Energy Costs and Temporal Organization of Torpor in the Rufous Hummingbird (<i>Selasphorus rufus</i>)

Abstract

Continuous records of nighttime O₂ consumption (V̇o2) 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 (Ta)from −1° to 24° 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, V̇o2 was inversely related to Ta over the entire temperature range. During steady-state torpor, V̇o2 reached its lowest value at a Ta of approximately 8° C (= $$T_{a\dot{V}o_{2}min}$$). The measured minimum body temperature (Tbmin) was approximately 13° C The comparatively low values of Tbmin and $$T_{a\dot{V}o_{2}min}$$ for this temperate-zone migrant species may represent an adaptation for surviving low nighttime Ta's in nature. Duration of the entryphase was inversely related to Ta, but linear regression showed that total O₂ consumption (Vo2) during entry was not systematically related to Ta. Total oxygen consumption during arousal was inversely related to Ta, but duration of the arousalphase peaked at approximately 8° C, decreasing at both lower and higher Ta's. Calculations using these data show that, at Ta'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 ofsteady-state torpor.