This animal freezes its brain each winter and comes back to life

Picture a mammal that can deliberately shut down most of its brain functions for most of the year, then wake up in the spring with its memories and intellectual faculties fully intact. This is precisely what the Arctic ground squirrel (Urocitellus parryii) does every year during winter.

This fascinating creature inhabits some of the coldest parts of the world, where winter conditions can last for as long as eight months. Rather than trying to counter the effects of the cold, this squirrel chooses to celebrate it by entering one of the most extreme states of hibernation ever recorded in mammals.

During this time, its body temperature goes below the freezing point of water, its heart rate slows down considerably, and most of its brain functions come to a standstill. And then, in the spring, it miraculously comes back to life with no long-term ill effects.

What happens to the brain during long hibernation?

In the human case, even a small reduction in brain temperature or oxygen levels can lead to irreversible damage or cell death. For instance, research has shown that human brain cells are highly sensitive to a lack of oxygen, or ischemia, which is often associated with strokes and traumatic injuries. Under normal circumstances, such stress would trigger a cascade of cellular damage that would be hard to reverse. A study, “The Arctic Ground Squirrel Brain Is Resistant to Injury From Cardiac Arrest During Euthermia”, published in the journal Stroke, illustrates just how sensitive human brain cells are to a lack of oxygen, a contrast to the Arctic ground squirrel’s ability to withstand such conditions.

What the Arctic ground squirrel does is remarkable. It reduces the activity, blood flow, and temperature of its brain to such low levels that the firing of its neurons almost ceases, but the neurons remain alive. This is made possible by controlled physiological processes that turn off synaptic communication and slow down metabolism, a condition referred to as “torpor.”

How the squirrel’s brain survives what would kill ours

The secret to this deep torpor response is the extreme reduction in cellular activities that are otherwise vital for life. A study published in the American Journal of Physiology, “Effects of ambient temperature on metabolic rate, respiratory quotient, and torpor in an arctic hibernator”, have found that in hibernating mammals, the process of protein synthesis, which is a vital cellular activity, is reduced to almost undetectable levels during hibernation. This is a part of the overall plan that the animal has devised to safeguard its cells under conditions of low oxygen and nutrient availability.

The brain of the Arctic ground squirrel enters a “low-power mode” due to the reduction in protein synthesis, blood flow, and oxygen consumption. The cells of the brain are protected from the toxic effects of hypoxia (low oxygen levels) and energy deficiency, which would result in cell death in humans.

The physiology of hibernation: More than just sleeping

Metabolic processes in Arctice ground squirrel are tightly regulated when in hibernation, allowing the animal to survive long periods without food or water. In hibernation, the Arctic ground squirrel’s body goes through extreme changes:

• The body temperature can drop to as low as −2.9 °C (26.8 °F), which is the lowest known in any mammal.
• The heart rate, which was 200 per minute, slows down to less than ten.
• The rate of breathing slows down considerably, and the metabolism rate drops to a fraction of what it was.

The squirrel is able to survive for long periods without eating or drinking by relying on its stored fat and by regulating fluid balance very carefully. Every few weeks, it manages to warm its body up to normal temperatures for a short period, which takes up most of the energy that the squirrel has been conserving during hibernation. This process is believed to help repair any cellular damage that may have occurred.

Why are scientists so interested in this squirrel

The Arctic ground squirrel is more than just an interesting biological phenomenon; it is also a model organism for understanding how life can potentially survive extreme physiological stress. How the Arctic ground squirrel protects its brain during torpor has been found to have applications in the following areas:

• Stroke and neuroprotection research
• Emergency medicine
• Long-duration space travel
• Therapeutic hypothermia and organ preservation

Studying animals that go into extreme torpor can help scientists investigate ways to protect human brains.