From 6 to 8 months each year, the striped ground squirrel remains in its burrow underground and completely refrains from eating or drinking.
Below the grasslands of North America, the striped ground squirrel hibernates to survive the cold season without storing food or water. They do not consume anything during this entire period. In a study published in the journal Science, scientists are exploring how and why they do this, according to Popular Science.
Specific brain regions associated with thirst activation are strongly suppressed in hibernating ground squirrels, even during the transition period when they are active. Combined with previous findings from the same group of experts, the new research clarifies the underground survival strategies of striped ground squirrels over such long periods.
Striped ground squirrels have an extremely long hibernation period. (Photo: Gracheva Laboratory).
In most cases, thirst is viewed as a key adaptation for survival. All mammals need water for circulation, cellular activity, waste elimination, and temperature regulation. When ion concentrations in the blood reach a critical point, when blood volume is too low, and when the kidneys begin to feel pressure, hormones and other signals trigger the brain to feel thirsty. After drinking, the balance is restored.
However, for brown-furred ground squirrels, the urge to leave the burrow and seek water can be fatal. This increases their risk of predation, according to Elena Gracheva, a professor of neuroscience and cellular and molecular physiology at Yale University and a co-author of the study. Furthermore, the freezing conditions are also a threat to them. However, hungry predators roaming the ground pose the greatest risk. They are ready to pounce on any ground squirrel that ventures out of its burrow during the winter months when prey is scarce and there is no cover.
Thus, avoiding thirst becomes an unusual way to survive, even if the squirrel could access drinking water. Previous research by Gracheva and colleagues found that hibernating squirrels maintain a stable ion density, like salt, in their blood, equivalent to that of active squirrels, by conserving water and sequestering ions elsewhere in the body. Hormones such as oxytocin and vasopressin enable water storage and play a role in inhibiting urination. The brain region responsible for producing these hormones remains highly active during hibernation, despite the low body temperature of the ground squirrels.
However, the physiological mechanisms do not fully explain the complete absence of thirst. Other signals that stimulate thirst, such as hormones related to kidney pressure and low blood volume, still circulate in the mammal’s body. Nevertheless, even when provided with water during hibernation, ground squirrels still avoid drinking.
Hibernation is not merely a state of sleep. For several weeks at a time, hibernating ground squirrels significantly reduce their metabolic rate and nearly become immobile. Their body temperature decreases by 2 to 4 degrees Celsius, and they exist in a physiological state known as torpor. Throughout the months of hibernation, these long torpor periods of 2 to 3 weeks alternate with 1 to 2 days of wakefulness. Suddenly, the squirrel appears active, and its body temperature rises to normal levels. However, they do not leave the burrow or eat. The active periods are thought to be crucial for ground squirrels to eliminate waste and maintain oxygen supply to the cardiovascular system, according to Gracheva.
To determine why ground squirrels do not experience thirst or seek water when awake, Gracheva and her team conducted a series of molecular and behavioral experiments. First, they provided hibernating ground squirrels with fresh water or saline solution when they woke up. They found that the squirrels showed more interest in the saline solution rather than water. This indicates that they may require salt to increase blood volume without diluting ion concentrations. Upon further examination of the squirrels’ brains, the research team discovered that neurons could still respond to thirst signals during hibernation, but something continuously occurring in the brain inhibits their responses.
Research on hibernation has the potential for broad applications, such as improving the effectiveness of transplant surgeries or heart operations. If we can better control metabolic activity and find ways to eliminate uncomfortable needs, humans could leverage the hibernation state for long-distance space travel to Mars and beyond.
