Here’s how hibernating squirrels live for months without water

As the weather cools, one species of squirrel in the U.S. Midwest is gearing up for one of the most intense naps in the animal kingdom. For up to 8 months, the tiny mammals won’t eat or drink anything at all—and now scientists know how they do it.

Most squirrels don’t hibernate—instead, they stash food for the cold season and spend the winter snug in their nests. Not the 13-lined ground squirrel (Ictidomys tridecemlineatus), whose heart rate, metabolism, and body temperature dramatically plummet during their long rest—similar to bears, woodchucks, and other hibernating animals.

To find out how the squirrels suppress their thirst—a powerful force that could potentially wake them up—researchers measured the blood fluid, or serum, of dozens of squirrels, divided into three groups: those that were still active, those that were in a sleep-of-the-dead hibernation state called torpor, and those that were still hibernating, but in a drowsy in-between state.

Generally, a high serum concentration makes animals, including humans, feel thirsty. The sleeping squirrels’ serum concentration was low, preventing them from waking up for a drink. Even when researchers roused the torpid squirrels, they wouldn’t drink a drop—until the team artificially increased the concentration of their blood serum.

Next, the researchers wanted to know how the squirrels’ blood concentration dropped so low. Perhaps the squirrels drank a lot of water prehibernation to dilute their blood, the researchers thought. But when they filmed squirrels preparing for their winter snooze, they found the animals actually drank less water than they normally did.

Instead, chemical tests revealed the squirrels regulate their blood concentration by removing electrolytes like sodium and other chemicals like glucose and urea and storing them elsewhere in the body (possibly in the bladder), the researchers reported last month in Current Biology . The finding could also explain how other hibernating animals stay hydrated.

This new knowledge might one day help humans with conditions such as diabetes, or astronauts who have launched on long space flights. Unfortunately, even if people can figure out how to drop their serum concentrations, it’s unlikely they’ll ever be as cute as sleeping squirrels.