Cooling by cutaneous water evaporation in the heat-acclimated rock pigeon (Columba livia)

The present study provides an up-to-date overview of the cutaneous water-evaporation cooling mechanism in the rock pigeon. Cutaneous water evaporation fully replaces the classic respiratory cooling mechanism in the resting, heat-acclimated bird, and is more economical in terms of water conservation. It enables the pigeon to maintain homeostasis, and to breed successfully in harsh environments. Adrenergic signaling is involved in the initiation of this novel mechanism, either by deactivation of the β-adrenergic receptors (ARs), or activation of the α-AR. The adrenergic signaling results in a marked increase in cutaneous blood flow and in the arterial-to-venous blood-flow ratio. This is associated with alterations in the cutaneous capillary wall ultrastructure, which increase its permeability to plasma proteins and water. The end result of this process might be an increase in water efflux from the capillary lumen. The properties of β-ARs were measured in the cardiac muscle of thermal-acclimated pigeons. Significant down-regulation in the density of β-ARs, associated with increased affinity of these receptors, was measured in the heat-acclimated pigeon. Concomitantly, changes in the skin ultrastructure and lipid composition were found in very well defined patches in the epidermis of heat-acclimated pigeons. These suppress the skin resistance to water transfer. We suggest that this cooling mechanism involves finely orchestrated adjustments in the ultrastructure of the skin and the cutaneous capillaries, and in skin blood flow. Adrenergic signals are among those factors that regulate this cooling mechanism during exposure to a hot environment.