Phenotypic flexibility in cutaneous water loss and lipids of the stratum corneum

When vertebrates invaded land during the Carboniferous period, they were exposed not only to new ecological opportunities but also to a desiccating environment. To maintain cellular water homeostasis, natural selection modified the integument of pioneering terrestrial animals, enabling them to reduce water loss through the skin. In desert environments, where ambient temperatures (T(a)) can reach 50 degrees C, relative humidities are low and drinking water is scarce, integumentary modifications that reduce cutaneous water loss (CWL) could be fundamental to survival. Previous research has shown that hoopoe larks (Alaemon alaudipes) from the Arabian desert reduced CWL when acclimated to 35 degrees C compared with individuals at 15 degrees C, but skylarks (Alauda arvensis) and woodlarks (Lullula arborea), from The Netherlands, and Dunn's larks (Eremalauda dunni), also from the Arabian desert, did not. Here, we test the idea that hoopoe larks acclimated to 35 degrees C would alter the lipid composition of their stratum corneum (SC), resulting in a decrease in CWL, but that skylarks, woodlarks and Dunn's larks would not. Specifically, we hypothesized that hoopoe larks, acclimated to 35 degrees C, would increase the proportions of polar ceramide content and decrease the proportions of free fatty acids in their SC compared with individuals acclimated to 15 degrees C. Results showed that hoopoe larks at 35 degrees C had lower CWL and higher proportions of total ceramides but lower proportions of free fatty acids and sterols in their SC. We demonstrate that adjustments in ratios of lipid classes in the SC are associated with changes in CWL in hoopoe larks.