How do animals regulate their water loss in response to extreme heat and aridity?

Maintaining water balance is a crucial physiological task, particularly in arid environments where ambient temperatures are high and humidity is low. Research on birds and reptiles has shown that animals living in arid environments have lower rates of evaporative water loss than related species living in mesic environments. Does this pattern reflect genetic divergence, or physiological acclimation? And what physiological adaptations underlie the differences in water loss between arid and mesic species?

Related Papers

Cox, R.M., A. Munoz-Garcia, M. Jurkowitz, and J.B. Williams. 2008. Beta-glucocerebrosidase activity in the stratum corneum of house sparrows following acclimation to high and low humidity. Physiological and Biochemical Zoology 81: 97-105.

Munoz-Garcia, A., R.M. Cox, and J.B. Williams. 2008. Phenotypic flexibility in cutaneous water loss and lipids of the stratum corneum in house sparrows (Passer domesticus) following acclimation to high or low humidity. Physiological and Biochemical Zoology 81: 87-96.

Cutaneous water loss. In birds and reptiles, evaporative water occurs during respiration (respiratory water loss, RWL) and via evaporation across the skin (cutaneous water loss, CWL). Joe Williams and his group at Ohio State have found that variation in CWL explains much of the difference in evaporative water loss rates between arid and mesic birds. Moreover, variation in CWL is related, in part, to variation in the amounts and proportions of two types of lipids (ceramides and cerebrosides) located in the stratum corneum of the skin. The conversion of cerebrosides to ceramides is regulated by the enzyme beta-GlcCer’ase. Joe, Agus Munoz-Garcia and I set out to determine whether house sparrows can alter the rates of CWL, the proportions of ceramides and cerbrosides, and the activity of beta-GlcCer’ase after exposure to arid conditions.

Figure 1. Evaporative water loss can be partitioned into respiratory (RWL) and cutaneous (CWL) fractions by fitting house sparrows with a mask to trap their respiratory output.

Acclimation experiment. After acclimation to arid conditions (15% relative humidity) for 21 days, house sparrows exhibited an impressive 45% reduction in cutaneous water loss. However, birds acclimated to humid conditions (95% relative humidity) also experienced a smaller (23%) reduction in CWL (Fig. 2a). Both wet- and dry-acclimated birds exhibited a substantial increase in beta-GlcCer’ase activity (Fig. 2b). These changes in CWL and enzyme activity were mirrored by decreases in cerebroside lipids in both groups and a significant increase in ceramide lipids in the dry-acclimated group. These experiments illustrate how the impressive phenoypic plasticity of CWL is mediated by underlying changes in enzyme activity and skin lipids. However, the fact that both dry-and wet-acclimated birds exhibited these changes suggests that other factors, such as temperature, influence CWL acclimation in addition to humidity per se.

Figure 2. Left panel: difference in pre- and post-treatment cutaneous water loss (CWL) for sparrows acclimated for 21 days to arid (dry) or humid (wet) conditions. Right panel: Beta-GlcCer’ase activity for non-acclimated sparrows was substantially lower than for birds acclimated to dry or wet conditions. Modified from Cox et al. (2008) and Munoz-Garcia et al. (2008).