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β‐Glucocerebrosidase Activity in the Stratum Corneum of House Sparrows following Acclimation to High and Low Humidity

Skin is an important avenue of water loss in terrestrial birds, so environmental conditions that necessitate water conservation should favor physiological mechanisms that reduce cutaneous water loss (CWL). Skin resistance to CWL is conferred by a barrier of lipid molecules located in the stratum corneum (SC), the outer layer of the epidermis. In mammals, SC barrier function depends on the conversion of cerebrosides to ceramides by the enzyme β‐glucocerebrosidase (β‐GlcCer’ase). Avian SC contains both cerebrosides and ceramides, suggesting that observed plasticity in CWL may be mediated by changes in β‐GlcCer’ase activity and resultant SC lipid composition. We tested the hypothesis that changes in ambient humidity would alter β‐GlcCer’ase activity by acclimating house sparrows (Passer domesticus) to either dry (6.5 g H2O m−3 absolute humidity) or humid (31 g H2O m−3) conditions for 5 and 21 d at 30°C and then measuring β‐GlcCer’ase activity from SC homogenates. Our results provide the first characterization of β‐GlcCer’ase activity in any nonmammalian vertebrate. Relative to nonacclimated controls, both dry‐ and humid‐acclimated sparrows had significantly elevated β‐GlcCer’ase activity at 21 d postacclimation. Across individuals, we observed negative correlations between β‐GlcCer’ase activity and both CWL and SC ceramide content. Although dry‐ and humid‐acclimated sparrows did not differ in β‐GlcCer’ase activity, these results are consistent with our findings that both humidity treatments caused a reduction in CWL and similar changes in SC lipid composition. Our results demonstrate physiological plasticity in CWL and provide tentative support for a role of β‐GlcCer’ase in mediating this response.