Thermoregulation and ventilation in the tawny frogmouth, Podargus strigoides:: a low-metabolic avian species

Oxygen consumption ((V)over dot(O2)) and body temperature (T-b) were measured during daytime (corresponding to the normal resting phase) in the tawny frogmouth (Podargus strigoides, mean body mass of 341 g) at ambient temperatures (T-a) between -1 degrees C and 30 degrees C. Mean body temperature (over this range of T-a) was 37.8 degrees C and there was only a small (0.4 degrees C), and insignificant, day-night variation in T-b. Mean (V)over dot(O2) withthin thermoneutrality (25-30 degrees C) was 0.59 mL O-2 g(-1) h(-1), corresponding to a basal metabolic rate (BMR) of 3.32 W kg(-1). This value is only 61% of the predicted value for a non-passeriform bird. The minimal thermal conductance attained at T-a below thermoneutrality was 0.156 W kg(-1) degrees C-1, a value which is very close to the allometrically predicted value. The relatively low (V)over dot(O2) was paralleled by a low total ventilatory volume. This, in turn, was mainly the result of a low respiratory frequency (10.2 breaths min(-1), only 52% of that expected for a similar-sized bird) whereas tidal volume (6.6 mL [BTPS]) was 107% of the expected value. Thus, our results suggest that the changing ventilatory needs during the evolution of the low (V)over dot(O2) in the tawny frogmouth have been met primarily by changes in respiratory frequency.