Respiratory exchange and ventilation during nocturnal torpor in hummingbirds

In mammals that undergo torpor and hibernation, a period of CO2 storage is often a prelude to, and may be required for, the onset of a bout of torpor. Storage of CO2 has been hypothesized to induce an energy-conserving metabolic suppression in torpid mammals. It is unclear whether CO2 storage also occurs in birds that undergo torpor. To study these questions, we measured rates of oxygen consumption (VO2) and carbon dioxide production (VCO2) during normothermy, entrance into torpor, steady-state torpor, and spontaneous arousal in Selasphorus hummingbirds. The respiratory exchange ratio (RER) declined from about 0.86 to about 0.71 in both normothermic and torpid individuals, suggesting a shift from carbohydrate oxidation to lipid oxidation during the night. Torpor metabolism ranged from 5% to 42% of normothermic values. In most individuals that became torpid, a brief period of CO2 storage occurred during entrance into torpor, beginning after VO2 had fallen considerably below normothermic rates. The volume of CO2 stored averaged 0.048 mL CO2 g-1, somewhat less than reported for mammalian hibernators. Some individuals entered torpor without measurable CO2 storage. There was no abrupt change in RER at the start of arousal, although mean arousal RER exceeded the RER during steady-state torpor. The change in metabolic rate between normothermia and torpor had a Q10 of 1.9-2.2 in birds that did not regulate body temperature in torpor. We conclude that a normal Q10 effect is sufficient to explain the metabolic rate of torpid hummingbirds and that transient CO2 storage may occur during entrance into torpor but is not a prerequisite for entrance. Similarly, CO2 release is not a prerequisite for arousal.