Cutaneous water loss and lipids of the stratum corneum in house sparrows Passer domesticus from and and mesic environments

Birds that live in hot, dry environments must balance water intake with losses in order to maintain water homeostasis. The outer layer of the integument, called the stratum corneum (SC), consists of corneocytes embedded in a matrix of lipids. The SC serves as a barrier to water vapor diffusion through the skin. We measured cutaneous water loss (CWL) in two populations of house sparrow Passer domesticus L., one living in a desert environment in Saudi Arabia, and another living in a mesic environment in Ohio, USA. We found that CWL rates at 30 degrees C were lower in desert individuals (11.9 +/- 2.2 mg H(2)Ocm(-2) day(-1); N=11) than in mesic birds (16.0 +/- 2.6 mg H2O cm(-2) day(-1); N=14). We hypothesized that changes in the lipid composition of the SC could affect CWL. We analyzed four classes of lipids in the SC: ceramides, cerebrosides, cholesterol and free fatty acids, by thin layer chromatography. Compared to mesic sparrows, desert birds had a higher amount of ceramides (49.2 +/- 10.3 mg g(-1) SC dry mass in Saudi Arabia; 38.2 +/- 18.0 mg g(-1) SC dry mass in Ohio) and cerebrosides (101.2 +/- 48.9 mg g(-1) SC dry mass in Saudi Arabia; 56.5 +/- 34.0 mg g(-1) SC dry mass in Ohio), and a lower percentage of cholesterol (4.1 +/- 3.6% in Saudi Arabia; 5.4 +/- 2.5% in Ohio) in their SC. Although CWL was lower in sparrows from Arabia, and lipid composition of their SC differed, we could not detect differences between rates of water loss through non-living skin attached to glass vials (46.0 +/- 15.7 mg H2O cm(-2) day(-1) for sparrows in Saudi Arabia; 45.8 +/- 27.2 mg H2O cm(-2) day(-1) for sparrows in Ohio). These results suggest that biological control mechanisms interact with layers of lipids in the stratum corneum to adjust CWL to the environment.