Hibernation is a common phenomenon in the animal kingdom and occurs in over 200 species. Species that can hibernate and survive at low temperatures with only food (fat) reserves stored in their body have been fascinating scientists for years. Most species, like the female black bear, can even give birth and lactate while they're in torpor, which a form of hibernation.
Then, there is the Arctic ground squirrel (Urocitellus parryii) that has intrigued many scientists for its supercooling type of hibernation. Native to the icy tundra where temperatures dip below zero, the sun barely breaks the horizon, and trees struggle to take root in the icy soil, the arctic squirrel retreats into one meter-deep burrows.
Habitat and Burrowing
Their distribution ranges between Alaska, Yukon, northern British Columbia, and Siberia. They inhabit the bushy meadows, riverbanks, and lakeshores of the alpine and arctic tundra. These habitats are characterized by loose, well-drained soil that is favorable for burrowing. For this reason, they are also spotted along sandy coasts and roadsides.
This Rodent family member can be characterized by red fur on the face, white specks on its back, and a light brown or beige-colored underside. It has a bushier tail compared to other genera of squirrels that hibernate in tree nests. The strong limbs and sharp claws of the Arctic Ground Squirrel or AGS specialize in digging and burrowing. Hundreds of squirrels live in colonies with dominant, territorial males within a system of underground tunnels.
Hibernation of Arctic Ground Squirrel
Arctic ground squirrels hibernate in September and again through March with intermittent arousal. Females enter torpor before males in August. They snug in underground nests, lined with grass or lichen. The hardy creatures survive harsh winters by falling into the most extended sleep of any hibernating mammal.
In contrast to hibernation, in which an organism voluntarily enters a dormancy state to conserve energy, torpor involves a physiological change related to reduced body temperature and metabolic rate. The body temperature drops -2.9 Degrees Celsius below freezing. The Central Nervous System (CNS) activities and metabolic rates are maintained at a low, yet functioning level.
The normal course of hibernation or torpor consists of regular bouts of dwindling blood flow, plummeting body temperature, reduced metabolic activity, infrequent breaths, supercooled body fluids, and vanishing neural currents.
Do they wake up during hibernation?
Yes. The stocky squirrels cannot stay in torpor forever. It rouses from the torpor every fifteenth day to warm themselves up, which is known as an Interbout arousal (Serge Daan, 1991). The transition out of torpor is awe-inspiring. The blood flows to the cold muscles and brain. After shivering and shaking for several hours, they are back in their normal state. 90% of the total energy spent during hibernation accounts for warming and maintaining temperature during arousal.
Seven months later, the arctic squirrel wakes up from sleep---famished and ready to mate in May. The males emerge earlier than females. And, the circannual clock in their brains helps males begin the warming process.
It is really more of a cycle than an annual event. The squirrels maintain circadian rhythms throughout the Arctic summer. Their clocks are not light-bound (Bhowmick et al., 2017).
This supercooled hibernation is a biological miracle, a necessity to survive through harsh winters for Arctic squirrels, and an opportunity for humans to understand their own biological clock to cure diseases.
Torpor and Ischemic tolerance
The Arctic Ground Squirrel displays ischemic-like levels of body functions without any evidence of ischemic injury (Bhowmick, Moore, Kirschner, & Drew, 2017). The cerebral ischemic brain injury resulting from oxygen-glucose deficiency is the major cause of mortality in cardiac patients. About 60% of the patients expire despite fast emergency response and treatment.
Yet, the role of the hibernation season or the state in ischemia tolerance in mammalian hibernators is a matter of debate. Some studies suggest that the resistance to injury is dependent on hibernation, and other studies show contrasting results. Recent studies show that the AGS has a unique ability to resist brain damage independent of hibernation season, better than other species, i.e. rats (Bhowmick & Drew, 2017; Bhowmick et al., 2017).
What happens when a squirrel supercools its body?
How Arctic Ground Squirrels survive harsh winters without freezing to death has intrigued scientists. Brian M. Barnes from the University of Alaska, Fairbanks, conducted experiments on live-trapped squirrels in normal conditions. Simultaneously, some were placed in a laboratory to study the effects of temperature variations (Buck, 2000).
The laboratory squirrels, maintained at -3.4 degrees Celsius, had warmer necks (indicating active brain) compared to their colon, feet, and bellies. AGS's wakes up after every two or three weeks and shiver to attain their normal body temperature of 36-37 degrees Celsius. This transitional period is vital for their survival.
Researchers from Russia captured and studied Siberian AGS in temperature-controlled chambers. Their brains were sliced, and their hippocampus' were dissected at three distinct hibernation stages; during hibernation, 2 hours after arousal, and on the first day in spring.
While hibernation in a supercooled environment affects dendrites’ branching in the brain, the AGS that emerge from sleep can re-sprout their bushy dendrites and re-new the neural links in only two hours (PoPov, 1992). Their brain is in full bloom compared to the summer or springtime squirrels. This ability to regenerate the lost dendrites is what scientists refer to as “synaptic plasticity.”
AGS can not only refurbish their hippocampus but all of their brain. It is impressively resilient to brain damage. This process is even more interesting at the molecular level.
Protective Tau protein and resistance to brain damage
In a struggle to understand what triggers the AGS brain’s replenishment, Thomas Arendt from the University of Leipzig in Germany suggested some protective proteins are at work. There is a protein called the tau protein (Thomas Arendt, 2013). It is most commonly seen in patients with Alzheimer’s.
This protein was found in a hyperphosphorylated state in the brain of squirrels. The presence of this protein was tested by a stain that binds to a phosphorylated tau protein. The brains of hibernating and non-hibernating squirrels were observed after the application of stain. It was seen that hibernating squirrels’ brains were stained heavily, whereas non-hibernating were not (Arendt, 2013).
After arousal, the squirrels eliminate the tau proteins from their brain. Perhaps this is evidence that leads scientists to believe that as the squirrel’s metabolism grow slowly and the brain gets shrunken, the modified tau protein accumulation in the brain serves a protective function. This is for the undernourished brain cells and prevents them from going beyond rejuvenation. Intermittent arousal is necessary to reverse the phosphorylated tau from the brain (Thomas Arendt, 2013).
This paring of the brain that is the key to AGS hibernation would be destructive in non-hibernating animals. Scientists are exploring the prospects for treating Alzheimer’s disease, cardiac arrest/stroke, and other neurodegenerative disorders using this tau protein found in the Arctic Ground Squirrel.
Bhowmick, S., & Drew, K. L. (2017). Arctic ground squirrel resist peroxynitrite-mediated cell death in response to oxygen glucose deprivation. Free Radic Biol Med, 113,
203-211. doi: 10.1016/j.freeradbiomed.2017.09.024
Bhowmick, S., Moore, J. T., Kirschner, D. L., & Drew, K. L. (2017). Arctic ground squirrel hippocampus tolerates oxygen glucose deprivation independent of hibernation season even when
not hibernating and after ATP depletion, acidosis, and glutamate efflux. J Neurochem, 142(1), 160-170. doi: 10.1111/jnc.13996
Buck, C. L., B. M. B. (2000). Effects of ambient temperature on metabolic rate, respiratory quotient, and torpor in an arctic hibernator. The American Physiological Society.
Serge Daan, B. M. B., Arjen M. Strijkstra. (1991). Warming up for sleep?- ground squirrels sleep during arousals from hibernation. Neuroscience Letters.
Arendt, T. B. (2013). Neuronal plasticity in hibernation and the proposed role of the
microtubule-associated protein tau as a “master switch” regulating synaptic gain in neuronal networks. The American Physiological Society. doi:10.1152/ajpregu.00117.2013.-The
PoPov, L. S. B., A. G. BRAGIN. (1992).Repeated changes of dendritic morphology in the hippocampus of ground squirrels in the course of hibernation. Journal of Neuroscience.
Nida Riaz is a freelance blogger based in Pakistan. She started writing about her passion for the environment when the world came to a stop in early 2020.