If you spent the entire winter snoozing in bed, it’s unlikely you would be able to pop out your door on the first balmy day of spring and run a 5K. That’s because our muscles have some very specific rules, and one of them is “use it or lose it.” Working a muscle lets our body know what’s expected of it, so a human muscle develops pretty much exclusively through regular use and good nutrition. Squirrels, however, wake up from a winter of hibernation ready to hunt down grasshoppers with the athleticism of a lioness — no training period necessary. Scientists have found that the ground squirrel is able to build muscle even when it’s not getting any exercise or taking in nutrients — all thanks to gut bacteria. Typically, ground squirrels build up fat stores during the summer months, and by the time they shuffle out of their burrows in April, they’ve lost about 40% of their body weight. They sleep soundly, their body temperature plummets to match that of their burrow, and their metabolism drops to around 1% of what it was in summer. However, toward the end of winter, their leg muscles begin to build new tissue. This process is called urea nitrogen salvage, and it allows the body to retain nitrogen that would otherwise be sent out into the world in the form of urine. Nitrogen is essential to making protein, which is essential to building muscle. This process requires something else: an enzyme called urease, which animals can’t produce but bacteria can. So, when the squirrels wake up, their leg muscles are fit as a fiddle, they hear the sound of the theme from Rocky in their tiny little heads, and they're out of the burrow with the resolve of a cheetah on the hunt. Now you know way more than you ever wanted to about squirrels.
