From London: Science does not often overturn a working assumption politely. The emerging picture of how mammals manage body heat is doing precisely the opposite, and Australian wildlife is sitting at the very heart of the revision.
For most of recorded biology, the ability to maintain a stable internal temperature regardless of conditions outside, a trait called homeothermy, was seen as one of the great mammalian achievements. When the 18th-century British physician-scientist Charles Blagden famously sat in a room approaching 93 degrees Celsius and marvelled that his own temperature barely shifted, he was celebrating that very quality. Today, researchers are beginning to appreciate that for a remarkable number of species, allowing body temperature to slide around quite considerably may be just as clever a solution.
The strategy is called heterothermy, and a body of recent research published across several peer-reviewed journals suggests it is far more prevalent, and far more purposeful, than scientists once appreciated. Knowable Magazine has drawn together the threads of this work in a new synthesis that reveals just how many triggers can send an animal into a state of reduced metabolism and lowered body temperature, known as torpor.
"Because we're homeotherms, we assume all mammals work the way we do," Danielle Levesque, a mammalian ecophysiologist at the University of Maine, told Knowable Magazine. In recent years, as sensor technology allowed researchers to monitor small animals in the wild with far greater precision, she said, "we're starting to find a lot more weirdness."
The most familiar expression of heterothermy is classic hibernation, the deep, months-long dormancy used by bears and ground squirrels to outlast Northern Hemisphere winters. In that state, an animal's metabolism slows to a crawl and body temperature can drop to just above freezing. But hibernation, it turns out, is only the far end of a spectrum. Many mammals deploy much shorter and shallower bouts of torpor in response to immediate circumstances, and the triggers are far more varied than cold and food shortage alone.
Australian species have proven particularly instructive. Fritz Geiser, a comparative physiologist at the University of New England in Armidale who has spent decades studying the thermal biology of Australian animals, describes the field as "extremely complicated" and considerably more interesting than the homeothermy story ever was. His own research on sugar gliders, the small pink-nosed marsupials that glide between trees on flaps of skin, found that during a storm with winds reaching cyclone category 1 force and nearly 10 centimetres of rain falling in a single night, many gliders retreated to their tree-hole nests and entered torpor. Their body temperatures dropped from around 34.5 degrees Celsius to an average of about 19 degrees, allowing them to dramatically reduce their energy needs until conditions improved.
The eastern long-eared bat, another native Australian species, uses torpor even more responsively. Researcher Mari Aas Fjelldal of the Norwegian University of Life Sciences and the University of Helsinki attached tiny transmitters to 37 free-ranging bats in Australia and tracked their skin temperatures over time. The bats sank into torpor not just when temperatures fell, but also as rain and wind speeds picked up, as reported in the journal Oecologia in 2021. For an animal weighing less than a small packet of sweets, the energetic cost of flying through wind and rain, only to find fewer insects, can be prohibitive.
Fjelldal's work extended to a stranger finding still. Pregnant hoary bats have been observed entering torpor during unpredictable spring storms, effectively pausing their pregnancies until conditions improve. "It means that they can, to some degree, actually decide a bit when to give birth," Fjelldal noted, pointing out that since producing milk is metabolically expensive, timing birth to coincide with good food availability offers a genuine survival advantage.
The predator-avoidance dimension is perhaps the most counterintuitive. The edible dormouse, found across Europe, was long puzzling to researchers because it sometimes enters prolonged torpor in early summer, precisely when food is plentiful and temperatures are comfortable. After examining years of pooled data, a pair of scientists concluded the most likely explanation was owl predation: spring and early summer are peak periods for owls hunting small mammals, and the dormouse was opting to spend its dangerous nights safely torpid underground rather than exposed and active. Fjelldal's Australian bats showed a related pattern, spending more time in torpor as the moon waxed fuller and they became more visible to nocturnal predators.
The fat-tailed dunnart, a mouse-sized carnivorous marsupial native to Australia, rounds out the predator-defence picture. In experimental enclosures offering little ground cover, simulating higher predation risk, dunnarts foraged less and allowed their body temperatures to become more variable, effectively hiding physiologically when the risk of being eaten increased.
For animals living in hot, arid environments, heterothermy serves an entirely different purpose. Human sweating is a superb cooling mechanism, but for small mammals it risks rapid dehydration. Madagascar's leaf-nosed bats address this by entering mini bouts of torpor on warm days lasting just minutes, stretching to up to seven hours on the hottest days and allowing their metabolism to fall to less than 25 per cent of normal. Research on ringtail possums found that slightly elevating body temperature during a simulated heat wave saved an estimated 10 grams of water per hour, a significant margin for an animal weighing under 800 grams.
The scientific community has not entirely converged on how to interpret all of this. Some researchers maintain that daily torpor and full hibernation remain functionally distinct categories rather than a seamless continuum, and cluster analyses of 214 species in earlier studies did support that classical distinction. The debate matters because how scientists classify these states shapes how they model the resilience of species to environmental change.
That resilience question carries weight given current climate trajectories. Liam McGuire, a physiological ecologist at the University of Waterloo in Ontario, says heterothermy gives some animals a degree of buffer against environmental variability, but he is clear that it can only do so much. The pace of climate change is likely to outstrip what a flexible thermostat can compensate for.
What the research ultimately reveals is a more layered picture of animal physiology than biology textbooks have traditionally offered. Species that were assumed to be straightforward homeotherms are turning out to have tricks in reserve, deploying torpor in response to storms, moons, predators, and heatwaves in ways that researchers are only beginning to catalogue. For Australia, whose native mammals have evolved in some of the most climatically variable environments on Earth, that flexibility may have always mattered more than scientists gave it credit for. Whether it will be enough in the decades ahead is a different, and considerably more urgent, question.