Respiratory and cutaneous water loss of temperate-zone passerine birds Jennifer Ro ⁎, Joseph B. Williams Department of Evolution, Ecology and Organismal Biology, 318 West 12th Avenue, Aronoff Laboratory, Ohio State University, Columbus, OH 43201, USA abstract article info Article history: Received 2 December 2009 Received in revised form 10 February 2010 Accepted 10 February 2010 Available online 16 February 2010 Keywords: Bird Cutaneous water loss Respiratory water loss Lipid Skin Stratum corneum Temperate birds We measured respiratory water loss (RWL) and cutaneous water loss (CWL) of 12 species of passerine birds, all from a temperate environment, and related their CWL to classes of lipids within the stratum corneum (SC). We purposed to gain insight into the generality of patterns of CWL in birds that have been generated mostly from studies on species from deserts, and we addressed the hypothesis that CWL is a passive diffusion process. Despite taxonomic and ecological differences among 12 species of temperate birds, mass-specific RWL and surface-specific CWL were statistically indistinguishable across species. When the birds were dead, CWL was reduced by 16.3% suggesting that CWL is, in part, under physiological control. We found that ceramides, cerebrosides, dioscylceramides, cholesterol, cholesterol sulfate, fatty acid methyl esters, free fatty acid, sterol esters, and triacylglycerol constituted the intercellular lipids of the avian SC. CWL was positively associated with amount of ceramide 3, but this lipid class represented less than 2% of the total SC lipids. Combining direct measurements (n = 24) of RWL with indirect estimates (n = 25) yielded the equation log RWL (g H 2 O/d) =-0.86 + 0.73 (log body mass, g). © 2010 Elsevier Inc. All rights reserved. 1. Introduction To maintain an aqueous internal milieu in the face of a desiccating external environment, terrestrial animals have developed behavioral, morphological, and physiological mechanisms, produced by natural selection, to conserve body water (Bartholomew and Cade, 1963; Addo-Bediako et al., 2001; Lillywhite, 2006). Among the morpholog- ical and associated physiological mechanisms that terrestrial endotherms possess to reduce evaporative water losses are structures in the nares called nasal turbinates that are thought to recover water from the exhaled air stream, thereby reducing respiratory water loss (RWL), and a relatively impermeable integument with an attendant lower cutaneous water loss (CWL) (Hillenius 1992; Gorden and Olson, 1995; Tieleman et al., 1999; Tieleman and Williams, 2002). In many species of birds, evaporative water demand is relatively high because they are diurnal, exposed to higher solar radiation loads, higher ambient air temperatures (T a ), and increased wind speeds (Maclean, 1996). In addition, birds have high mass-specific rates of metabolism and as a result high oxygen demand, which influences RWL (Tieleman et al., 1999). The importance of understanding mechanisms of evaporative water loss in small birds is underscored when one considers that total evaporative water loss (TEWL), the sum of RWL and CWL, is their major avenue of water loss, accounting for up to 83% of total water loss at moderate temperatures (Willoughby, 1968; Bartholomew, 1972; Williams, 1996). At these same temperatures, CWL comprises over 60% of TEWL emphasizing the importance of this variable in the water economy of birds (Wolf and Walsberg, 1996; Tieleman and Williams, 2002; McKechnie and Wolf, 2004; Muñoz-Garcia and Williams, 2005b, 2007). Most investigators have indirectly estimated RWL by measuring the temperature of exhaled air and minute volume during respiration, and then calculating RWL as a product of minute volume and the saturated water vapor density at the temperature of exhaled air (Withers and Williams, 1990; Tieleman and Williams, 1999; Geist, 2000). CWL has also been indirectly estimated by using whole body plethsesmography (Withers and Williams, 1990), or by evaluating resistance of sites on the skin and calculating whole organism CWL from Fick's law of diffusion and estimates of skin surface area (Michaeli and Pinshow, 2001; Marder et al., 2003; Larcombe et al., 2003). Direct measurements of RWL and CWL of birds are few, mostly from desert species; RWL and CWL has been measured in only four non-desert species of birds (Tieleman and Williams, 2002; Muñoz- Garcia and Williams, 2005b, 2007). Whether water permeation through the skin is dictated entirely by passive diffusion, or if it is under active physiological control mechanism(s), remains a subject of debate (Chuong et al., 2002; Muñoz-Garcia and Williams, 2005b; Falkenberg and Georgiadis, 2008). Some authors have suggested that CWL is a passive diffusion process (Pinnagoda, 1994; Wilson and Maibach, 1994; Chuong et al., 2002), whereas others have suggested that water transport across the living epidermis can be physiologically altered by changing ion gradients, a process involving active transport (Falkenberg and Georgiadis, 2008), or by altering vascular blood supply to the dermis; Comparative Biochemistry and Physiology, Part A 156 (2010) 237–246 ⁎ Corresponding author. Tel.: + 1 614 292 3393; fax: +1 614 292 2030. E-mail address: ro.25@buckeyemail.osu.edu (J. Ro). 1095-6433/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2010.02.008 Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa