Tuesday, February 23, 2010

Why a three-legged bear walks upright

Recently a video of a bear walking upright has been popular on You Tube, with over 200,000 views.  The bear was missing one front leg--she apparently found it easier to walk on her two hind legs, than to hobble about on three. 

Earlier, I reported on Faith, the dog born without front legs.  Faith was taught to walk on her hind legs, but only with lots of training.  So how was a wild black bear able to teach herself to walk on hind legs, while a dog required so much training?  Bears and dogs are related, according to biologists, so you might expect similar behavior.

Part of the explanation for Faith's slow progress in learning to walk--is that, lacking front legs from birth, she couldn't learn to walk as a puppy, even on four legs. But the bear with three legs probably learned to walk normally on four legs as a cub.

Generalists VS specialists

The habits of bears and dogs give more insight.  Bears are generalists, in diet and behavior, while dogs are specialists.  Bears eat almost anything, from berries to ants to fish.  Dogs, the descendants of wolves, are much more restricted to eating meat.  Being specialists, their anatomy isn't so easily adapted to a variety of tasks.

Bears are much more flexible, in both behavior and gait.  They can use their big clumsy-looking paws for delicate tasks.  A black bear named "Yellow-Yellow" in the Adirondack Mountains recently learned to open the bearproof containers that hikers use for storing food. 

Dogs, in contrast, don't do much with their paws except run, dig, or hold a bone.

In the wild, bears occasionally rear up on their hind legs.  They may do this to appear more frightening, to see over obstacles, or to reach food.  Grizzly bears rear up to reach for cones of the pinyon pine.  Since occasional standing on two legs is part of their normal behavior, it's understandable that a three-legged bear could easily discover the benefits of walking for longer distances.

Efficiency matters

Wild chimpanzees also walk occasionally on their hind legs, especially when carrying food in their hands.  But in comparison to humans, walking on hind legs is very inefficient for chimps.   It takes far more energy for chimps to walk than it does for humans.  They have short legs and a bent-over posture.

I doubt if the efficiency of walking on two legs has been measured for bears--but it must be rather inefficient, because their hind legs are so short.  Nevertheless, if you're a bear with only three legs--two legs may beat three, especially over even ground.

But don't expect bears with four legs to take up strolling anytime soon.  They wouldn't get many miles to the calorie.

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Photo of wild grizzly opening a refrigerator.

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