Ground Squirrels and Hibernation
The temperature is -35° Celsius. Frigid Arctic drafts presage the wind chimes of death, ringing incessantly for all those still vulnerable out in the open. With the halo of northern lights shimmering over the monotonous white layout, life goes into a limbo for almost eight months. Survival in these extreme conditions requires specifically evolved traits that confer a selective advantage over other species, acquired through eons of natural selection coupled with ruthless persistence. Deep down in the ground, nuzzled in warm and cozy nests, lie animals that have devised a surreal solution to the unrelenting winter- Hibernators. Indifferent to the outside climate and the throes of the world, hibernators hitchhike with time and travel to the Arctic summers where the world is no longer cold and harsh and where gardens of Eden sprout seemingly out of nowhere, heralding the beginning of good times.
What is hibernation?
Technically speaking, hibernation is a unique and highly regulated physiological state characterized by profound, albeit periodically reversible, depression in body temperature, metabolism, and consciousness. Opposed to popular beliefs, hibernation isn’t deep sleep. The hibernator’s brain becomes too cold and idle to generate the electrical activity necessary to give rise to typical sleep-related wave patterns. In fact, sleep is one of the reasons hibernating animals wake up periodically! Long-time sleep deprivation is always life-threatening.
Some basic definitions first: ‘Torpor’ is the state with reduced physiological activities, usually by lowered body temperature and reduced metabolic rate; while ‘Arousal’ is the phase where the animal exits hibernation. Hibernating animals survive periods of reduced food availability by going into a state of torpor. Hibernation is also described as prolonged torpor. This is indeed a fascinating stage where the electroencephalogram (EEG)- detects electrical activity in the brain, flatlines, or becomes isoelectric (just like people in a comatose state) and action potentials (firing of neurons) cease throughout the nervous system. Cerebral glucose metabolism levels drop down to just 1–2% of the normal levels. Typically, for every 1° drop in temperature, the metabolic rate decreases by 5–7%.
Let’s understand this phenomenon a bit better by discussing one of the most mesmerizing examples of its display:
This snuggled epitome of endearment and comfort is an arctic ground squirrel.
Arctic ground squirrels are the largest and most northern of the North American ground squirrels. With their nests made of caribou hair, these animals (and at times cannibals) do seem cute at first glance. But they are extremely territorial animals who even kill each other over territory battles. However, other related females in the colony often care for orphaned youngsters.
Most small hibernating mammals — hamsters, hedgehogs, bats — turn down their body’s thermostat during hibernation, relinquishing one of the defining features of all mammals: warm blood. Typical hibernating ground squirrels include Arctic ground squirrels, golden-mantled ground squirrels, and long-tailed ground squirrels. Like many other arctic animals, arctic ground squirrels have unique physiological adaptations that allow them to survive during winter. Arctic ground squirrels are obligate hibernators, i.e., they must hibernate; otherwise, they can’t survive the upcoming winter. They spend 7 to 8 months in hibernation! The arctic squirrel is conscious for only 12 days during the whole winter.
To save energy, they curl up in balls in underground burrows. In order to slow their metabolism, they drop their temperatures way down, sometimes below 0°C. Now one may wonder, doesn’t its blood freeze up under such cold conditions? Well, the ground squirrel has another ingenious solution stored in its repertoire. Using a condition known as ‘supercooling’- the squirrel’s blood remains liquid even at -2.9° Celsius- which is the lowest temperature recorded for any mammal.
Ground squirrel hibernation is more of a cycle than an annual event. Every few weeks, they warm back their tiny bodies. This is done to ensure that their brain remains unharmed and doesn’t deteriorate because of oxygen deprivation. Researchers have estimated that many small hibernating mammals devote between 80 and 90 percent of all energy used during hibernation to keeping their brains alive. At intervals of two to three weeks, while maintaining a state of sleep, hibernating squirrels shiver and shake for 12 to 15 hours to create heat that warms them back to an average body temperature of about 36.4 degrees Celsius. Less than 24 hours later though, after the shivering and shaking stops, they return to their uber-cold suspended animation. This cycle continues until spring, when they start bulking up fat reserves, preparing for the following winter. This type of hibernation is rare among mammals, and scientists are still studying this unique physiological behavior.
Studies Performed
In the early 1990s, Russian scientists studied what was exactly happening in Siberian ground squirrels' brains during this cycle. They found that compared to fully active squirrels, animals 3–4 days into a bout of hibernation had shorter branching dendrites. Dendrites are the structures that extend from neuronal cell bodies and connect the neuron with other neurons and cells. The dendrites also had fewer knob-like contact points or spines. Additionally, the cell bodies of the neurons had shrunk by more than 30%. These studies were performed in the squirrel hippocampus because it is the primary part responsible for hibernation, and also, it’s relatively easier to study compared to other brain parts. Hippocampal synapses undergo pronounced remodeling in concert with torpor and arousal. During hibernation, the number of postsynaptic densities, apical dendritic branches, and spine densities decreases substantially in the hippocampus.
The results were completely different when the scientists looked at squirrels that had just woken up from hibernation just two hours earlier. The dendrites observed in those squirrels were in excellent condition, even more so than usual, and also, the cell bodies were bigger in size. This meant that the squirrels had resprouted from scratch in an extremely short duration of time, everything that they lost before! Later, other scientists confirmed that this process also happened in other parts of the squirrel brain and that this process repeated over and over again according to hibernation cycles. Technically known as ‘synaptic plasticity,’ this process is common even for us humans. Synaptic pruning occurs when our brain clears off synapses that are no longer required, i.e., deletes connections that are dormant for long and probably wouldn’t be required in the future. But the sheer speed at which synaptic pruning takes place in squirrels was unheard of. Rats take four months to do what these squirrels do in just two hours.
One of the hypotheses proposed for this is that a stripped-down brain could save a lot of energy, which is the crux of hibernation. But rebuilding the entire brain doesn’t seem especially efficient. Scientists have had the notion that reconstructing connections when it’s cold is fairly common, but it’s more destructive in non-hibernating animals. Mice, for instance, lose all of their dendritic spines when temperatures drop, while squirrels just lose a quarter. This is what makes hibernating squirrels special: they lose fewer connections than one would otherwise expect.
Benefits of studying hibernation
- When a hibernating mammal is awake in the spring and summer, its brain remains resistant to the kind of oxygen deprivation and neuronal damage that often results from heart attacks and stroke. This strategy can be used one day for treating human diseases encompassing cardiac arrests and strokes.
- Another interesting application of hibernation can help us cure Alzheimer’s disease once and for all. “Tau” proteins in the neurons of their brains are modified by a process called hyperphosphorylation. In humans, this has been found to trigger a tangling of such proteins in the neurons — a hallmark of Alzheimer’s disease. Tau proteins are normally involved in stabilizing long, ropelike components of a cell’s scaffolding called microtubules. Tau keeps the many threads in the rope tightly bundled. When, for unknown reasons, tau proteins become hyperphosphorylated, i.e., burdened with too many phosphate groups — they change shape and start clumping together inside neurons. As a result, microtubules grow slack, and cells lose their shape and stop functioning properly. Researchers know that misshapen tau proteins build up in the brain cells of people with various neurodegenerative disorders — notably Alzheimer’s. Still, it is not yet clear whether distorted tau proteins in part cause such disorders or whether they are a side effect of the true causes. After performing experiments in rodents, researchers observed that the more synapses the rodents’ brains lost during hibernation, the more hyperphosphorylated tau accrued in their neurons. However, in squirrels, they observed that within a few hours of emerging from hibernation into a period of arousal, the squirrels somehow removed tau from their brains. Hyperphosphorylated tau proteins probably accumulate during hibernation to prevent neurons from losing even more synapses than they do and possibly play a role in the swift recovery of synapses during arousal. Hyperphosphorylated tau also accumulates in the developing mammalian brain but largely disappears soon after birth, when the brain is pruning unnecessary connections. Perhaps tau usually protects neurons but malfunctions in the brains of people with Alzheimer’s, analogous to an overreactive immune system in people with autoimmune disorders.
- Studies in hamsters have pointed out a cryopreservation enzyme known as CBS- stands for ‘Cystathionine Beta Synthase.’ It stimulates hydrogen sulfide (rotten egg smell) production. Without CBS, hamsters could no longer enter torpor. The ones which were made to enter using induced hypothermia suffered the kind of kidney damage akin to the one which we, non-hibernators, would suffer. This was actually discovered by pure accident when a student accidentally left hamster cells at 5ׄׄ° Celsius in a fridge for two days. Leaving the egregious odor aside, the good news was that the hamster cells survived, which paved the way for such vital findings.
Can humans hibernate?
In 2004, a species of Madagascar lemurs- fat-tailed dwarf lemurs, were found to practice regular bouts of torpor. We, humans, share about 98% of our genes with them. Surely, it’s very unlikely to find the specific genes for hibernation in the left-out 2% of genes, right?
The normal human temperature is approximately 37°C. At 33°C, our heart starts to flutter and beat arrhythmically. At 25°C, it may completely stop beating. In the blood of hibernators, blood platelets disappear. Blood platelets are responsible for recruiting clotting factors and dislodging them when not required. Based on their prompt arrival after rewarming, it has been hypothesized that they must be stored somewhere rather than being discarded and resynthesized. Also, white blood cells or leucocytes are stored in lymph nodes in hibernators, which prevents an immune response of inflammation, which would damage the kidneys primarily and build up after rewarming. Within 90 minutes after awakening, they reappear. However, this is a risky strategy that leaves hibernators susceptible to opportunistic diseases during torpor. For instance, the fungus responsible for the white-nose syndrome in bats, which is currently wiping out entire colonies of bats in the US, takes advantage of this vulnerability in bats during their dormant stage.
Hibernation is an amazing biological feat and an opportunity to learn new ways of pushing the human body beyond its ostensible limits, as well as healing it when it breaks down. The far-reaching applications of human hibernation or suspended animation include everything from buying enough time to treat heart conditions and potentially saving countless lives to the enchanting realm of space travel. Hibernation in humans still remains a fantasy which, for now, seems a bit far-fetched but nevertheless can be an achievable feat one day. Further research in hibernating animals, enzymes and genes involved in hibernation, its impact on circadian rhythm and memory, hibernation proteins, and many more facets would definitely bring us closer to that wondrous dream of suspended animation and traveling to distant worlds and other unimaginable possibilities.
