Hibernation secrets: why animals don’t get weak over the winter

Photo courtesy Matthew Regan

Every year as the temperature drops, hibernating animals, fattened from a summer spent gorging on food, prepare to hunker down for the winter. When they wake up in the spring, they’re healthy and strong despite months of inactivity. So how do they do it?

This was the question that Matthew Regan, an assistant professor of biology at the University of Montreal, and his colleagues set out to answer in a new study recently published in Science. Hibernation has long intrigued scientists, he explains. There was speculation that something in the gut helped some hibernators preserve muscle. Regan and his colleagues tested this hypothesis on 13-lined ground squirrels in Wisconsin and found that bacteria play a key role in helping them stay strong while they hibernate. 

These ground squirrels are an “extreme example of hibernation,” Regan says. They are inactive for six months, during which time their metabolism is reduced by 99 per cent. When they wake up in mid-April, the squirrels “start eating furiously” and double their weight by the fall, he says. 

Like humans, ground squirrels have many different kinds of bacteria in their gut, and one group of microbes is doing them an important service. While the squirrels hibernate, urea, the main component in urine, is transported to their intestines where microbes break it down, releasing nitrogen, Regan explains. Nitrogen is essential for making protein, and this process allows the squirrels to preserve and build muscle through the winter. 

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There is already interest in how this process could help humans prevent muscle-wasting—whether they’re patients on bed rest or astronauts in zero-gravity environments. Regan has received funding from the Canadian Space Agency to study the possible applications for space travel. However, the human gut microbiome is complex and delicate, and disruptions could have negative impacts, he says. So, careful research must be done before applying this mechanism in space.

For now, this process is best left to the squirrels. While it may be tempting to leave food out to help the little hibernators fatten up, Regan says it’s safer to “let them do what they’re evolved to do.”

Quiz: how much do you know about hibernation?

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