Assessing sources of variation in amphibian skin thickness: ecological and evolutionary implications
The skin is the largest organ of the body and provides many functions. Among tetrapod vertebrates, amphibian skin is semi-permeable and responsible for a greater proportion of water absorption and gas exchange. Myriad factors affect the physiological performance of amphibian skin. Morphological traits linked with amphibian skin physiology or ecology have remained difficult to discern because of a lack of quantitative comparative research and the discovery of sources of intraspecific variation that are mostly ignored in study designs. This thesis aims to address the effects of these sources of variation using a trait that is known to vary between sexes, among seasons, and among body regions and thought to be linked with physiology or ecology, skin thickness. The first source of variation addressed is sexual dimorphism. Specimens of the white-lipped treefrog, Litoria infrafrenata, that display sexual dimorphism in body size and skin thickness were used to test if body size was the main determinate of sexually dimorphic skin thickness. Size corrected values did not significantly differ between males and females, although the sample size was small. Seasonal variation in skin thickness has also been documented in some species, so the American bullfrog (Lithobates catesbeianus), the Northern leopard frog (L. pipiens), and the spring peeper (Pseudacris crucifer) from multiple months of the year were sampled to determine if skin thickness increased in the autumn or winter months. Seasonal skin thickening was only detected in L. catesbeianus, and skin from autumn and winter was significantly thicker than from earlier in the year. This pattern was also detectable in museum specimens collected 80 years ago, although the signal was damped, possibly due to preservation. Using a dataset of 10 species and published data, a general pattern was uncovered whereby the dorsal skin is the thickest region and the ventral thigh region is the thinnest. However, this pattern is not always true for every individual of every species (L. pipiens and P. crucifer) and in some species the dorsal skin is thinnest (Bokermannohyla alvarengai and Litoria infrafrenata). The same dataset found that skin thickness is significantly related to body size, as was found in the chapter on Litoria infrafrenata. Summer specimens of Lithobates catesbeianus were outliers below the interspecific regression line and winter specimens fell within the range of variation of other species, hinting that seasonal skin thickening could be renamed seasonal skin thinning in this species. Finally, a link between ecology and skin thickness was tested using the 10 species from previous analyses and data from the literature. At a phylogenetially broad scale, body size explained a greater amount of the variation in environmental parameters than skin thickness. At smaller taxonomic scales, skin thickness appears more closely linked with ecology. It is concluded that amphibians generally follow an allometric trend for skin thickness and when faced with suboptimal conditions over long periods of time, evolve integumentary structures like iridiophores to compensate for any physiological disadvantage of an ‘ideal’ skin thickness. In the interim, however, skin thickness may change, thus sacrificing e.g. mechanical support.