A Comparative Analysis of Plasticity in Larval Development in Three Species of Spadefoot Toads

Abstract

We used spadefoot toad larvae to evaluate assumptions and test predictions of the Wilbur and Collins model for plasticity in age and size at metamorphosis. The model assumes that there is a threshold during larval development after which it is possible to metamorphose and to accelerate metamorphosis in response to certain environmental stimuli. The threshold presumably sets the minimum size and/or age at metamorphosis. Development after the threshold is modified according to environmental conditions, with growth rate serving as an index of environmental quality. Rapidly growing larvae are predicted to delay metamorphosis to take advantage of a favorable larval environment. Slower growers are predicted to transform closer to the threshold, thus enabling them to escape from a poor larval environment. Groups of three larvae from all three species (Experiment 1) or isolated larvae from two species (Experiment 2) in the genus Scaphiopus were reared at three food levels. At predetermined ages, subsamples of larvae had their food supply eliminated. The cessation of feeding represents a decline in the quality of the larval environment. After food elimination, larvae either completed development or entered a developmental stasis, making no further progress toward metamorphosis. The ability to complete development was demarcated by a specific stage of development or a narrow range of stages, thus defining a threshold. The threshold for successful metamorphosis was arrived at earlier at higher food levels and is best described as a function of both stage and mass. If food was eliminated after the threshold had been reached, the starved larvae completed larval development at an earlier age than continuously fed controls at each food level except the lowest food treatment of S. intermontanus. The magnitude of this acceleration of development was greatest in fast-growing (high food) larvae and smallest in slow growers (low food). These results confirm that all three species respond to a decline in the quality of the larval environment as predicted by Wilbur and Collins. Furthermore, the interspecies comparison implies an adaptive shift in this plasticity in the form of a change in the stage of development that serves as a threshold, the degree of acceleration in development rate after the threshold, and the flexibility in the amount of growth that is seen after the threshold. S. couchii, which is derived from the most ephemeral environment, reaches the threshold at an earlier stage of development. It also displays a greater proportional acceleration in development rate after the threshold, matched by a larger trade-off in reduced body size at metamorphosis in comparison with species from less ephemeral environments.