Emperor penguins breed during the cold antarctic winter. The males incubate the single egg while fasting for up to 4 mo and losing some 20 kg of their body mass. Fasting captive birds under outdoor conditions lost from 0.145 to 0.434 kg day -1. Mean resting metabolic rate, 49.06 W for 24.8 kg body mass, is 7 and 27%, respectively, higher than predicted from general metabolic equations for birds. Minimal thermal conductance, 1.31 W m-2 degrees C-1, is within the range for other birds. The lower critical temperature is about -10 degrees C; this can be related to large body size (20-40 kg) and to body shape, giving a smaller relative surface area than for other birds. Rigidity of the feathers explains why winds of moderate speed (up to 5 m s-1) have little effect on heat loss. At very low temperatures the behavior of huddling close together is essential in reducing metabolic rate. Without this behavior, survival during the long fast (up to four mo) at winter temperatures would be impossible.