Miniaturization and the anuran skull: An integrative study of developmental constraint

The relationship between body size and skull morphology in anurans is explored. In Chapter 1, general trends in the skull morphology of miniature anurans are summarized. These trends include the reduction or absence of skull elements that appear late in a generalized ontogeny of the anuran skull. In Chapter 2, a method for studying size-related developmental constraint is explored with preliminary data on skull shape in anurans. Fifty-nine species of anurans are sampled to determine if skull shape variation lies within a restricted range. Small frogs have disproportionately narrow skulls compared to large frogs. Ontogenetic data from several species parallel those found between species. An experimental method is employed to test if the size-related variation in skull shape could be the result of truncation of development. Treatments with low doses of thyroxin produced experimental specimens of Rana blairi that are significantly smaller than conspecific controls. The experimental animals have disproportionately narrow skulls relative to the larger control specimens. In Chapter 3, the approach outlined in Chapter 2 is used to study the effect of size on anuran skull morphology in more detail than in the previous chapter. Pseudacris ocularis is a miniature hylid frog with a skull that differs markedly from other North American hylids. These differences correspond to the trends summarized in Chapter 1. Analysis of the ontogeny of the skull in P. ocularis and P. triseriata reveals that the unique adult skull morphology of the former species appears to be the result of truncation of development relative to larger-bodied, closely related species. However, while the differences between adults are indeed paedomorphic, the ontogenetic alterations responsible vary from one skull element to another. Thus, simple truncation of development is not an adequate explanation for the variation in skull morphology between P. ocularis and other members of the genus. Finally, the specimens generated by the experimental work in Chapter 2 are reexamined in light of the trends summarized in Chapter 1 and supported in the first part of Chapter 3. The skulls of experimentally generated miniature Rana blairi have many features in common with those of naturally miniature frogs.