Hibernating Bears' Urine

Stupid Question ™
Feb. 4, 1999
By John Ruch
© 1999

Q: Do hibernating bears ever have to wake up to urinate, or do they just “hold it” until spring?
—Patrick Murray


A: In this case, a bear doesn’t do it in the woods.

Amazingly, they neither wake up nor “hold it.”

No, they’re not bedwetters. Hibernating bears produce no urine at all—a unique power among mammals.

Urine is 95 percent water. Most of its solid component is urea—a nitrogen-based waste product of protein metabolism.

Since the point of hibernation is to survive winter’s lack of food and water, urination is restricted as much as possible.

In animals such as woodchucks and squirrels, the metabolism shuts down, body temperature drops to near freezing and they live off extra body fat. Because they drink no water, and their super-low metabolism produces very little urea, there’s little need to urinate.

However, these animals to awaken a few times during hibernation to urinate out the urea that has built up (and sometimes to eat a sort of midnight snack). Otherwise, the wastes would poison them.

Due to their great body size, bears aren’t able to shut down as completely as smaller animals. They do shut down their digestive system (developing a mysterious, disgusting “anal plug” in the process), but their temperature drops only slightly and they are in effect only sleeping deeply.

They drink no water and reduce urea production to about 20 percent of the summertime rate. But this isn’t nearly good enough to conserve protein and avoid repeated wake-ups during their six-month hibernation.

So bears have evolved a urea-recycling system. Urea built up in the blood enters the bladder and is immediately reabsorbed through the bladder wall back into the blood, where the nitrogen is used to build protein again. This keeps the bears’ muscles from deteriorating. (They also recycle calcium to maintain their bones.)

Since they’re not expending much energy, bears may actually come out of hibernation with more protein than they had going in.

While there’s some evidence that squirrels have mild urea-recycling capability, only the bear uses it as a self-contained system during hibernation.

When they’re not hibernating, bears generally urinate normally. But it seems the potential for recycling is always there.

Incidentally, polar bears don’t hibernate at all—except for pregnant females, which hole up and sleep through pregnancy. Despite urinating normally for the four years prior to sexual maturity, the hibernating pregnant female will automatically start recycling her urea.