Metamorphic Responses to Changing Food Levels in Two Species of Hylid Frogs

Abstract

We investigate how changes in food availability during development affected the timing of and body size at metamorphosis in two closely related species of tree frogs that use different larval habitats. We raised tadpoles of Hyla gratiosa (a temporary—pond breeder) and Hyla cinerea (a permanent—pond breeder) at two different temperatures on either constant resources or a regime in which we altered food levels at one of three different times during development. For both species, larval period was affected only by early changes in food level; early increases shortened larval period, and decreases lengthened it. The timing of metamorphosis of Hyla gratiosa showed greater plasticity than that of Hyla cinerea, because of its greater overall response to any food—level change and not because of any difference between species in the duration of the sensitive period. The two species showed comparable levels of plasticity in body size at metamorphosis; increases in food level produced larger body sizes, and decreases produced smaller sizes. However, in contrast to the pattern seen in larval period, later changes in food level had the greatest effect on body size. These results force a reexamination of current ideas about the adaptive significance of plasticity in the timing of metamorphosis in response to food availability in larval anurans. We offer a model of dynamic allocation that accommodates the extant data on this issue.