Lipids Show No Waterproofing Function in Anurans

Water loss can lead to dehydration, which can reduce fitness by constraining opportunities for important activities, such as foraging and reproduction. Minimizing water loss is thus beneficial to organisms and, in birds and mammals, is partially accomplished by lipids in the skin. This suggests that waterproofing lipids act as a mechanism for reducing water loss, which should be an important step in the evolution of terrestriality. We used paired liquid chromatography-mass spectrometry lipidomics to test the presence and relative concentration of waterproofing lipids such as cerebrosides and ceramides in the skin tissue, fat body (fat storage organ), and parotoid (poison) gland secretions of toads. We found these lipids in all tissue types in nominal quantities; however, their relative abundances were not correlated with rates of evaporative water loss. Results suggest that the presence of waterproofing lipids alone may be insufficient to explain variation in water loss and that other mechanisms, such as skin structure, may play a role. High-sensitivity tests like LC-MS have rarely been used before and have never been used for lipid analysis in amphibians. Our novel methods clarify the relationship between lipids and water loss in previously untested taxa and tissue sources. Studying these relationships in the pivotal taxa for the evolution of terrestriality will help us understand universal constraints on terrestrial life.