WATCH: Israeli Bats Destroy Theory of Hibernation

The mouse-tailed bat slumbers the winter away in true hibernation – in toasty warm caves with a temperature of 20 degrees Celsius, Tel Aviv scientists discover.

Ezra Hadad

Like wood frogs, deer mice and hamsters (yes, seriously, Hammie isn't dead), Israeli bats have been discovered to hibernate in the winter – but they're doing it in warm caves, throwing the entire theory of hibernation into disarray. Apparently, that deep "winter sleep" is not driven by temperature at all.

The discovery by scientists at Tel Aviv University began with a mystery: every winter, Israel's population of mouse-tailed bats vanishes, only to reappear in the spring. Professor Noga Kronfeld-Schor and Dr. Eran Levin set out to find where the little insectivores were going.

What they found is that the mouse-tailed bats were hibernating in warm caves in the Great Rift Valley, inside which the ambient temperature - fed by the Earth's internal heat - is a permanent 20 degrees Celsius. Moreover, the bats are in true hibernation, not that fake thing bears do (see below).

The Israeli mouse-tailed bats were spending October to February fast asleep, breathing only once every 15 to 30 minutes, as true hibernators do.

"Their body temperature didn't drop, because their environment is warm and body temperature won't fall below the ambient temperature. But as true hibernators, they are expending very little energy," Kronfeld-Schor told Haaretz.

Bats from the Rhinopomatidae family
Dr. Eran Levin

Previously, hibernation in mammals had been thought confined to animals living in low-temperature climes, who evolved it as a mechanism to survive when it's too cold and generally horrible out to find food. At least enough food to sustain themselves. Not so.

Science had though the trigger for mammalian hibernation was the combination of low temperature and environmental conditions. Given the vagaries of the Israeli mouse-tailed bat, now they have no idea what triggers hibernation, Kronfeld-Schor says. It might boil down to the biological clock, which is also evidently what wakes hibernating beasts.

One clue that environment isn't the only factor is that the warm caves are shared by other species of bats that are not in hibernation. While our mouse-tailed bats snore the winter away, the other species are active as usual.

The caves are near the Sea of Galilee, which lies in the Great Rift, roughly along the northernmost border of the mouse-tailed bat's territory. It could also be that the winter around the Sea of Galilee – though temperate for humans – is too cold for this species.

Don't poke the bear

What can be said is that if the hibernating mouse-tailed bat is rudely removed from the cave and put into conditions below 20 degrees, it goes into stress, says the professor. It leaves the state of hibernation, as your hamster will if you shake it, by the way.

Other hibernating bat species had been a lot more typical, doing so under conditions of low temperature. "For the hibernation to be efficient and energy expenditure to be low, most bats go to places of nearly zero degrees," explains Kronfeld-Schor. "Bats in North America, for instance in Massachusetts and New York, fly to mountains in Vermont where the temperature is even lower. They need the cold so their hibernation is efficient."

Another element that surprised the Israeli bat researchers is that the mouse-tail really is achieving minimal energy expenditure in hibernation despite that high body temperature. During hibernation, an animal does not eat and exists solely on its body fat. At temperatures of 20 degrees, other mammals can't hibernate at all, Kronfeld-Schor says.

In contrast to popular opinion, while the local climate is temperate all year, winters in Israel can get cold, often featuring snow in the Golan Heights and Jerusalem. Some winters are warm throughout and others cold and rainy, so averages are a bit misleading. Elevation also counts.That said, if there's an Israeli winter average, it's about 50-60 degrees Fahrenheit, which is 10-15 degrees Celsius, during the day and colder at night. Though being warm-blooded (they're mammals), bats prefer things a little warmer.

By the way, no, bears do not hibernate in the winter, though many northern folk think they do. Bears do enter a stuporous state which is commonly mistaken for hibernation, but technically it is no such thing. A bear in torpor can easily be woken up, much like your hibernating hamster, and neither will be amused.

"Their body temperature does drop a little but not much," Kronfeld-Schor explains the difference. Therefore, their energy use doesn't drop that much either. In short, the mosquito-eating bat is really fast asleep in that warm Israeli cave and that bear giving you the evil eye really is awake. Now you know.