The Evolution of Cutaneous Water Loss and Lipids of the Skin of Birds

Working in the Netherlands and in Saudi Arabia, members of our group discovered that desert larks have a reduced water loss through their skin compared with more mesic species (Williams and Tieleman 2005), a finding that prompted us to investigate possible mechanisms that could operate to produce this result. The outer layer of the epidermis, the stratum corneum, is thought to form the barrier to water vapor diffusion from animal to environment. We documented that larks from desert environments had a higher proportion of ceramides, a lipid known as a sphingolipid, and fewer free-fatty acids in their stratum corneum. This is an exciting result, because it appears that subtle changes in the ratios of lipid classes in the stratum corneum can alter movement of water vapor through the skin. Moreover we have shown that cerebrosides, ceramides with a sugar molecule attached, are common in the stratum corneum of birds, and we are now exploring how these molecules with their associated hydroxyl groups are involved in water transport through skin. High concentrations of cerebrosides in the skin of mammals produce a pathological condition, yet in birds these molecules seem important in skin function. We are currently exploring phenotypic plasticity of cutaneous water loss in both adult and nestling birds. Our work suggests that nestling sparrows exposed to dry air develop a less permeable skin than individuals growing in moist air, and that this reduction in CWL is associated with predictable changes in lipids of the stratum corneum. We have recently embarked on an exploration of the role of developmental plasticity, adult acclimation, and adaptation in constructing the lipids of the stratum corneum of horned larks from an array of environments ranging from the hot dry deserts of Saudi Arabia to the cold dry tundra.

As part of our study of lipids in the skin, we have developed an exciting new method that identifies and quantifies every lipid molecule in the stratum corneum of birds using Reverse Phase High Performance Liquid Chromatography coupled with Atmospheric Pressure Photo-Ionization Mass Spectrometry (Muņoz-Garcia et al. 2006). Instead of identifying lipid classes using thin layer chromatograph, as investigators have done in the past, we now have the ability to identify and quantify every lipid molecule in the stratum corneum of skin. Our mass spectrometry method provides a power lens though which we can observe lipid molecules in the skin. We are applying our method to birds from the lowland tropics, from temperate Ohio , and from the deserts of Saudi Arabia to gain an appreciation how natural selection has sculpted skin of birds from these areas to regulate cutaneous water loss. In addition, we are now seeking collaborations to help us understand how these lipid molecules combine in the stratum corneum to form a barrier to water vapor diffusion.

The Life History-Physiology Nexus

  As part of a large collaborative effort with colleagues from 4 other universities, our lab is looking at the linkages between physiology and life history of birds in the tropics and in temperate environments. We have shown that tropical birds have a lower basal metabolism, a lower peak metabolic rate, and on average longer survival, than do temperate birds (Wiersma et al. 2008). In addition we are investigating the association between metabolism and life-history attributes of birds, such as clutch size, the number of clutches laid per year and the rate of nestling growth. Research in the tropics takes place at the Smithsonian Tropical Research Institute, Gamboa , Panama , whereas our work on temperate birds occurs in Ohio and Michigan .