In 1774, British physician-scientist Charles Blagden received an unusual invitation from a fellow physician: to spend time in a small room that was hotter, he wrote, “than it was formerly thought any living creature could bear.”

Many people may have been appalled by this offer, but Blagden was delighted by the opportunity for self-experimentation. He marveled as his own temperature remained at 98° Fahrenheit (approximately 37° Celsius), even as the temperature of the room approached 200°F (about 93°C).

Today, this ability to maintain a stable body temperature—called homeothermy—is known to exist among myriad species of mammals and birds. But there are also some notable exceptions. The body temperature of the fat-tailed dwarf lemur, for example, can fluctuate by nearly 45°F (25°C) over a single day.

In fact, a growing body of research suggests that many more animals than scientists once appreciated employ this flexible approach—heterothermy—varying their body temperature for minutes, hours, or weeks at a time. This may help the animals to persist through all sorts of dangers.

“Because we’re homeotherms, we assume all mammals work the way we do,” says Danielle Levesque, a mammalian ecophysiologist at the University of Maine. But in recent years, as improvements in technology allowed researchers to more easily track small animals and their metabolisms in the wild, “we’re starting to find a lot more weirdness,” she says.

The most extreme—and well-known—form of heterothermy is classic hibernation, which has been most extensively studied in critters who use it to save energy and so survive the long, cold winters of the Northern Hemisphere. These animals enter long periods of what scientists call deep torpor, when metabolism slows to a crawl and body temperature can drop to just above freezing.

But hibernation is just one end of what some scientists now consider a spectrum. Many mammals can deploy shorter bouts of shallow torpor—loosely defined as smaller reductions in metabolism and smaller fluctuations in body temperature—as the need arises, suggesting that torpor has more functions than scientists previously realized.

“It’s extremely complicated,” says comparative physiologist Fritz Geiser of the University of New England in Australia. “It’s much more interesting than homeothermy.”

Australian eastern long-eared bats, for example, adjust their torpor use based on day-to-day changes in weather conditions. Mari Aas Fjelldal, a bat biologist at the Norwegian University of Life Sciences and the University of Helsinki, used tiny transmitters to measure skin temperatures as 37 free-ranging bats in Australia went about their daily lives. Like many heterothermic species, the bats spent more time in torpor when it was cold, but they also sank into torpor more often as rain and wind speeds picked up, Fjelldal and colleagues reported in Oecologia in 2021. This hunkering down makes sense, says Fjelldal: Wind and rain make flying more energetically demanding—a big problem when you weigh less than a small packet of M&M’s—and make it more costly to find the insects the bats eat.

There are even reports of pregnant hoary bats entering torpor during unpredictable spring storms, a physiological maneuver that basically pauses their pregnancies. “It means that they can, to some degree, actually decide a bit when to give birth,” says Fjelldal, “which is really handy when you’re living in an environment that can be quite harsh in the spring.” Fjelldal, who wasn’t involved in that study, notes that producing milk is expensive metabolically, so it’s advantageous to give birth when food availability is good.

Other animals, like sugar gliders—tiny, pink-nosed marsupials that “fly” through the trees using wing-like folds of skin—rarely use torpor but seem able to take advantage of it in the case of major weather emergencies. During a storm with category 1 cyclone winds of nearly 100 kilometers per hour and 9.5 centimeters of rain falling in a single night, the gliders were more likely to stay cuddled up in their tree-hole nests, and many entered torpor, reducing body temperature from 94.1°F (34.5°C) to an average of about 66°F (19°C), Geiser and colleagues found.

Similarly, in response to an accidental flooding event in the lab, researchers observed a highly unusual period of multiday torpor in a golden spiny mouse, its temperature reaching a low of about 75°F (24°C).

This more flexible use of torpor can help heterotherms wait out a catastrophe, Geiser says. In contrast, homeothermic species can’t just dial back their need for food and water and may not be able to outlast challenging conditions.

“Maybe there’s no food, maybe no water, it may be really warm,” says ecophysiologist Julia Nowack of Liverpool John Moores University in England, a coauthor on the sugar glider study. Torpor, especially in the tropics, has “lots of different triggers.”

Threats of a different sort, such as the presence of predators, can also prompt hunkering down. The (perhaps perfectly named) edible dormouse, for example, sometimes enters long periods of torpor in early summer. At first, this behavior puzzled researchers—why snooze away the summer, when temperatures are comfortable and food abundant, especially if it meant forgoing the chance to reproduce?

After looking at years of data collected by various scientists, a pair of researchers concluded that because spring and early summer are especially active periods for owls, these small snackable critters were likely opting to spend their nights torpid, safely hidden in underground burrows, to avoid becoming dinner. In what is thought to be a similar strategy to avoid nocturnal predators, Fjelldal’s bats alter their torpor use slightly depending on the phase of the moon, spending more time torpid as the moon grows fuller and they become easier to spot.

The fat-tailed dunnart, a mouse-like carnivorous marsupial native to Australia, is a third species to lie low when it feels more at risk of being eaten. In one study, researchers placed dunnarts in two types of enclosures: Some had lots of ground cover in the form of plastic sheeting, simulating an environment protected from predators, while other enclosures had little cover, simulating a greater risk of predation. In the higher-risk settings, the animals foraged less and their body temperatures became more variable.

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