Insensible Evaporative Water Loss Control by an Australian Arid-Habitat Psittacine, the Mulga Parrot (<i>Psephotellus varius</i>)

AbstractFor terrestrial animals, evaporative water loss (EWL) is a crucial component of osmotic balance and thermoregulation. At high ambient temperatures, augmented EWL for thermoregulation is well recognized, but there is growing evidence that insensible EWL at temperatures within and below thermoneutrality is also regulated and does not just passively conform to Fick's Law. We investigated the capacity of the arid-habitat mulga parrot (<i>Psephotellus varius</i>) to physiologically regulate its insensible EWL within or below thermoneutrality (15°C-30°C) using open-flow respirometry and manipulating two physical parameters, the water vapor pressure deficit (by varying atmospheric relative humidity) and the water diffusion coefficient (using a helox atmosphere). Both experimental approaches provided evidence for physiological control of EWL. There was no significant effect of relative humidity on EWL despite the reduced water vapor pressure deficit at high relative humidity, and EWL did not increase in response to a higher vapor diffusion coefficient in helox despite expected increases in thermal conductance, metabolic rate, and ventilatory parameters. Adjustments of both respiratory and cutaneous EWL contributed to the control of EWL, achieved by modifying a combination of ventilatory parameters, expired air temperature, skin resistance, and, to a lesser extent, skin temperature. These results are important for understanding how animals function in, and respond to, their environment.