Squirrels can sleep for more than 6 months without drinking water.

For six to eight months each year, striped ground squirrels do not leave their small underground burrows and do not need to eat or drink at all .

Under the grasslands of North America, striped ground squirrels hibernate to wait out the cold without storing up food or water. They don't eat or drink anything during that time. In a study published in the journal Science, scientists are exploring how and why they do it, according to Popular Science .

Specific brain regions involved in thirst activation are strongly inhibited in hibernating ground squirrels, even during the transition period when they are actively active. Combined with previous findings from the same team, the new study sheds light on the striped ground squirrel's strategy for surviving underground for such long periods.

Striped ground squirrels have extremely long hibernation periods. (Photo: Gracheva Lab).

In most cases, thirst is considered a key adaptation for survival. All mammals need water for circulation, cellular activity, waste removal, body temperature regulation, and more. When the ion concentration in the blood reaches a critical point, when blood volume is too low, when the kidneys begin to be stressed, hormones and other signals trigger the brain to feel thirsty. After drinking water, the balance is restored.

But for brown ground squirrels, the urge to leave their burrows and find water can be a death sentence. That puts them at increased risk of being eaten, said study co-author Elena Gracheva, a professor of neuroscience and cellular and molecular physiology at Yale University. Cold conditions are also a threat. But the biggest threat is hungry predators roaming the ground, ready to pounce on any squirrel that emerges from its burrow during the winter months, when prey is scarce and there is no cover.

Suppressing thirst thus becomes an unusual way to survive, even if the squirrel has access to drinking water. Previous research by Gracheva and colleagues found that hibernating squirrels maintain stable concentrations of ions such as salt in their blood, similar to those in active squirrels, by conserving water and sequestering ions elsewhere in the body. Hormones such as oxytocin and vasopressin enable water storage and act as an inhibitor of urination. The brain regions responsible for producing these hormones remain highly active during hibernation, despite the ground squirrels' low body temperatures.

However, this physiological mechanism alone is not enough to explain the complete absence of thirst. Other thirst-inducing signals, such as hormones related to renal pressure and low blood volume, circulate in mammals. However, even when provided with water during hibernation, ground squirrels avoid drinking.

Hibernation isn't exactly sleep. For several weeks at a time, hibernating ground squirrels dramatically reduce their metabolic rate and become nearly frozen. Their body temperature drops to 2 to 4 degrees Celsius, and they enter a physiological state called torpor . During the months of hibernation, these 2- to 3-week periods of torpor are interspersed with 1 to 2 days of wakefulness. Suddenly, the squirrels appear to become active, their body temperature rising to normal. But they don't leave their burrows and don't eat or drink. The periods of activity are thought to be crucial for ground squirrels to sleep, eliminate waste, and maintain oxygen supply to their cardiovascular system, according to Gracheva.

To determine why ground squirrels don't experience thirst or seek water when they wake up, Gracheva and his colleagues conducted a series of molecular and behavioral experiments. First, they offered hibernating ground squirrels either water or a saline solution when they woke up. They found that the squirrels preferred the saline solution over the water, suggesting that they likely need salt to increase blood volume without diluting ions. When they examined the squirrels' brains more closely, they found that neurons could still respond to thirst signals during hibernation, but something was happening in the brain that was constantly blocking their response.

Hibernation research has potential applications as far-reaching as improving the efficiency of transplant surgery or heart surgery. If we can better control our metabolism and find ways to eliminate unpleasant needs, humans could use hibernation to travel long distances in space to Mars and beyond.