If you can't stand the heat, you're not an echidna, as the saying (almost) goes. These egg-laying mammals are unusual for several reasons. One of those reasons, it turns out, is that their ability to lower their body temperatures makes them largely indifferent to their homes burning down around them. The short-beaked echidna, Tachyglossus aculeatus, is one of four living species of echidna. Like the platypus, echidnas are Australian mammals that lay eggs instead of bearing live young. They do feed their young milk, in the mammalian tradition, though it oozes straight out of their nipple-less skin. Unlike a platypus, a short-beaked echidna doesn't have a duckbill or swim in streams. But it makes up for those deficits with spines and a long, bendy tongue built for catching bugs. T. aculeatus also has the power to dramatically reduce its body temperature. It often enters a chilly state called torpor. During periods of torpor, an echidna slows its metabolism, eating and moving less. Short-beaked echidnas enter long stretches of torpor during Australia's hot season. At other times of the year, they may go into torpor more briefly at midday. It's sort of like taking a siesta, if your afternoon nap also made you extremely hypothermic. Julia Nowack, of the University of New England in New South Wales, Australia, and her coauthors wondered whether this trick might help echidnas survive fires. The researchers studied 10 short-beaked echidnas living in a Western Australia woodland that was due for a controlled burn. Half of the animals lived within the burn area, and the other half lived outside it. Scientists captured the spiny animals, anesthetized them, and temporarily implanted devices that would measure their body temperatures. They also attached a radio transmitter, a GPS device, and an external temperature logger to each animal. Thus outfitted, the echidnas were sent back into the wild about three weeks before the scheduled burn. A few weeks after the fire, the researchers tracked down their subjects and removed their data loggers. One echidna, sadly, hadn't survived the blaze. The internal body-temperature loggers showed that the echidnas had gone into a brief torpor almost every day. The animals' normal body temperature is between 28 and 35 degrees Celsius (82 to 95 degrees Fahrenheit). During torpor, their temperatures dropped as low as 11.6 C (52.9 F). After the fire, the control animals—those living outside the burn area—kept up their normal habits. But echidnas in the burn zone adopted a new, chillier lifestyle. They lowered their body temperatures, spending more time in torpor each day and moving around less. And they sank more often into multi-day torpor stretches. Immediately following the fire, the surviving animals lay low for between 2 and 5 days. Yet when they roused themselves and started moving around again, the echidnas didn't travel to new territory. They stayed on the same charred land, searching for bugs to eat, even as logs and trees smoldered and burned around them for weeks. The researchers think echidnas' remarkable skill at slowing down their metabolisms helps them survive in fire-prone areas. They can hunker down in a fire's aftermath. When they reemerge, they can slow their metabolisms to deal with a food shortage. The same may have been true millions of years in the past. Echidnas are a lot like the small mammals that survived the mass extinction of about 65 million years ago, Nowack and her coauthors say. After the meteor strike, the world was dim and fire-prone. If the strange, ancient mammals alive then were as indifferent to blazes as today's echidnas are, it would help explain how mammals became so successful while the dinosaurs disappeared. There's one potential downside to torpor, the researchers add: being too relaxed in an emergency. The echidna that died in the fire was resting inside a log that caught fire. At the moment the log erupted—according to the data the researchers recovered—the echidna was in torpor. Earlier research found that smoke can wake an animal from torpor, but things didn't work out that way for this particular animal. As the echidna died, another study animal resting inside the same log roused from its torpor and fled—becoming one more mammal survivor.
Image: by Leo (via Flickr)
Nowack, J., Cooper, C., & Geiser, F. (2016). Cool echidnas survive the fire Proceedings of the Royal Society B: Biological Sciences, 283 (1828) DOI: 10.1098/rspb.2016.0382