The mitochondrial physiology of torpor in ruby-throated hummingbirds, Archilochus colubris.

Abstract

Hummingbirds save energy by facultatively entering torpor, but the physiological mechanisms underlying this metabolic suppression are largely unknown. We compared whole-animal and pectoralis mitochondrial metabolism between torpid and normothermic ruby-throated hummingbirds (Archilochus colubris). When fasting, hummingbirds were exposed to 10℃ ambient temperature at night, they entered torpor; average body temperature decreased by nearly 25℃ (From ∼37 to ∼13℃), and whole-animal metabolic rate (VO2) decreased by 95% compared to normothermia, a much greater metabolic suppression compared with mammalian daily heterotherms. We then measured pectoralis mitochondrial oxidative phosphorylation (OXPHOS) fueled by either carbohydrate or fatty acid substrates at both 39℃ and 10℃ in torpid and normothermic hummingbirds. Aside from a 20% decrease in electron transport system complex I-supported respiration with pyruvate, the capacity for OXPHOS at a common in vivo temperature did not differ in isolated mitochondria between torpor and normothermia. Similarly, the activities of pectoralis pyruvate dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase did not differ between the states. Unlike heterothermic mammals, hummingbirds do not suppress muscle mitochondrial metabolism in torpor by active, temperature-independent mechanisms. Other mechanisms that may underly this impressive whole-animal metabolic suppression include decreasing ATP demand or relying on rapid passive cooling facilitated by the very small body size of A. colubris.