In the tinder-dry bush of Australia, wildfire can tear through vegetation at terrifying speeds, incinerating almost everything in its path and leaving little more than blackened desert in its wake.
Most animals have a natural, ingrained fear of fire that compels them to flee the flames. But one strange creature has a rather surprising tactic for dealing with blazes. It does nothing.
Short-beaked echidnas – odd little hedgehog-like critters that lay eggs instead of live young – can enter an inactive state known as torpor, which is used by many animals to help them conserve energy. When in torpor, echidnas reduce their metabolic rate and lower their body temperature.
This, according to research published in April 2016, gives them an uncanny knack for surviving bushfires. It might also have helped their distant ancestors survive a mass extinction.
In 2013 a catastrophic fire swept across Warrumbungle National Park in eastern Australia. Julia Nowack, then based at the University of New England in New South Wales, Australia, studied the aftermath.
Staying inactive during a fire is likely to be the safer option
She and her colleagues noticed that short-beaked echidnas were among the few survivors. Even in areas that had been reduced to ash, the spiny mammals were roaming around, apparently unperturbed.
Echidnas often nest underground in burrows or inside fallen logs, which protects them from the heat of the flames. But fires often have a longer-term impact. Echidnas mostly eat ants, and any such six-legged snacks either flee or get roasted by the blaze.
This led Nowack and her colleague Fritz Geiser to wonder if the echidnas were using torpor to help them survive the fires.
"Foraging activity during a fire can lead to animals being trapped in burning areas or being hit by falling trees," says Nowack. "So staying inactive during a fire is likely to be the safer option."
To find out, the researchers took advantage of a controlled burn being conducted by the authorities in woodland south-east of Perth in Western Australia. They identified 10 short-beaked echidnas living in and around the area that was due to be burned.
Torpor allows the echidnas to save energy while they wait for their insect food to return
Using small temperature loggers implanted into each echidna's abdomen and GPS trackers glued to the spines on the animals' backs, Nowack and her colleagues were able to study the echidnas for 21-25 days before the fire and up to 31 days after it. Five of the animals lived inside the burn area and five lived outside it.
In the days leading up to the fire, the echidnas dropped into short bouts of torpor almost every day.
But on the night of the fire, those living in the fire area entered a multi-day torpor that saw their body temperatures drop by up to 20C over the next 31 days, far lower than they had before the fire. This continued for at least three weeks after the fire.
Outside the burn area, the echidnas continued a similar pattern to before.
"After the fire, the body temperature of the echidnas in the fire areas was on average lower than the body temperature in the control groups," said Nowack.
Most examples of torpor seem to have evolved to help animals survive extreme cold
Lowering their body temperature in this way might help the echidnas cope with increased temperatures, as the area continues to smoulder after a fire.
"The fire produced large amounts of smoke, and because there was little ground cover, it mainly affected bushes, logs and trees, causing trees to collapse and leading to the presence of burning logs for weeks after the fire," says Nowack. "Smoking trees were observed until the end of our study, 31 days after the burn.
But perhaps more importantly, torpor allows the echidnas to save energy while they wait for their insect food to return.
"Lowering their body temperature is connected to a reduction of energy expenditure," says Nowack. "It enables echidnas to stay inactive for longer and therefore allows them to stay hidden and protected."
Echidnas' ability to maintain a regular body temperature and then switch to a variable one is called "heterothermy".
It is a common trick. Many small mammals, such as bats, ground squirrels and mice, as well as some birds including hummingbirds, can go into torpor for anything from a few hours to several weeks.
Previous studies have shown that echidnas can hibernate for months
However, most examples of torpor seem to have evolved to help animals survive extreme cold. During a deep freeze, food is scarce and it takes lots of energy to maintain a high body temperature.
Nowack and Geiser now believe torpor may also play a key role in helping animals survive fires too.
In 2015, working with Clare Stawski, they found that the brown antechinus, another small insect-eater, also appears to enter torpor for longer periods in the days after a fire. Without cover to hide them from predators, they also became active in the night rather than the day.
Also in 2015, they found that small possums called sugar gliders enter torpor during cyclonic storms.
Torpor, they argue, may be a key way for animals to survive disasters. It may have even given mammals the edge they needed to survive in the aftermath of the dinosaur-killing asteroid that struck Earth 66 million years ago.
The ensuing mass extinction "wiped out three-quarters of the plants and animal species on Earth," says Geiser. "Reduced light levels and recurring fires continued for a year or more, and must have affected animal survival for a long period."
Under both scenarios, fire and ice, torpor is a successful strategy
Torpor could have helped. "Although the torpor bouts observed in our study only lasted a few days, previous studies have shown that echidnas can hibernate for months," says Geiser.
After surviving the extreme conditions that marked the end of the Cretaceous period, mammals went on to dominate the Earth.
The researchers argue that heterothermy evolved early in mammal evolution, before the echidnas' group split from the other mammals.
Only later, they say, did some mammals lose the ability to undergo torpor – perhaps because their sizes and lifestyles made it unnecessary.
"The idea that a state of torpor may have helped some mammal groups survive the K-T extinction event is quite reasonable," says palaeontologist Tyler Lyson of the Denver Museum of Nature and Science in Colorado, who was not involved in the research.
This chilled-out approach to wildfires may well give echidnas an edge
"They not only had to deal with a thermal pulse and fire immediately after the impact," he says, "but also the subsequent nuclear winter caused by the debris that was blasted out into the atmosphere, which blocked the Sun for several months. Under both scenarios, fire and ice, torpor is a successful strategy."
The mammals may not have been the only ones. "In fact, a state of torpor is also employed by other winners of the K-T extinction event, including turtles and crocodiles," Lyson says.
However, while this chilled-out approach to wildfires may well give echidnas an edge, it does not work every time.
During their study, Nowack's team found three echidnas that had perished in the blaze. One was an animal they were tracking, which seems to have dropped into torpor inside a fallen log that caught fire.
Previous research has shown that echidnas can wake from torpor and move off at speed when threatened by smoke. In fact, another echidna resting in the same log woke up and fled – but its companion was not so lucky.
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