Physiological and Ecological Aspects of Hypothermia

Abstract

Daily cycles of body temperature in pigeons were discovered in 1843 by Chossat (Aschoff 1970). It is now known that a daily rhythm of 1–2°C in body temperature occurs in virtually all birds, and that many small birds exhibit an extreme decline in nocturnal body temperature (Prinzinger et al. 1991). Hypothermia is defined as any core temperature below the set-range specified for the normal active state of the species, while torpor is defined as a state of inactivity and reduced responsiveness to stimuli (e.g., during hypothermia, hibernation, or estivation) (Commission for Thermal Physiology of IUPS, 1987). Since the criteria for torpor are usually met at core temperatures below 30°C, hypothermia is described in one of two forms: nocturnal hypothermia, with a shallow depression of body temperature to 30–38°C, and torpor, in which body temperature ranges between 4°C to 30°C. Nocturnal hypothermia does not include the normal, sleep-related reduction of body temperature.