Assessing the Presence, Pattern, and Environmental Correlates of Seasonal Skin Thickening in Anurans

Amphibians are the vertebrates most sensitive to environmental change, in part due to their relatively thin skin. As such, research seeks to better understand variation in amphibian skin traits. One source of variation that is poorly understood is seasonal variation in amphibian skin thickness that has been reported in two anurans and one caudatan. It is currently unknown how widespread this trait is among amphibians, or the pattern it follows throughout the year, because existing studies have only sampled specimens at two or three times of the year. We use museum specimens of three sympatric anurans from the northern United States to test for the presence of seasonal changes in skin thickness and which functional hypothesis best explains the pattern: either an environmental function/response or an intrinsic life history-related pattern. Seasonal changes in skin thickness were detected in all skin regions and layers measured in the American bullfrog (Lithobates catesbeianus), in some of the skin layers of the northern leopard frog (L. pipiens), but such changes were not detected in spring peepers (Pseudacris crucifer). The results favored the life history functional hypothesis, where skin is thinnest after brumation, increasing in thickness throughout the year ahead of hibernation. The differences between skin thickness variables between early- and late-year specimens of L. catesbeianus were as high as 40%, indicating that this source of variation has the potential to skew the results of morphological studies of amphibian skin thickness if not controlled for.