How do squirrels stay hydrated for months without drinking water, Current Biology
As the weather cools, one species of squirrel, the 13-lined ground 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. Their hibernation consists of prolonged periods of torpor, which are characterized by low body temperature and suppressed metabolism and can last up to 18 days. This torpidity is interspersed with short periods of arousal, lasting up to 48 h, during which squirrels temporarily return to an active-like state and lose small amounts of water to urination and evaporation.
To study how the squirrels manage to stay hydrated without drinking water, researchers divided dozens of squirrels 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-and measured the blood fluid, or serum, of each group of squirrels. Generally, high serum concentration means more water is needed and makes animals, including humans, feel thirsty. However, 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. More surprisingly, instead of drinking a lot of water before hibernation, researchers found that the animals actually drink less water than they normally do.
The chemical test revealed that 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), as reported by researchers on a recent publication on Current Biology. During hibernation, osmolytes are depleted from the extracellular fluid. Then during the brief period of arousal, serum osmolality (a fancy word for concentration) levels are restored, preventing the kidney from producing urine, but thirst remains suppressed. This decoupling of thirst and diuresis enables water retention by the kidney while suppressing the drive to leave the safety of the underground burrow in search of water.