Torpor is a regulated state of physiological depression in which an animal temporarily lowers its metabolic rate, body temperature, and heart rate to conserve energy. This strategy is employed by various mammals and birds to survive periods when food is scarce or environmental conditions are harsh, such as during cold nights or drought. The duration of this state is highly variable across the animal kingdom, ranging from just a few hours to several months depending on the species and the specific environmental challenge it faces.
Differentiating Short-Term Torpor from Hibernation
Short-term torpor, often called daily torpor, is a brief state typically lasting less than 24 hours, frequently used overnight by smaller animals. This state is generally controlled by the animal’s circadian rhythm, allowing for quick entry and exit.
Hibernation is an extended form of torpor, where the animal enters a deeper, multi-day hypometabolic state that can last for weeks or months. This long-term commitment requires extensive physiological preparation, including significant fat accumulation, and a more profound drop in body temperature and metabolic rate than daily torpor.
The primary physiological difference lies in the depth of metabolic suppression. During hibernation, the metabolic rate can drop to as low as 1–5% of the normal basal rate, and body temperature can fall close to the ambient temperature. In contrast, daily torpor usually reduces the metabolic rate to about 30% of the normal rate, with a less dramatic drop in body temperature.
Environmental and Internal Drivers of Duration
An animal’s maximum possible torpor duration is dictated by its stored energy reserves. For seasonal hibernators, the amount of adipose tissue accumulated before winter directly determines how long they can survive without foraging. When these fat stores are depleted, the animal must arouse from torpor to replenish its fuel supply.
Ambient temperature is an external factor that influences the duration of a torpor bout. If the environmental temperature drops too low, the animal must increase its metabolism to prevent its body temperature from falling close to the freezing point. This metabolic increase consumes more energy, shortening the length of the torpor bout.
Body mass plays an inverse role. Smaller animals lose heat faster due to a higher surface-area-to-volume ratio. This rapid heat loss means small animals must use torpor more frequently and for shorter durations to conserve energy. Conversely, larger hibernators can sustain longer torpor bouts because their mass helps retain heat more effectively, slowing the rate of energy expenditure.
The duration is also regulated internally by a hypothesized “hourglass mechanism.” Metabolic byproducts or the depletion of certain metabolites accumulate during the torpid state, eventually triggering a spontaneous arousal. This energy-expensive process temporarily restores the animal to a normal body temperature to clear the metabolic imbalance before re-entering torpor.
Animals Employing Daily Torpor Cycles
Many small animals with high metabolic demands utilize daily torpor as an energy conservation measure, especially when food availability is unpredictable. This state lasts for a few hours, most often during the animal’s rest phase. Hummingbirds, for example, have a high metabolism that requires them to feed almost constantly during the day.
To survive the night without feeding, a hummingbird enters torpor, dropping its heart rate and body temperature. The duration of this nightly torpor is usually between 4 and 12 hours, allowing them to save up to 60% of their nightly energy expenditure. Small insectivorous bats employ daily torpor during the daytime, their rest period, or during sudden drops in evening temperature.
Small rodents and marsupials, such as pocket mice and dunnarts, use daily torpor to manage their energy budget. In arid environments, some of these animals may extend their torpor bouts up to around 20 hours in winter to maximize energy savings without committing to a full seasonal hibernation.
Animals Utilizing Seasonal Periods of Torpor
Seasonal torpor, which includes hibernation in winter and estivation in summer, lasts from several weeks to many months. Hibernators, such as ground squirrels, woodchucks, and hedgehogs, enter a deep state of torpor that can span up to seven months. The Arctic ground squirrel maintains torpor bouts that can last for two to three weeks at a time before a brief, metabolically expensive arousal.
Bears enter a lighter form of seasonal torpor, sometimes called winter lethargy, with a less profound drop in body temperature. Their body temperature only decreases by a few degrees, allowing them to wake up and become active quickly. This milder suppression means their torpor bouts are continuous for the entire season, as they do not need the frequent arousals seen in smaller hibernators.
Estivation is a form of prolonged torpor that occurs during hot, dry periods to cope with heat and lack of water. The desert hedgehog, for example, can estivate for several weeks during periods of heat and food shortage. The Australian blossom bat uses prolonged torpor in summer, likely in response to reduced water availability and the resulting scarcity of nectar.