The strange animals that control their body heat

Source: Ars Technica

Introduction

Some creatures can dramatically alter their internal temperature and outlast storms, floods, and predators.

![Image: An edible dormouse]
Credit: DeAgostini / Getty Images

Early Experiments in Extreme 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 formerly thought any living creature could bear.”

Blagden was delighted by the opportunity for self‑experimentation. He marveled as his own temperature remained at 98 °F (≈ 37 °C), even as the temperature of the room approached 200 °F (≈ 93 °C).

Homeothermy vs. Heterothermy

Today, the ability to maintain a stable body temperature—called homeothermy—is known to exist among myriad species of mammals and birds. However, notable exceptions exist. The fat‑tailed dwarf lemur can fluctuate its body temperature by nearly 45 °F (25 °C) over a single day.

A growing body of research suggests that many more animals employ heterothermy, varying their body temperature for minutes, hours, or weeks. This flexibility may help them persist through various 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.”

Hibernation and Torpor

The most extreme—and well‑known—form of heterothermy is classic hibernation (see Knowable Magazine, 2022). Animals enter long periods of deep torpor, slowing metabolism and allowing body temperature to drop to just above freezing.

Hibernation is just one end of a spectrum. Many mammals can deploy shorter bouts of shallow torpor, defined as smaller reductions in metabolism and body temperature, suggesting torpor serves more functions than 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.”

Torpor in Bats

Australian eastern long‑eared bats adjust their torpor use based on daily weather changes. Bat biologist Mari Aas Fjelldal measured skin temperatures of 37 free‑ranging bats with tiny transmitters.

• Bats spent more time in torpor when it was cold.
• They sank into torpor more often as rain and wind speeds increased, because wind and rain make flying energetically demanding and reduce insect availability.

There are reports of pregnant hoary bats entering torpor during unpredictable spring storms, effectively pausing 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.”

Other Species Using Torpor

• Sugar gliders rarely use torpor but do so during major weather emergencies. During a Category 1 cyclone (≈100 km h⁻¹ winds, 9.5 cm rain), many entered torpor, reducing body temperature from 94.1 °F (34.5 °C) to about 66 °F (19 °C) (Geiser et al., Scientific Reports).
• A golden spiny mouse exhibited a multiday torpor period after accidental lab flooding, with temperature dropping to about 75 °F (24 °C) (Flood‑induced multiday torpor in golden spiny mice).
• The edible dormouse enters long periods of torpor in early summer to avoid nocturnal owl predation (Spring torpor study).
• Fjelldal’s bats also adjust torpor based on moon phase, spending more time torpid during fuller moons when they are more visible to predators (Moon‑phase torpor study).
• The fat‑tailed dunnart, a carnivorous marsupial, shows more variable body temperatures and reduced foraging in high‑risk (low‑cover) environments (Study).

Heterothermy and Climate Adaptation

Levesque notes that even small temperature variations can save water and energy. In hot weather, evaporative cooling can quickly dehydrate small mammals, making torpor a valuable strategy.

• Madagascar’s leaf‑nosed bats use mini bouts of torpor lasting minutes to hours, reducing metabolism to <25 % of normal and allowing body temperatures up to 109.2 °F (42.9 °C) (Source).
• Ringtail possums raised body temperature by ~3 °C (5.4 °F) during simulated heat waves, saving an estimated 10 g of water per hour (Source).

Physiological ecologist Liam McGuire (University of Waterloo) emphasizes that heterothermy provides a buffer against environmental variability, though it may not fully offset rapid climate change.

Historical Perspective

Blagden marveled at human capacity to maintain steady temperature, even “generating cold” in extreme heat. Modern research now recognizes that flexible body temperature regulation is key to survival for many mammals.

This story originally appeared at Knowable Magazine.

Knowable Magazine explores the real‑world significance of scholarly work through a journalistic lens.

33 Comments