‘We’re starting to find a lot more weirdness’: These strange animals can control their body heat
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 once thought a living creature could stand.”
Many people may have been dismayed by this offer, but Blagden was thrilled at the opportunity to self-experiment. He was surprised to find that his own temperature remained at 98 degrees Fahrenheit (about 37 degrees Celsius), even as the room temperature approached 200°F (about 93°C).
Today, this ability to maintain a stable body temperature – called homeothermy – is known to exist in 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 the course of a single day.
In fact, a growing body of research suggests that many more animals than scientists once believed use this flexible approach—heterothermy—by varying their body temperatures for minutes, hours, or weeks at a time. It can help animals survive all kinds of dangers.
“Because we are homeothermic, we assume that all mammals function like us,” explains Danielle Lévesquemammal ecophysiologist at the University of Maine. But in recent years, as technological advances have made it easier for researchers to track small animals and their metabolisms in the wild, “we’re starting to discover a lot more weirdness,” she says.
The most extreme – and best known – form of heterothermy is classic hibernationwhich has been most studied in creatures that use it to save 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 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 periods of superficial torpor – loosely defined as smaller reductions in metabolism and smaller fluctuations in body temperature – as needed, suggesting that torpor has more functions than scientists previously thought.
‘It’s extremely complicated,’ says comparative physiologist Fritz Geiser from the University of New England in Australia. “It’s much more interesting than homeothermy.”
Eastern Australian long-eared bats, for example, adjust their use of torpor based on daily changes in weather conditions. Mari Aas Fjelldalbat 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, bats spent more time in torpor when it was cold, but they sank into torpor more often as rain and wind speeds accelerated, Fjelldal and colleagues reported in Ecology in 2021. The decline makes sense, Fjelldal says: Wind and rain make flying more energy-intensive — a big problem when you weigh less than a small package of M&Ms — and make it more expensive to find the insects that bats eat.
There are even reports of pregnant hoary bats entering torpor during unpredictable spring storms, a physiological maneuver that essentially terminates their pregnancies. “This means that they can, to some extent, decide a little bit when to give birth,” says Fjelldal, “which is very handy when you live in an environment that can be quite harsh in the spring.” Fjelldal, who was not involved in this study, notes that producing milk is metabolically expensive, so it is advantageous to give birth when food availability is good.
Other animals, like sugar gliders—tiny pink-nosed marsupials that “fly” through trees using wing-like folds of skin—rarely use torpor but appear capable of taking advantage of it during a major weather emergency. 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, gliders were more likely to stay huddled in their nests in trees, and many have entered torporreducing body temperature from 94.1°F (34.5°C) to an average of about 66°F (19°C), Geiser and his colleagues found.
Similarly, in response to an accidental flooding in the laboratory, researchers observed a strong unusual period of torpor lasting several days in a golden spiny mouse, its temperature reaching a minimum of about 75°F (24°C).
This more flexible use of torpor may help heterotherms wait out a catastrophe, Geiser says. In contrast, homeothermic species cannot simply reduce their food and water requirements and may not be able to survive harsh conditions.
“Maybe there’s no food, maybe no water, maybe it’s very hot,” says an ecophysiologist. Julia Nowack from Liverpool John Moores University in England, co-author of the sugar glider study. Torpor, especially in the tropics, has “many different triggers.”
Threats of another type, such as the presence of predators, can also encourage retreat. The (perhaps perfectly named) edible dormouse, for example, sometimes enters long periods of torpor at the beginning of summer. At first, this behavior intrigued researchers: why sleep in summer, when temperatures are comfortable and food is plentiful, especially if it meant giving up the chance to reproduce?
After reviewing years of data collected by various scientists, two researchers concluded that because spring and early summer are particularly active times for owls, these little nibbling creatures were likely choosing to spend their nights numbhidden safely in underground burrows, to avoid becoming dinner. In what is thought to be a similar strategy for avoiding nocturnal predators, the bats modify their use of torpor slightly depending on the phase of the moon, spending more time numb as the moon fills and they become easier to spot.
THE big cock dunnarta carnivorous, mouse-like marsupial native to Australia, is the third species to hide when at greater risk of being eaten. In one study, researchers placed dunnarts in two types of enclosures: some had heavy plant cover in the form of plastic sheeting, simulating an environment protected from predators, while others had little cover, simulating a greater risk of predation. In higher risk contexts, animals fed less and their body temperatures became more variable.
Levesque, who studied similar temperature flexibility without torpor in large tree shrews, says that even small variations in body temperature can be important for saving water and energy.
Indeed, water loss in hot weather can pose serious risks for many mammals, and hetethermy is an important conservation tool for some. As Blagden observed, people are wonderfully capable of maintaining stable temperatures even in extremely hot environments, thanks in large part to our ability to sweat. But this is not necessarily a good strategy for small mammals: such evaporative cooling in a sweltering climate can quickly lead to dehydration.
Instead, creatures like Leaf-nosed bats of Madagascar use torpor. In hot weather, bats enter mini-episodes of torpor that last only a few minutes. But on particularly hot days, bats become twisted for up to seven hours, reducing their metabolism to less than 25 percent of normal and allowing their body temperatures to rise as high as 109.2°F (42.9°C). And in an experiment with ring-tailed possumsBy slightly increasing their body temperature by about 3°C (5.4°F) during a simulated heatwave, the animals saved about 10 grams of water per hour, which is a lot for a creature weighing less than 800 grams.
This heterothermal lifestyle gives some animals a certain buffer when it comes to coping with the variability of their environment, says a physiological ecologist. Liam McGuire from the University of Waterloo in Ontario, Canada. But that can’t do much, he said; Heterothermy is unlikely to exempt them from the challenge of rapidly changing weather patterns brought about by climate change.
As for Blagden, he considered the human body remarkable in its ability to maintain a constant temperature, even while “generating cold” when the ambient temperature rose too high. Today, however, scientists are beginning to understand that for many mammals, allowing body temperature to be a little more flexible may also be essential for survival.
This article was originally published in Knowable Magazine, a non-profit publication dedicated to making scientific knowledge accessible to all. Subscribe to the Knowable Magazine newsletter.
