20Despite their small size and large relative surface area, many hibernating bats have the 21 ability to thermoregulate and defend their body temperature (T b ) often below 10°C by an 22 increase in metabolic rate. Above a critical temperature (T crit ) animals usually thermoconform. 23We investigated the physiological responses above and below T crit for a small tree dwelling 24 bat (Chalinolobus gouldii, ~14g) that is often exposed to subzero temperatures during winter. 25Through simultaneous measurement of 2 ) we 26 show that the relationship between oxygen transport and cardiac function is substantially 27 altered in thermoregulating torpid bats down to -2°C, compared with thermoconforming torpid 28 bats at mild ambient temperatures (T a 5-20°C). T crit for this species was T a 0.7 ± 0.4°C, with 29 a corresponding T b of 1.8 ± 1.2°C. Below this T crit animals began to thermoregulate, indicated 30 by a considerable but disproportionate 2 . The maximum increase 31 in HR was only 4-fold greater than the average 32 -2 . 2 to low T a was represented 33 by a 15-fold increase in oxygen delivery per heart beat (cardiac oxygen pulse). During torpor 34 at low T a , thermoregulating bats maintain a relatively slow HR and compensate for increased 35 metabolic demands by significantly increasing stroke volume and tissue oxygen extraction. 36Our study provides valuable new information on the relationship between metabolism and 37 HR in an unstudied physiological state and further advances our knowledge of the 38 thermogenic capacity of small bats. 39 40