Influence of air temperature on ventilation rates and thermoregulation of a flying bat

To assess the involvement of the ventilatory system in thermoregulation during flight, breathing frequencies and tidal volumes were measured from three Phyllostomus hastatus undertaking steady wind tunnel flights at a constant speed over a range of air temperatures (Ta) from 17.7 to 31.1 degrees C. Mean breathing frequency was independent of Ta, and tidal volume increased only modestly with increasing Ta. Consequently, minute ventilation rate increased insignificantly over the range of Ta values investigated. Mean rectal temperature showed a direct linear relation to Ta and increased significantly from 39.1 to 41.9 degrees C over the range of Ta values investigated. The highest rectal temperatures measured from flying P. hastatus are approximately 3 degrees C less than those of flying birds. In contrast to flying birds, flying P. hastatus does not modulate its rate of respiratory evaporative heat loss to any significant extent in response to environmental heat stress and only loses an estimated 14% of its metabolic heat load by this route. Cutaneous heat loss channels must therefore be very important to these animals. Some reasons for the observed differences in the thermoregulatory responses of flying bats and birds are discussed as well as the relative advantages and limitations of each group's solutions to their thermoregulatory challenges.