Environmental Causes of Correlations between Age and Size at Metamorphosis in Scaphiopus Multiplicatus

We examined environmental effects on the correlation between age and size at metamorphosis in spadefoot toad tadpoles (Scaphiopus multiplicatus). These tadpoles occur in ponds of variable duration as two environmentally induced, trophic morphs: a large, rapidly developing carnivore morph and a smaller, more slowly developing omnivore morph. Drawing on data from both natural ponds and artificial pools we examined for both morphs the environmental determinants of, and the intercorrelations among, three life history parameters: age at metamorphosis, size at metamorphosis, and survivorship (both pre— and postmetamorphic). Censuses of four natural ponds revealed that age and size at metamorphosis for both morphs were inversely related, in contrast to many other amphibians and insects with complex life cycles. In artificial pools in which we simultaneously varied two diet components and pond duration, age and size at metamorphosis were negatively correlated in high food treatments and positively correlated in low food, long pond duration treatments. Ephemeral larval habitats should favor rapid differentiation; achieving large body size depends upon the quality and quantity of available food and the time available to develop. A negative relationship between age and size at metamorphosis may occur often S. multiplicatus since this species is exposed to a rapidly diminishing food resource (anostracans). We predict that other organisms with complex life cycles faced with these conditions would possess an inverse relationship between size and age at metamorphosis.