Reduced Rates of Water Loss and Chemical Properties of Skin Secretions of the Frogs Litoria caerulea and Cyclorana australis

Abstract

We measured the rates of water loss in two Australian hylid frogs: the arboreal Litoria caerulea and the terrestrial burrowing frog Cyclorana australis. We measured the latter species with and without cocoons. Both species showed reduced rates of water loss compared with ‘typical’ amphibians that lose water as if from a free surface. Cocooned C. australis had very low rates of water loss. We examined the chemical composition of skin secretions rinsed (using only high-pure water) from both species and the cocoon material from C. australis. The chemical composition of the material from these three sources was generally similar and consisted of 5–10% neutral lipids and 78–85% proteinaceous material. The fact that the terrestrial species has a high resistance to water loss is unusual given that almost all other known species of non-cocooned frogs with reduced rates of water loss are arboreal. The chemical similarity of the skin secretions and cocoons from this species suggest that the reduced rate of water loss in this species is linked to its ability to form a cocoon. Amino acid composition of the material indicated that a sclerotisation process may occur upon oxidation of the secretions. This would result in a physical barrier to water loss in the cocoons and possibly a thin physical proteinaceous barrier on the skin of both species in the absence of cocoons. We suggest that the high proportion of proteins in the skin secretions cannot be ignored, and that it may, in conjunction with the lipids, produce an effective waterproofing barrier in both species. We suggest that chemical components other than lipids also may be important in frogs from other continents, and complete compositional analyses of frog ‘mucus’ are required before we can fully understand the nature of the mechanisms involved in reduced rates of water loss in amphibians with and without cocoons.