Ecological Significance of Body Size and Skin Resistance to Terrestriality in Frogs

Many amphibians evaporate water freely across their skin. This creates a challenge to thermoregulation: how to simultaneously regulate water and temperature balances, when thermoregulatory behaviors may simply result in changes in water loss rates, rather than changes in temperature. The apparent conflict between predictions based on the physics of heat exchange, and observations of apparently thermoregulatory behavior has resulted in a debate for many years. It is likely that frogs with extreme resistance to water loss have the capacity to thermoregulate and be partially emancipated from the need to find free water, and therefore have the means to become arboreal. Controlling rates of desiccation in frogs appears to be accomplished by three mechanisms: (1) having cutaneous resistance to water vapor transport (e.g. Litoria bicolor), (2) having a large body size which effectively reduces the surface area to volume ratio, and thus, reduces the rate dehydration (e.g. L. caerulea), and (3) behavioral or ecological selection of wet environments (such as rainforests or streamsides; e.g. Platymantis vitiensis). We have developed biophysical models to calculate water loss in frogs in relation to cutaneous resistance to water vapor loss and body size, and the simulations from this modeling shows that it is possible to achieve the same resistance to loss of hydration level by having a high cutaneous resistance, or by having large body size. This is consistent with the hypothesis that there is more than one means by which the ability to avoid desiccation can evolve, thereby providing potential for thermoregulation. We have constructed a relatedness tree of the Hylid frogs in Australia and we have found that arboreality has evolved more than once in the evolutionary history of Australian Hylid frogs. Additionally, it is clear from the relatedness analysis that the trait of arboreality has been achieved in different ways in different clades.