Hibernating ground squirrels recycle urine to maintain their muscles

Hibernating animals find it hard to get the nitrogen they need to maintain muscles – but ground squirrels have gut microbes that can break down urea to free up the nitrogen it contains

Life 27 January 2022

Hibernating thirteen-lined ground squirrel

Hibernating thirteen-lined ground squirrel

Robert Streiffer

Hibernating ground squirrels stay in shape by recycling urea, the main compound in urine, with the help of their gut microbes.

When animals hibernate, they enter a state of suspended animation, reducing their energy use and food intake. But the lack of food can lead to a dearth of nitrogen, an essential building block for establishing and maintaining muscle.

Now, Matthew Regan at the University of Montreal, Canada, and his colleagues have discovered how thirteen-lined ground squirrels (Ictidomys tridecemlineatus) overcome the problem. Their gut microbiomes harvest nitrogen from urea, which is typically a waste product.


Regan and his team examined several groups of ground squirrels at various stages of their hibernation cycles, but gave half of the animals microbiome-depleting antibiotics. In the untreated squirrels, the researchers tracked nitrogen compounds like ammonia and glutamine all the way from the squirrels’ stomachs to muscles and proteins in their livers. For the depleted squirrels, these compounds weren’t found in significant numbers in their muscles and liver.

“The microbes are important in all this because they have this ability to break urea down into its component parts, freeing up nitrogen to then be used again at a time when there is no new nitrogen coming into the animal at all,” says Regan.

The urea-eating bacteria also benefit from this process, which could explain how the mutually beneficial arrangement came about. “It’s a symbiosis that allows both of them to effectively emerge from hibernation in good shape,” he says.

Humans have also demonstrated the capacity to salvage nitrogen from urea, though not in amounts as significant as those seen in squirrels. Understanding how squirrel microbiomes recycle urea could help us mitigate muscle loss and nutrient depletion in a wide variety of scenarios, including the muscle loss associated with old age and with space flights in microgravity.

“We can look to these animals to learn a lot about the basic biology of nitrogen recycling,” says Kevin Kohl at the University of Pittsburgh in Pennsylvania. “Some of these things might not exist in humans, but it’s going to give us such a better understanding of the process. We might not know what bits we’re going to pull over to the human side, but as we learn more, it’s going to allow us to do that.”

Journal reference: Science, DOI: 10.1126/science.abh2950

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