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Many hibernating animals thermoregulate during torpor and defend their body temperature ( T  b ) near 0°C by an increase in metabolic rate. Above a critical temperature ( T  crit ), animals usually thermoconform. We investigated the physiological responses above and below T  crit for a small tree-dwelling bat ( Chalinolobus gouldii , ∼14 g) that is often exposed to sub-zero temperatures during winter. Through simultaneous measurement of heart rate ( f  H ) and oxygen consumption ( V̇  O 2  ), we show that the relationship between oxygen transport and cardiac function is substantially altered in thermoregulating torpid bats between 1 and -2°C, compared with thermoconforming torpid bats at mild ambient temperatures ( T  a 5-20°C). T  crit for this species was at a T  a of 0.7±0.4°C, with a corresponding T  b of 1.8±1.2°C. Below T  crit , animals began to thermoregulate, as indicated by a considerable but disproportionate increase in both f  H and V̇  O 2  The maximum increase in f  H was only 4-fold greater than the average thermoconforming minimum, compared with a 46-fold increase in V̇  O 2  The differential response of f  H and V̇  O 2   to low T  a was reflected in a 15-fold increase in oxygen delivery per heart beat (cardiac oxygen pulse). During torpor at low T  a , thermoregulating bats maintained a relatively slow f  H and compensated for increased metabolic demands by significantly increasing stroke volume and tissue oxygen extraction. Our study provides new information on the relationship between metabolism and f  H in an unstudied physiological state that may occur frequently in the wild and can be extremely costly for heterothermic animals.

Cold-hearted bats: uncoupling of heart rate and metabolism during torpor at subzero temperatures