If you spent the entire winter snoozing in bed, it’s unlikely you’d be able to pop out your front door on the first balmy day and run a 5K. That’s because human muscles have some very specific rules, and one of them is “use it or lose it.” That’s not the case for animals, however, and squirrels are the perfect example. They wake up in the Spring ready to hunt down grasshoppers with the athleticism of a lioness — no training period necessary. A 2022 study found that ground squirrels are able to build muscle, even when they’re not getting any exercise or taking in nutrients. That’s thanks to a special friend called gut bacteria. The fat stores of hibernating squirrels decline sharply in February, and by the time they shuffle out of their burrows in April, the have lost about 40% of their body weight from the autumn before. The squirrels slept soundly, and their body temperatures plummeted to match that of their chilly burrows, their metabolism dropping to around 1% of what it was in the summer months. However, toward the end of winter, their leg muscles begin to build new tissue, thanks to urea nitrogen salvage. That allows the body to retain nitrogen that would normally be expelled in the form of urine. The nitrogen is essential to making protein, which is essential to building muscle. What’s interesting is, if humans spend just 10 consecutive days in bed, it can lead to a 14% loss in muscle power. Squirrels, on the other hand, come out of their burrows after hibernation as lean, mean, fighting machines, jacked up and ready to chase down their first meal.
