The Function of Torpor in Hummingbirds

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Abstract
We constructed estimates of daily energy budgets for male Eugenes fulgens and female Archilochus alexandri from daily time budgets, energetic rate functions, measurements of food intake, and nocturnal oxygen consumption. These budgets permitted us to examine the relationship between net energy gains and the occurrence of torpor. Results indicated that torpor was used only in "energy emergency" situations at a minimum "threshold" of energy reserves and not to reduce nocturnal energy expenditures when net energy gains during the day were sufficient for overnight expenditures.The feeding behavior of hummingbirds together with a threshold model of torpor generates a scheme of daily energy balance which produces the relationship between torpor and reduced net energy gains that is characteristic of several species exhibiting daily torpor. The absence of torpor except in energy emergencies suggests either a loss in energetic efficiency due to torpor or a constraint from entering torpor due to predation or some physiological risk.