consistent with a fixed-rate model of amphibian metamorphosis.Abstract
Growth profiles of the 12 treatment groups. Each point represents the mean mass determined every 10 days. Profiles terminate at the mean metamorphic date for each treatment. Letters refer to feeding regimen for each feeding period; H = high food, L = low food. A “t” indicates groups that received thyroxine supplements when tadpoles reached Gosner stages 32–35 (approximately 26–29 days posthatching). The first food switch occurred after 15 days; the second switch was implemented when tadpoles reached Gosner stages 32–35. Bars represent “+” or “-” 1 SE. (A) Begun on low food (no thyroxine supplements), (B) begun on high food (no thyroxine supplements), (C) begun on low food (received thyroxine supplements), and (D) begun on high food (received thyroxine supplements)
Summary of a posteriori Tukey's HSD contrasts. Brackets indicate groups that did not differ significantly (α = 0.05). Lines connect nonthyroxine groups with thyroxine sister treatments to emphasize the effects of thyroxine-induced acceleration of metamorphosis. Refer to Figure 1 for explanation of codes