The Role of Larval Diet in Anuran Metamorphosis (original) (raw)

1997, Integrative and Comparative Biology

Focus on tadpole diet and foraging behavior offers potential for integrating ecological and endocrinological approaches to understanding anuran metamorphosis. Natural larval diets vary widely in relative amounts of protein, carbohydrate and lipid, factors known to influence thyroid hormone function, which in turn is essential for metamorphosis to occur. Previous work has shown that tadpoles fed high protein diets grow and develop quickly. This pattern is consistent with findings from other classes of vertebrates that some aspects of thyroid function, e.g., activity of 5'D monodeiodinase, an enzyme that converts thyroxine (T4) to the more potent metamorphosis-inducer 3,5,3'- triiodothyronine (T,), are proportional to availability of dietary protein. In field experiments at a northern California river, I found that nutritional variation among algal taxa routinely consumed by tadpoles (chlorophytes, diatoms, cyanobacteria, etc.) influenced metamorphosis several ways. Growth rate was positively correlated with percent protein content of a food type. Tadpoles fed filamentous green algae with epiphytic diatoms developed more quickly and metamorphosed at larger sizes than tadpoles raised on other diets. The dominant diatoms in this system are high in protein because they host nitrogen fixing cyanobacterial endosymbionts. Resource quality also mediated the effects of competition and predation. Changes in abundance of high quality algae caused by invading bullfrog (R. catesbeiana) tadpoles explained most of their competitive effects on metamorphosis of native tadpoles (Hyla regilla and (Rana boylii). In choice experiments, tadpoles foraged selectively on the algal foods that promoted most rapid growth and development. In the presence of garter snake predators, however, tadpoles avoided such patches thus decreasing growth. Direct examination of thyroid hormone production in tadpoles consuming different diets may reveal a proximate mechanism linking diet quality to size at and time to metamorphosis.