In early winter, brown bears find a comfy spot in a cave or burrow that's well-protected from the cold. Then they curl up, fall asleep, and don't awaken or eat for 5 to 7 months. It's called hibernation, and until now we weren't really sure what happened in bears' bodies during that time. How could they survive for half a year without food? Today, Science published the first comprehensive study of bear hibernation, which revealed that many of our assumptions about it were wrong.

What we learned will help scientists studying suspended animation for humans on long space voyages. For one thing, they won't be packed in ice boxes like River Tam was on Firefly. Here's why this study means you'll have to revise everything you've learned about cryo-sleep in science fiction.


Studying the behavior of enormous, deadly bears has always been difficult, and especially so when it comes to hibernation. But a few years ago, some scientists took the plunge. They rounded up several "nuisance" animals - bears who come into towns in early winter and endanger humans - and put them in a nice preserve with high tech hibernation boxes. The scientists write:

Bears hibernated inside 0.8 m3 wooden nest boxes with straw for bedding and equipped with infrared cameras, activity detectors, and telemetry-receiving antennas. Food and water were not provided. Air was continuously collected from the closed hibernacula to record O2 consumption (a measure of metabolic rate). After the spontaneous emergence of bears from their dens in spring, Tb recordings were continued, and minimum metabolism after 24 hours of fasting was determined every four nights for 1 month.

Basically they monitored everything from the bear's temperature and body positions, to how much oxygen they took in while breathing.

Their first discoveries were not too surprising: The bears remained curled up, to preserve heat and water. They got up every day or so and repositioned themselves in their straw.


In this video of a hibernating bear, you can hear how slowly he's breathing. Listen to how long the pause is between snores. This guy isn't taking in much oxygen.

Warm sleepers

The biggest surprise for the researchers was that the bears stayed fairly warm throughout their hibernation - even though their metabolisms slowed to 25 percent of normal. Their bodies underwent more profound shutdown than expected, and stayed a lot warmer.


Other animals who go into suspended animation-like states lower their body temperatures quite a bit, but the bears' temperatures lowered by only an average of 5 or 6 degrees - as you can see in the chart, their temperature was the lowest during the coldest parts of winter, then went back up as the season warmed. One bear, who was pregnant during hibernation, had a completely normal temperature until she gave birth - and then it lowered once the cub was born. Probably this was to preserve the health of her fetus.

Nobody is quite sure why bears slow down their metabolism so much while still keeping their temperature up. The researchers speculate that it might be to preserve neural functioning.


So what would human hibernation look like?

Dreams of space travel often include some kind of technology or drug that will induce hibernation in humans so we can endure years of space travel. And these bears make a good model organism because they are fairly large and are mammals (unlike many hibernating creatures).


One thing we've learned right off the bat is that all those movies that show people in icy suspended animation chambers are most likely wrong. We will probably stay at or near normal temperature. But we might, like River, be curled into a fetal position during our travels. So all those movies like Alien that show people lying down neatly on cots - probably not realistic.

It's also likely that we won't be strapped down like the marines are when they come out of their suspended animation chambers in Avatar - we'll need to move around every few days, so we'll want a bit of space in those warm suspended animation boxes.

The big question is how we'd induce hibernation in humans. Would it be through gene therapy that gave us a few bear proteins? And how will we extend the bear's half-year hibernation to potentially decades of suspended animation in space travel? This study, and others after it, can help answer those questions.


Read the full scientific paper at Science.