We measured body temperatures in two large hibernating mammals, the eutherian alpine marmot (Marmota marmota) and the egg-laying echidna (Tachyglossus aculeatus) from unrestrained animals in their natural environment. In both species hibernation is broken every 13 days on average by rewarming to euthermic temperatures. We found that the time course of a rewarming Could be closely fitted with a sigmoid curve, allowing calculation of peak rewarming rate and corresponding body temperature. Maximum rewarming rates were twice as high in marmots as in echidnas (12.1 +/- 1.3 degrees C h(-1), n = 10 cf. 6.2 +/- 1.2 degrees C h(-1), n = 10). Peak rewarming rates were positively correlated with body temperature in echidnas, but negatively correlated in marmots. (C) 2009 Elsevier Ltd, All rights reserved.