ENERGY COSTS AND TEMPORAL ORGANIZATION OF TORPOR IN THE RUFOUS HUMMINGBIRD (SELASPHORUS-RUFUS)

Continuous records of nighttime O2 consumption (VO2) from rufous hummingbirds (Selasphorus rufus) were used to determine O2 consumption during normothermic nighttime rest, entry into torpor, steady-state torpor, and arousal from torpor over a range of air temperatures (T(a)) from -1-degrees to 24-degrees-C. Whereas entry into torpor occurred anytime during the first 9.5 h of the 12-h night, arousals were consistently initiated within 2.5 h of the end of the night in the absence of known environmental cues. During normothermic rest, VO2 was inversely related to T(a) over the entire temperature range. During steady-state torpor, VO2 reached its lowest value at a T(a) of approximately 8-degrees-C (= T(aVO2min)). The measured minimum body temperature (T(bmin)) was approximately 13-degrees-C. The comparatively low values of T(bmin) and T(aVO2min) for this temperate-zone migrant species may represent an adaptation for surviving low nighttime T(a)'s in nature. Duration of the entry phase was inversely related to T(a), but linear regression showed that total O2 consumption (VO2) during entry was not systematically related to T(a). Total oxygen consumption during arousal was inversely related to T(a), but duration of the arousal phase peaked at approximately 8-degrees-C, decreasing at both lower and higher T(a)'s. Calculations using these data show that, at T(a)'s likely to be encountered at night in nature, rufous hummingbirds can save energy by entering torpor even if they arouse immediately without an intervening period of steady-state torpor.