Ecological, Physiological, and Biochemical Aspects of Torpor in Mammals and Birds

Abstract

As a physiological tactic to temporarily escape the brunt of cold and shortage of food and water, many mammals and birds have evolved the ability to exhibit torpor. Torpor in these heterothermic endotherms is characterized by a periodic, facultative lowering of the “thermostat” for body temperature regulation resulting in the achievement of a hypometabolic state for energy/water conservation. Depending on the species and ecological demand, the decrease of body temperature in torpor may range from 4° to 35°C and the period of torpor from a few hours to many weeks. Unlike torpor in the ectotherms, torpid endotherms are capable of spontaneously exiting from the depressed metabolic state using exclusively endogenously produced heat to restore euthermia (Fig. 1). This unique solution to a temporal energy/water crisis has been successfully employed by diverse groups of birds and mammals inhabiting the arctic, boreal and temperate regions where seasonal cold is predictable. Torpor has also been utilized by species inhabiting the deserts and semi-arid lands, where shortages of water and food may also manifest periodically. Somewhat surprisingly, however, torpor is also being deployed by those which inhabit the subtropical and tropical climates. In this case, daily, in addition to seasonal adversities may interfere with feeding and create a temporary energy shortage and the ability to exhibit torpor may prove to be advantageous. Extensive studies on the evolutionary, physiological and biochemical aspects have firmly established that torpor in birds and mammals is polyphyletic, and represents an advanced form of thermoregulation rather than a reversion to primitive poikilothermy (see Dawson and Hudson 1970; Hudson 1973; Lyman et al. 1982; Wang 1986, 1988; for reviews and references).