VENTILATORY AND METABOLIC DYNAMICS DURING ENTRY INTO AND AROUSAL FROM TORPOR IN SELASPHORUS HUMMINGBIRDS

Both ventilation frequency (f) and the rate of oxygen consumption (VO2) change dramatically as Selasphorus hummingbirds enter and arouse from torpor. Changes in f and VO2 are generally coincident, but the proportional changes in the two parameters are not identical, and their relationship varies at different ambient temperatures (T(a)) and between entry and arousal. Ventilation frequency and VO2 may be substantially decoupled, particularly during the latter stages of entry into torpor and also during arousals of birds with steady-state torpor body temperatures (T(b)) > 20-degrees-C. Contrary to what generally bas been reported in birds, the time of active inspiration (T(INV)) in hummingbirds usually exceeds the time of expiration (T(EX)). The large decrease in f that occurs as hummingbirds go torpid is accomplished by lengthening T(INV), by slightly increasing T(EX), and by inserting a nonventilatory pause (NVP) into the inspiratory portion of the ventilation cycle. The NVP occurs in the middle of inspiration such that inspiratory airflow occurs in two separate stages. At the lowest f's seen in torpor (1-2 breaths/min), ventilation occurs in bursts of breaths separated by NVPs that lengthen into apneas lasting up to 5 min.