Energy balance and constancy of body temperature are controlled by the brain in relation to changing conditions of an animal's body and environment. An attempt will be made here to investigate the characteristics of this regulatory system by studying an exceptional situation: the brief nightly period of 1-10 hours when the normal constancy of body temperature is abandoned as an energy conserving measure by one of the smallest known warm- blooded animals, a hummingbird. Like hibernating mammals, hummingbirds can enter a physiological state of torpor, in which metabolic rate and body temperature are greatly reduced. The kind of information about the internal state of the bird and the environment that is integrated by the physiological control system that initiates torpor will be determined. The special role of torpor in making possible the rapid and repeated weight gain necessary to achieve long-distance migration will also be investigated. Although hummingbirds enter the deepest state of torpor known among birds, almost every species of bird studied, as well as many mammals, reduce nighttime body temperature and metabolic rate to some degree in times of energy shortage. Torpor also appears to be analagous to human sleep. Thus torpor in hummingbirds provides an opportunity to study series of generally important physiological fundamentals with the special experimental advantages afforded by an exception to the rule of constant regulation.
