Torpor

Torpor is a physiological state in which an animal markedly decreases its metabolic rate and lowers its body temperature to conserve energy during periods of limited food availability or harsh environmental conditions. The extent of metabolic suppression and the drop in core temperature vary among species, and torpor can affect behavior as well as physiology.

There are two main forms of torpor. Daily torpor is a short-term reduction that lasts hours and is used by many small mammals and some birds to cope with cold or scarce resources. Hibernation is a longer, seasonal form of torpor in which body temperature remains depressed for extended periods, typically weeks to months, during winter in certain species.

Physiologically, torpor involves a substantial reduction in metabolic rate, heart rate, and respiration. Thermogenesis decreases and body temperature often drops toward ambient levels. In mammals, brown adipose tissue can contribute to heat production during the arousal from torpor. Arousal is an active process that requires energy and time, and animals can resume normal activity once conditions improve.

Triggers for torpor include energy deficit, low ambient temperatures, and reduced food availability, with the strategy enhancing survival in challenging climates. It occurs in a range of small endotherms, particularly mammals and birds; some nocturnal and seasonally active species employ torpor as a regular energy-saving tool, while others use it only under extreme conditions.

Torpor is distinct from sleep, though it may involve periods of reduced responsiveness. It is a reversible, regulated state that allows animals to balance the costs of maintaining metabolic activity with the risks and benefits of waking to forage or reproduce.