When vertebrates invaded land during the Carboniferous

period, they were exposed not only to new ecological opportunities but also to a desiccating environment (Williams and Tieleman, 2002). To maintain cellular water homeostasis in such an environment, natural selection modified the integument of pioneering terrestrial animals, enabling them to reduce water loss through the skin. Nowhere is this modification of the integument more important than for animals living in hot, desert environments, where ambient temperatures (Ta) can reach 50°C, relative humidity is low and drinking water is scarce (Noy-Meir, 1973; Williams and Tieleman, 2002). Given these harsh conditions, one might imagine that phenotypes with minimal evaporative water loss are favored in desert environments. The sum of evaporative water loss from respiratory passages and from skin – total evaporative water loss (TEWL) – is the major avenue of water loss to the environment, especially for small species of birds in which TEWL is five times greater than urinary/fecal water loss (Bartholomew, 1972; Dawson, 1982; Williams and Tieleman, 2000). There exists substantial evidence that TEWL is reduced in species of birds that live in deserts, with mesic species having rates of TEWL nearly double those of species from deserts (Williams, 1996; Williams and Tieleman, 2000; Tieleman et al., 2003). Early investigators assumed that most evaporative water loss took place across respiratory passages and that cutaneous water loss (CWL) played a minor role in thermoregulation (Mount, 1979; Bartholomew and Cade, 1963). Subsequent work has shown that CWL can equal or exceed evaporation from respiratory passages at moderate Tas (Dawson, 1982; Tracy and Walsberg, 2001; Webster and King, 1987; Wolf and Walsberg, 1996). In a study on larks, Tieleman and Willliams (2002) found that CWL accounted for 50–70% of TEWL at 25°C and that surface-area-specific CWL was about 30% lower in arid species compared with ones living in mesic environments. They hypothesized that natural selection has equipped resident desert birds with a mechanism(s) that reduces water loss through the skin, at least at moderate Tas, as a water conservation mechanism. Natural selection could be responsible for reduced TEWL and CWL found in desert larks (Bennett, 1997; Feder et al., 2000; Williams and Tieleman, 2001; Tieleman et al., 2003). 3581 The Journal of Experimental Biology 206, 3581-3588 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00596