Winter is coming, and a plethora of animals — and some humans — are stocking up on the comfort food to make it through the harshest months.
But while bears gorge on berries and fish to try amass enough fat stores to keep them alive during periods of hibernation, other animals employ a more unusual survival strategy to make it through the winter. In a new paper, scientists shed light on one of these creatures' curious skill — and suggest it may hold useful lessons for humans as we attempt to survive increasingly lengthy trips into space.
What's new — Arctic ground squirrels uniquely recycle their own body's nutrients to stay alive during their long hibernation, according to the new study. The paper was published Monday in the journal Nature Metabolism.
The findings demystify the metabolic processes which keep the body alive during hibernation. Arctic squirrels hibernate for up to eight months of the year — far longer than most other hibernating species. Because of the sheer length of their winter sleep and the fact the squirrels' body temperatures drop down significantly more than bears', how their bodies manage to make it through unscathed hold lessons to understanding hibernation in other species, too.
"Arctic ground squirrels get colder during hibernation than bears, so these metabolic processes are less diluted by other background metabolic processes and may be easier to see in hibernating Arctic ground squirrels compared to bears," Kelly L. Drew, co-author on the study and director of the Center for Transformative Research in Metabolism at University of Alaska Fairbank, tells Inverse.
What they did — Both squirrels and bears likely undergo a similar bodily process in hibernation known as 'nitrogen recycling,' according to Drew.
While peacefully hibernating, the Arctic ground squirrel's body is hard at work.
To observe what happens to squirrels' bodies during hibernation, researchers performed 'isotope tracing' on nine wild Arctic ground squirrels. This technique involves using radioactive markers to track chemical reactions taking place in the squirrels.
Using this method, the researchers wanted to understand how the squirrels were metabolizing amino acids — the building blocks of proteins — during hibernation. The squirrels' metabolism is key to how they regulate nitrogen use in the body, even while on autopilot.
What they discovered — When they are not hibernating, the squirrels' metabolic processes break down more collagen — essential connective tissue protein — than skeletal muscle to fuel their bodies.
Surprisingly, this study found the opposite may be happening in hibernation — more skeletal muscle breaks down during the squirrel's overwintering than collagen. They also observed suppression of metabolic activity which might otherwise deplete precious energy reserves.
Arctic ground squirrels don't eat or drink water during their hibernation, and they only take one breath per minute.
To stay alive, the squirrels forgo food and instead recycle urea, the research confirms. Urea is a waste product normally excreted in urine. They also recycle nitrogen — specifically free nitrogens, otherwise known as molecular nitrogen.
It seems the squirrels' bodies recycle free nitrogens into essential amino acids they can then convert into proteins. As a consequence, they suffer from significantly less muscle deterioration than might otherwise be expected.
Why it matters — On the face of it, hibernating squirrels have little to do with humans — but dig a little deeper, and this study shows how much we stand to learn from animals.
The possibility of human hibernation opens doors to entirely new ways of living. What if you could lie in bed for months on end without your muscles deteriorating? Studies like this could one day enable us to do this without endangering our health. This kind of ability may sound trivial, but consider it in the terms of missions to space — humans will need to endure exceedingly long trips on spacecraft to reach destinations like Mars. Putting humans into a hibernation-like state may be one way we can actually do it.
"A long-term goal is to mimic the metabolic adaptations in hibernation in humans," Drew says. "Towards this end, we need to know what metabolic processes contribute to the unique metabolic phenotype of hibernating animals which this paper reveals."
What's next — "Next steps are to understand how these [metabolic] processes are regulated, the role of the gut microbiota in the process," Drew says. "Then, we can identify molecular targets or gut microbiome manipulations to promote these metabolic processes in humans."
For right now, the researchers hope others will take the study's findings and try to apply them to humans. Ultimately, the goal would be to try and activate nitrogen recycling in humans who may remain sedentary for long periods, including patients who are confined to their beds, people who live in care homes, and astronauts in space.
"The information may ultimately inform feeding regimens or other therapies to facilitate nitrogen recycling in critical care, long-term convalescence, and even space travel," Drew says.
Abstract: Hibernation is a state of extraordinary metabolic plasticity. The pathways of amino acid metabolism as they relate to nitrogen homeostasis in hibernating mammals in vivo are unknown. Here we show, using pulse isotopic tracing, evidence of increased myofibrillar (skeletal muscle) protein breakdown and suppressed whole-body production of metabolites in vivo throughout deep torpor. As whole-body production of metabolites is suppressed, amino acids with nitrogenous side chains accumulate during torpor, while urea cycle intermediates do not. Using 15N stable isotope methodology in arctic ground squirrels (Urocitellus parryii), we provide evidence that free nitrogen is buffered and recycled into essential amino acids, non-essential amino acids and the gamma-glutamyl system during the inter-bout arousal period of hibernation. In the absence of nutrient intake or physical activity, our data illustrate the orchestration of metabolic pathways that sustain the provision of essential and non-essential amino acids and prevent ammonia toxicity during hibernation.