In 1774, British physician and scientist Charles Blagden received an unusual invitation from a fellow physician. They would spend time in a small room that was “hotter than previously thought any living creature could tolerate.”
While many may have been appalled by the offer, Blagden welcomed the opportunity for self-experimentation. He was surprised to find that his body temperature remained at 98 °F (about 37 °C) even though the temperature in the room approached 200 °F (about 93 °C).
Today, we know that this ability to maintain a stable body temperature, called homeothermia, exists in countless mammals and birds. However, there are some notable exceptions. For example, a fat-tailed lemur’s body temperature can fluctuate by nearly 45°F (25°C) over the course of a day.
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In fact, a series of studies suggests that far more animals than scientists once appreciated employ this flexible approach of changing body temperature for minutes, hours, or weeks at a time, or poikilothermy. This can help animals overcome all kinds of dangers.
“Because we are warm-blooded animals, we assume that all mammals work the same way we do,” says Daniel Levesque, a mammalian ecophysiologist at the University of Maine. But in recent years, advances in technology have made it easier for researchers to track small animals and their metabolism in the wild, and “we’re starting to find more weird things,” she says.
The most extreme and well-known form of cold-blooded animals is classical hibernation, and the most extensively studied of organisms that use hibernation to conserve energy and survive the long, cold winters of the northern hemisphere. These animals enter what scientists call deep torpor, a long period in which their metabolism slows down abnormally and their body temperature can drop to below freezing.
But hibernation is just the tip of what some scientists currently think is the spectrum. Many mammals can briefly cycle through shallow torpor (roughly defined as a state of low metabolic decline and small fluctuations in body temperature) as needed, suggesting that torpor has more functions than scientists previously realized.
“It’s very complex,” says comparative physiologist Fritz Geyser of the University of New England in Australia. “It’s much more interesting than thermotherapy.”
For example, the Australian eastern long-tailed bat adjusts its torpor state based on daily changes in weather conditions. Bat biologist Mari Ars Feldal from the Norwegian University of Life Sciences and the University of Helsinki used a small transmitter to measure the skin temperature of 37 free-ranging bats in Australia as they went about their daily lives. Like many cold-blooded animals, bats spend more time in a dormant state when it’s cold, but they also do so more often when it rains or wind speeds increase, Fjelldal et al. reported in a 2021 paper in Oecologia. This crouching position makes sense, Fjelldal says. Wind and rain require more energy to fly, which is a big problem when you weigh less than a bag of M&M’s, and transportation costs are higher, he said. Find the insects that bats eat.
There have even been reports of pregnant great bats falling into a coma during unpredictable spring storms, essentially a physiological maneuver that suspends pregnancy. “This means that you can have some control over when your children give birth, which is very useful when you live in a very harsh environment in the spring,” Ferdal says. Fjelldal, who was not involved in the study, points out that producing milk is metabolically costly, so it is advantageous to give birth when food availability is good.
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Other animals, such as sugar gliders, small pink-nosed marsupials that use wing-like skin folds to “fly” through trees, rarely take advantage of torpor, but appear to be able to take advantage of it in case of severe weather emergencies. During the storm, which brought Category 1 cyclonic winds of about 100 kilometers per hour and 9.5 centimeters of rain overnight, the gliders were likely huddled in nests in tree holes, with many entering a coma and their body temperatures dropping 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 laboratory, researchers observed a highly unusual multi-day torpor in golden spiny rats, with their temperatures reaching a minimum of around 24 degrees Celsius.
This more flexible use of torpor could help cold-blooded animals wait out catastrophes, Geiser says. In contrast, warm-blooded species are unable to suppress their food and water needs and may be unable to overcome difficult conditions.
“They might not have food, they might not have water, and it might be very hot,” says Julia Nowak, an ecophysiologist at Liverpool John Moores University in the UK and co-author of the sugar glider study. There are “various triggers” for anhedonia, especially in the tropics.
Other types of threats, such as the presence of a predator, can also prompt you to lay low. For example, the (perhaps perfectly named) edible dormouse can fall into a long coma in early summer. At first, this behavior puzzled researchers. Why snooze in the summer, when temperatures are comfortable and food is plentiful, especially if it means missing out on breeding opportunities?
After reviewing years of data collected by various scientists, the two researchers concluded that because spring and early summer are particularly active times for owls, these small snack animals are likely choosing to spend the night unconscious, hidden safely in their underground burrows to avoid being turned into dinner. In what is thought to be a similar strategy for avoiding nocturnal predators, Feldal bats use sleeplessness slightly differently depending on the phase of the moon, spending more time in a sleepless state as the moon waxes and waning, making them more likely to be spotted.
The fat-tailed dunnart is a rat-like carnivorous marsupial native to Australia, and is the third species to lie low when in danger of being eaten. In one study, researchers placed durnats in two types of enclosures. Some enclosures covered the ground in the form of plastic sheets, simulating a protected environment from predators, while others had little cover, simulating a greater risk of predation. In higher-risk environments, animals foraged less and had greater body temperature fluctuations.
Lévesque, who studied similar non-comatogenic temperature flexibility in large shrews, says even small changes 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, making cold-blooded animals an important conservation tool for some mammals. As Blagden observed, humans are able to maintain a surprisingly stable body temperature even in frighteningly hot environments, thanks in large part to their ability to sweat. However, this is not necessarily a good strategy for small mammals. This evaporative cooling in sweltering climates can quickly lead to dehydration.
Instead, creatures like Madagascar’s spotted bat take advantage of coma. On warm days, bats go into mini-seizures that last only a few minutes. However, on particularly hot days, bats can go into a coma for up to seven hours, with their metabolism dropping to less than 25% of normal and their body temperature rising to 109.2°F (42.9°C). Additionally, an experiment using ring-tailed possums that slightly increased body temperature by about 3°C (5.4°F) during a simulated heat wave saved an estimated 10 grams of water per hour. This is a large amount for a creature weighing less than 800 grams.
Liam McGuire, a physiological ecologist at the University of Waterloo in Ontario, Canada, says this cold-blooded lifestyle gives some animals a bit of a buffer when dealing with environmental fluctuations. But there are limits to what it can do, he says. Cold-blooded animals are unlikely to be exempt from the challenges of rapidly evolving weather conditions posed by climate change.
As for Blagden, he thought that the human body is amazing in its ability to maintain a constant body temperature, even by “producing cold” when the ambient temperature rises too much. But today, scientists are beginning to realize that for many mammals, being able to make their body temperatures a little more flexible may be the key to survival.
This article originally appeared in Knowable Magazine, a nonprofit publication dedicated to making scientific knowledge accessible to all. Sign up for Knowable Magazine’s newsletter.
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