In 1774, the British physician-scientist Charles Blagden received an unusual invitation from a fellow physician: to spend time in a small room that was warmer, he wrote, “than it was previously believed that any living creature could endure.”
Many people may have been horrified by this offer, but Blagden was delighted by the opportunity for self-experimentation. He marveled that his own temperature remained at 98 degrees Fahrenheit (about 37 degrees Celsius), even when the temperature in the room approached 200 °F (about 93 °C).
Today, this ability to maintain a stable body temperature – called homeothermy – is known to exist among countless species of mammals and birds. But there are also some notable exceptions. The body temperature of the fat-tailed pygmy walleye, for example, can fluctuate by almost 25 °C during a single day.
In fact, a growing body of research suggests that many more animals than scientists once appreciated use this flexible approach – heterothermy – varying their body temperature for minutes, hours or weeks at a time. This can help the animals to continue through all kinds of dangers.
“Because we are homeotherms, we assume that all mammals function the way we do,” says Danielle Levesquea mammalian ecophysiologist at the University of Maine. But in recent years, as improvements in technology allowed scientists to more easily track small animals and their metabolism 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 sleep modewhich has been most extensively studied in animals that use it to conserve energy and thus 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 only one end of what some researchers now consider a spectrum. Many mammals can use 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 the comparative physiologist Fritz Geiser at the University of New England in Australia. “It’s much more interesting than homeothermy.”
Australian eastern long-eared bats, for example, adjust their torpor based on daily changes in weather conditions. Mari Aas Fjelldala bat biologist at Norway’s University of the Environment and Life Sciences and the University of Helsinki, used tiny transmitters to measure skin temperatures while 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 too sunk in thorn more often as rain and wind increased, Fjelldal and colleagues reported Ecology in 2021. This hunking down makes sense, Fjelldal says: Wind and rain make flying more energy-intensive – a big problem when you weigh less than a small pack of M&Ms – and make it more expensive to find the insects the bat eats.
There are even reports of pregnant gray bats entering torpor during unpredictable spring storms, a physiological maneuver that basically puts pregnancy on hold. “That means that to a certain extent they can decide when to give birth,” says Fjelldal, “which is very practical when you live in an environment that can be quite harsh in the spring.” Fjelldal, who was not involved in that study, notes that milk production is metabolically expensive, so there is an advantage to giving birth when food availability is good.
Other animals, such as sugar gliders—tiny pink-nosed marsupials that “fly” through the trees using wing-like folds of skin—rarely use torpor, but appear to be able to take advantage of it in the event of extreme weather. During a storm with Category 1 cyclonic 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 huddled in their tree-hole nests, and many went into torporwhich reduces body temperature from 34.5 °C (94.1 °F) to an average of about 19 °C, Geiser and colleagues found.
Similarly, in response to an accidental flooding event in the laboratory, researchers observed a high degree of unusual period of multi-day torpor in a golden hedgehog, the temperature reaches its lowest at approx. 75°F (24°C).
This more flexible use of torpor may help heterotherms wait out a catastrophe, says Geiser. In contrast, homeothermic species cannot simply reduce their need for food and water, and they may not be able to survive challenging conditions.
“Maybe it’s not food, maybe not water, it might be very hot,” says the ecophysiologist Julia Nowack at Liverpool John Moores University in England, a co-author on the sugar glider study. Torpor, especially in the tropics, has “many different triggers.”
Threats of a different kind, such as the presence of predators, can also induce prowling. The (perhaps perfectly named) edible dormouse, for example, makes an occasional appearance long periods of dizziness in early summer. At first, this behavior puzzled scientists—why hibernate in the summer, when temperatures are pleasant and food plentiful, especially if it meant giving up the chance to reproduce?
After looking at several years of data collected by various researchers, a pair of researchers concluded that because spring and early summer are particularly active periods for owls, these little snackable creatures chose to spend their nights comfortedsafely hidden in underground caves, to avoid becoming dinner. In what is believed to be a similar strategy to avoid nocturnal predators, Fjelldals bats change the use of torpor slightly depending on the phase of the moon, spend more time wet as the moon gets fuller and they become easier to spot.
The fattail dunnarta mouse-like carnivorous marsupial native to Australia, is a third species that lies low when it feels more vulnerable to being eaten. In one study, researchers placed dunnarts in two types of enclosures: Some had a lot of bottom cover in the form of plastic wrap, which simulated an environment protected from predators, while other enclosures had little cover, which simulated a greater risk of predation. In the higher-risk settings, the animals searched less and their body temperature became more variable.
Levesque, who has studied similar non-torpor temperature flexibility in large shrews, says that even small variations in body temperature can be important for conserving water and energy.
Indeed, water loss during hot weather can pose a serious risk to many mammals, and heterothermy is an important conservation tool for some. As Blagden observed, humans are amazingly capable of maintaining stable temperatures even in horribly hot environments, largely due to our sweating abilities. But this is not necessarily a good strategy for smaller mammals – such evaporative cooling in a starving climate can quickly lead to dehydration.
Instead like creatures Madagascar leaf-nosed bat use torpor. On hot days, the bats go into mini bouts of torpor that last only a few minutes. But during particularly hot days, the bats become lethargic for up to seven hours, reducing their metabolism to less than 25 percent of normal and allowing their body temperature to rise as high as 42.9 °C (109.2 °F). And in an experiment with ringtail possumsby slightly increasing their body temperature by about 3 °C (5.4 °F) during a simulated heat wave, the animals saved an estimated 10 grams of water per hour—a lot for a creature weighing less than 800 grams.
This heterothermic way of life gives some animals a bit of a buffer when it comes to coping with variations in their environments, says the physiological ecologist. Liam McGuire at the University of Waterloo in Ontario, Canada. But it can only do so much, he says; heterothermy is unlikely to exempt them from the challenge of rapidly evolving weather conditions caused by climate change.
As for Blagden, he saw the human body as remarkable in its ability to maintain a steady temperature, even by “generating cold” when ambient temperatures climbed too high. Today, however, scientists are beginning to understand that for many mammals, allowing their body temperature to be a little more flexible may also be the key to survival.
This article originally appeared in Knowledgeable magazine, a non-profit publication dedicated to making scientific knowledge accessible to all. Sign up for Knowable Magazine’s newsletter.
