Scientists have explained why ground squirrels in hibernation ignore their thirst.

To survive in adverse conditions, many mammals, rodents, and some primate species enter hibernation. This adaptive mechanism slows down metabolism and other life processes within the organism. Most mammals occasionally awaken to replenish their water balance, while thirteen-lined ground squirrels (Ictidomys tridecemlineatus) hibernate in their burrows, disregarding thirst. Until now, scientists have not understood how they manage to do this.

Now, a research team led by Madeleine S. Junkins has discovered that the answer lies in brain structures responsible for regulating fluid and electrolyte balance, particularly in the circumventricular organs of the brain—structures located along the borders of the third ventricle that provide a connection between the central nervous and circulatory systems.

The results of the study, published in the journal Science, indicate the presence of a special mechanism in the animals' brains that prevents thirst signals from reaching the areas responsible for the sensation of thirst.

Previously, researchers found that during hibernation, the ion levels in the blood of the squirrels are maintained at levels close to those of awake rodents, allowing the body to conserve water and relocate ion reserves to parts of the body where they will not enter the bloodstream.

Since the main mystery was the ability of thirteen-lined ground squirrels to ignore thirst, the team examined neural activity, protein expression in the brain, and the neurons' responses to hormones that trigger thirst. The squirrels rejected offered water immediately after waking from hibernation (or during short periods of wakefulness) and showed no interest in the liquid. However, upon a second offer, they did accept the water.

The results established that the brain regions responsible for fluid and electrolyte balance (and hormone production) remained as active during hibernation as in awake rodents. Moreover, some hormones acted as antidiuretics, helping the body retain water.

However, the activity of neurons in the studied areas significantly decreased. This indicates that thirst signals were not transmitted to the responsible areas, despite hormonal and physical signs of dehydration. Researchers have yet to fully uncover the nature of this mechanism and plan to continue their investigations.

The authors of the study noted that the findings are important for a better understanding of the physiology of hibernation in animals. Further research into the ability to control thirst and fluid balance in these animals may aid in developing new treatments for water metabolism disorders in humans and improving outcomes in prolonged surgical procedures.

Understanding how mammals endure hibernation with reduced body temperature (which can drop to a few degrees above zero in thirteen-lined ground squirrels) may also prove useful in developing necessary technologies for human hibernation during long space flights. The findings significantly expand our understanding of how the brain controls vital functions necessary for survival in extreme conditions.