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

Abstract

Oxygen consumption (V_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 -1ºC and 30ºC. Mean body temperature (over this range of T_a) was 37.8ºC and there was only a small (0.4ºC), and insignificant, day-night variation in T_b. Mean V_O2 within thermoneutrality (25-30ºC) was 0.59 mL O₂ 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 ºC^-1, a value which is very close to the allometrically predicted value. The relatively low V_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_O2 in the tawny frogmouth have been met primarily by changes in respiratory frequency.