Cold-hearted bats: Cardiac function and metabolism of small bats during torpor at subzero temperatures

Despite their small size and large relative surface area, many hibernating bats have the ability to thermoregulate and defend their body temperature (T b ) often below 10°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 , ∼14g) that is often exposed to subzero temperatures during winter. Through simultaneous measurement of heart rate (HR) and oxygen consumption ( ) we show that the relationship between oxygen transport and cardiac function is substantially altered in thermoregulating torpid bats down to −2°C, compared with thermoconforming torpid bats at mild ambient temperatures (T a 5-20°C). T crit for this species was T a 0.7 ± 0.4°C, with a corresponding T b of 1.8 ± 1.2°C. Below this T crit animals began to thermoregulate, indicated by a considerable but disproportionate increase in both HR and . The maximum increase in HR was only 4-fold greater than the average thermoconforming minimum, compared to a 46-fold increase in . The differential response of HR and to low T a was represented by a 15-fold increase in oxygen delivery per heart beat (cardiac oxygen pulse). During torpor at low T a , thermoregulating bats maintain a relatively slow HR and compensate for increased metabolic demands by significantly increasing stroke volume and tissue oxygen extraction. Our study provides valuable new information on the relationship between metabolism and HR in an unstudied physiological state and further advances our knowledge of the thermogenic capacity of small bats.