Ask a Bear: What if You Have a Cub Due During Hibernation?

Q: What happens to pregnant bears who have a cub due while they’re hibernating? —Sleepy Sally

A: Plot twist: We always have our cubs while we’re hibernating. Crazy, right? Okay, let me blow your mind again: Grizzly cubs are born weighing a single pound each—smaller than most human preemies.

There are a lot of benefits to this system. Baby gets a warm, protected environment to spend its first few months. Mama gets to offload the cub, technically a survival liability, into the world a little sooner. It’s all pretty dialed—provided that mama bear weighs enough when hibernation rolls around. After all, most grizzles use up about 1.6 pounds of fat per day during hibernation. Mama grizzlies need about 2.8. Fortunately, we’ve got a system for that, too.

Let’s talk the bears and the bees. Bear conception works differently from human conception. We tend to get busy around May and June (who doesn’t love a summer fling?), but we don’t actually get “pregnant” until fall. In spring the egg gets fertilized and the cell divides a few times, but then it just hangs out there, waiting to see if the prospective mama gets fat enough to grow and nurse a cub once she stops eating for the winter.

Too scrawny come sleepy time? Our bodies will just ditch the embryo before it implants, letting us survive to try again next year. Conversely, the fatter I get, the more cubs I’ll produce. Get me up to 50 percent bodyfat and, what the hell? I’ll have six.