How Ground Squirrels Shut Off Thirst During Hibernation

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Thirteen-lined ground squirrels have an unusual way of getting through winter: they can spend roughly six to eight months hibernating without drinking water. Their survival is not simply a matter of turning off thirst. Instead, several parts of their fluid-control system change during hibernation. The animals conserve water, and their brain circuits that normally create the urge to drink become unusually quiet. During hibernation, the squirrels alternate between long periods of torpor and brief periods called interbout arousals. Torpor can last a few weeks, with body temperature falling to only a few degrees above freezing and metabolism dropping sharply. The animals periodically warm back up for roughly one to two days before returning to torpor, yet they still do not leave their burrows looking for water. Research shows that they can have signs of a substantial fluid deficit while their normal thirst response remains strongly reduced. The key finding is in brain regions involved in sensing the body’s fluid balance. During hibernation, neurons in the subfornical organ and organum vasculosum of the lamina terminalis show much less activity than they do in active squirrels. These neurons normally help detect dehydration-related signals and drive thirst. The hibernating squirrels can still respond to some of the same chemical signals in laboratory tests, suggesting that the system is not simply broken. Rather, its activity is actively suppressed during the hibernating state. Water conservation is also part of the strategy. Earlier work found that changes in body fluids help the squirrels remain hydrated during torpor, while the hormone system that limits water loss becomes active during the transition out of torpor. Antidiuretic hormone release begins early as the animals warm up, helping retain water during these brief arousal periods. That distinction matters. The research does not show that water is unnecessary, or that merely suppressing thirst would let an animal survive dehydration. The squirrels have evolved a coordinated set of physiological changes that reduce water loss and suppress the motivation to seek water while they remain safely underground. Understanding how these systems are controlled could eventually offer clues about fluid balance and other aspects of mammalian physiology, although any medical applications remain speculative.