One GPS-collared grizzly bear in Alaska's Brooks Range stayed inside its den for 233 days. That is nearly eight months. Sixty-four percent of the year spent underground without eating, drinking, urinating, or defecating. A 2025 study published in Wildlife Biology by Deacy and colleagues found that Brooks Range brown bears denned for an average of 206 days, the longest average ever documented using GPS collars. Pregnant females averaged 211. One bear pushed it to 233, close to what the researchers estimated as a theoretical #physiological maximum of 241 days. Ten more days underground and the bear's body would have begun consuming itself. What makes 233 days possible instead of fatal is what happens to the waste. A human confined to bed for eight months would lose roughly half their #muscle mass. The body breaks down protein during inactivity. The nitrogen released from that breakdown is converted to urea in the liver, filtered by the kidneys, and flushed out in urine. The nitrogen is gone. The muscle it came from is gone. Over months, the loss compounds until the patient can barely stand. Astronauts on the International Space Station lose measurable muscle despite exercising two hours a day. Inactivity destroys tissue, and the waste products of that destruction leave the body permanently. A hibernating bear does not let the nitrogen leave. Its kidneys slow to a fraction of normal. It produces almost no urine. The urea that a waking bear would excrete stays in the blood, circulates to the gut, and is broken down by intestinal bacteria into ammonia. The ammonia is absorbed back into the bloodstream, where it combines with glycerol released from the bear's fat reserves. The combination produces new amino acids. The amino acids are assembled into new proteins. The proteins rebuild the muscle that would otherwise waste away during five to eight months of absolute stillness. The #bear is running a closed loop. Muscle breaks down. The waste nitrogen from that breakdown is captured by gut bacteria, converted back into building blocks, and reassembled into the same tissue it came from. The waste goes out as urea. It comes back as muscle. Nothing is lost. Nothing leaves. The system is sealed the same way the den is sealed. Blood tests on hibernating bears confirm this. Creatinine, a marker of muscle #metabolism, rises sharply during hibernation, proving the muscles are still metabolically active. But urea, the waste product that should be rising alongside it, drops by half. The bear's muscles are turning over protein the entire time it sleeps. The waste just never makes it to the exit. Total amino acid levels in the blood stay constant for the full duration of hibernation despite zero food intake. The bear is not eating. It is not losing protein. Something inside is catching the waste and sending it back. That something is the gut. A hibernating bear's intestinal bacteria do not shut down when the bear stops eating. They switch jobs. Instead of digesting food, they digest urea. The ammonia they produce is the raw material the bear's body uses to manufacture the amino acids it needs to maintain muscle, organ tissue, and immune function while lying motionless in a hole for eight months. The fat reserves provide the energy. The gut bacteria provide the nitrogen. Between the two of them, the bear holds together. A 600-pound #grizzly enters its den in September carrying enough fat to burn for fuel and enough bacteria in its gut to turn its own metabolic waste back into the protein that keeps its muscles from collapsing while it sleeps. It does not eat. It does not drink. It does not urinate. For 233 days it lies in the dark running a recycling system that no human hospital has been able to replicate, and in late April or May it stands up and walks out on legs that work. Source: Deacy et al. (2025), Wildlife Biology / Mugahid et al. (2019), Scientific Reports / Stenvinkel et al. (2013), PLOS ONE / Frontiers in Physiology (2021). - @idespondent