Cutaneous Resistance to Water-Vapor Diffusion in Pigeons and the Role of the Plumage

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Abstract
To determine the cutaneous water-loss responses of birds to environmental temperature, we measured transcutaneous evaporation, skin and body temperature, and oxygen consumption in pigeons over a wide range of air temperatures. We used a general Fick diffusion model to estimate integumental plus boundary layer resistance to diffusion of water vapor. Cutaneous water loss (mean ± SE) increased from 1.4 ± 0.09 mg m⁻² s⁻¹ at 0 C to 3.0 ± 0.09 mg m⁻² s⁻¹ at 30 C and increased sharply to 9.0 ± 0.32 mg m⁻² s⁻¹ at 40 C. Whole-body vapor diffusion resistance of pigeons (mean ± SE) varied with air temperature, decreasing from 130.0 ± 8.2 s/cm at 0 C to 97.0 ± 4.1 s/cm at 30 C and to 45.0 ± 3.4 s/cm at 40 C. Measurements of the vapor resistance of the plumage made in isothermal diffusion chambers and estimates of the boundary-layer resistance to evaporation obtained from predictive equations indicate that the feather coat and associated layer of still air are together only 6.2%-25.8% of total vapor resistance and that the skin is therefore the primary barrier to cutaneous water loss in birds. Temperature-dependent reductions in skin vapor resistance that we observed in the pigeon may result from vasomotor adjustments, stearic changes in skin lipids, or hydration of the stratum corneum.