ABSTRACT
Some small homeotherms have evolved the capacity to enter daily torpor as a means of conserving energy. Many laboratory investigations and a few field studies have established that daily torpor occurs throughout the annual cycle in hummingbirds, even on occasion in incubating females. In hummingbirds, torpor may occur in response to conditions that limit food consumption or increase energy expenditure: short days, low ambient temperatures, storms that prevent feeding, or experimentally restricted food availability. Such observations are consistent with the view that torpor is a response to an "energy emergency", i.e., that it is used when necessary to prevent critical loss of energy reserves. Further research is necessary not only to discover how torpor is controlled in hummingbirds under seasonally varying physiological circumstances (migration, molt, and breeding), but also to determine if daily torpor is under similar kinds of proximate controls in mammals and birds.
