Figure 4.16 Torpor. The tiny West Indian hummingbird has a high surface-to-volume ratio, causing it to lose much of the heat it generates during periods of cold air temperatures. To save energy, measured as the amount of energy consumed to fuel the metabolism of its food, the bird enters torpor when it is resting at night.

Data from F. R. Hainsworth and L. L. Wolf. Regulation of oxygen consumption and body temperature during torpor in a hummingbird, Eulampis jugularis. Science 168 (1970): 368–369.