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.