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The capacity for nonshivering thermogenesis (NST) was examined in 26- to 27-day cold-acclimated (CA) muscovy ducklings reared for 21 days at 4 degrees C. Metabolic rate and integrated electromyographic (EMG) muscle activity were measured at ambient temperature ranging from -10 to 28 degrees C. Compared with controls reared at 30 degrees C, CA ducklings were more resistant to cold and had higher peak metabolic rate in extreme cold. Shivering threshold temperature of CA ducklings was 14.2 degrees C lower than lower critical temperature, whereas for controls the two temperatures were similar. Thus CA ducklings exhibited an NST in moderate cold. In addition, at temperatures that produced shivering, EMG activity in CA duckling muscle was lesser than that of controls, even at a higher metabolic rate. Because these ducklings are devoid of brown adipose tissue, these results indicated an increased thermogenic efficiency of muscular activity in CA ducklings.

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.