Cold-hearted bats: uncoupling of heart rate and metabolism during torpor at sub-zero temperatures.

Many hibernating animals thermoregulate during torpor and defend their body temperature () near 0°C by an increase in metabolic rate. Above a critical temperature (), animals usually thermoconform. We investigated the physiological responses above and below for a small tree-dwelling bat (, ∼14 g) that is often exposed to sub-zero temperatures during winter. Through simultaneous measurement of heart rate () and oxygen consumption ( ), 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 ( 5-20°C). for this species was at a of 0.7±0.4°C, with a corresponding of 1.8±1.2°C. Below , animals began to thermoregulate, as indicated by a considerable but disproportionate increase in both and The maximum increase in was only 4-fold greater than the average thermoconforming minimum, compared with a 46-fold increase in The differential response of and  to low was reflected in a 15-fold increase in oxygen delivery per heart beat (cardiac oxygen pulse). During torpor at low , thermoregulating bats maintained a relatively slow 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 in an unstudied physiological state that may occur frequently in the wild and can be extremely costly for heterothermic animals.