Torpor: a state of suspended animation
Some animal species enter a state of suspended animation that is called ‘torpor’. And while it might look like torpid animals have just gone to sleep for a while, they’ve actually undergone profound physiological changes that include a dramatic drop in their body temperature.
Is torpor like hibernation?
Sort of. Some animals can enter ‘daily torpor’ for a few hours, but hibernating species remain torpid for days or weeks. Although often described as ‘multi-day torpor’, a hibernation period will also include cycles of sleep and wakefulness, and it’s unknown whether the torpid phases are exactly the same as daily torpor. While hibernating, small animals such as chipmunks are sustained by seeds stored in their burrows, whereas large hibernators, such as the black bear, must burn body fat (bears won’t eat, drink or defecate but females may give birth and suckle cubs).
What’s the difference between torpor and sleep?
Both are states of physical inactivity, but it takes longer to exit torpor: if being awake is like driving, arousal from torpor is analogous to starting an old car on a cold day, while sleep is akin to keeping the engine running. Torpor is clearly different from rapid-eye movement (REM) but does resemble non-REM (NREM) sleep, which is also associated with changes such as a lower body temperature. Brain waves are fast and erratic during REM (similar to wakefulness) but slow and synchronised during NREM or torpor – except below 25ºC in torpid animals, where neural activity isn’t always detected. The fact that hibernating species can suffer from sleep deprivation suggests that vital functions of sleep, such as helping learning and memory, can only be performed at normal body temperature.
Which animals undergo torpor?
Mainly warm-blooded vertebrates, especially mammals and birds. Some species are ‘homeothermic’ and can only maintain their high body temperatures using thermal insulation or generating heat through processes such as shivering or burning fat reserves, whereas ‘heterothermic’ animals can allow temperatures to drop through torpor. Hibernation has been observed in various mammal groups but is known in only one bird species, the common poorwill.
Why is torpor useful?
Being heterothermic allows an animal to stay active so they can exploit their environment’s resources without relying on favourable external conditions. But maintaining high internal temperatures is a double-edged sword: when the supply of resources, such as food, become limited or unpredictable – because they aren’t available or foraging is restricted by the risk of predators – it’s difficult to fuel metabolism. Torpor enables animals to survive hard times by temporarily slowing their metabolic rate to conserve energy. A mouse spends over 30 per cent of its energy on generating heat at an ambient temperature of 22ºC, but enters torpor if it’s too cold or can’t consume enough calories for an active lifestyle.
How does physiology change?
The most noticeable effects are reductions in body temperature (hypothermia) and metabolic rate. In species with daily torpor, temperatures fall from about 38ºC to 18ºC on average, while basal metabolic rate (BMR) drops to 30 per cent. In hibernators, average temperature is 5ºC, while metabolism is only 5 per cent of BMR. Smaller animals experience extreme changes: the core temperature of Arctic squirrels can reach -3°C. Other physiological changes include reduced heart rate and breathing.
When do animals enter torpor? Nocturnal species tend to undergo daily torpor during the day, whereas diurnal species are typically torpid at night. And though the word comes from the Latin hibernare – meaning ‘to spend the winter’ – only 8 per cent of species hibernate in winter alone, and almost half do so from autumn to spring. Torpor occurs in all climate zones: the subtropical Australian blossom bat, which feeds on nectar, enters deep torpor in summer, probably due to water availability. Such an example shows how the timing and duration of torpor depends on local challenges to resources.