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Getting inside bears' brains
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Getting inside bears' brains

Grizzlies' powerful scent system amazes former neurosurgeon

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WEST GLACIER, Mont. - George Stevenson's brain appeared to be sweating, glistening on a bright red plastic picnic plate there in the heat of the day.

It was big as a big man's fist, and all around it, on other picnic plates, were slivers of other brains, like so many thin-sliced neural hors d'oeuvres. Some looked like spreading river deltas, carved deep with winding channels. One looked just like an elk hoof, stuffed tight with morel mushrooms.

With them on the table was a bone-white grizzly bear skull, top lopped to show inside, where Stevenson's juicy brown brain used to be. And beside that was a fully furred grizzly head, guillotined with eyes closed, ears perked up, sharp teeth curving over soft black lips.

These are the grisly tools Dr. Stevenson needs for his presentation: "Grizzly Bear Brain, Central Nervous System Structures."

"These bears are amazing creatures," Stevenson said. "I believe they have the most impressive olfactory system of any animal on the planet. Their nose is the very best."

Generally speaking, national park management, while admittedly complicated, is not brain surgery. Except today, when Stevenson, a pioneering neurosurgeon, has packed the room with Glacier National Park staffers for a brown-bag lunch seminar. Not a few of those brown bags were tucked quickly under chairs as Stevenson pried open the bear head and reached elbow-deep inside to point out its finer details.

Mostly, he was pointing out the snout, a full nine inches of highly evolved scent detection. It is, he said, like no other nose in the world.

A run-of-the-mill dog's sense of smell is roughly 100 times greater than a human's. A good hound dog's nose is perhaps 300 times better.

But a bear's scent system, Stevenson said, is at least seven times better than the hounds.

"It's remarkable," he said. "It's how they know the world."

When humans think about their hometowns, they think in terms of visual maps - down this street to that avenue, turn left at the bank, right at the stoplight. But bears don't see things that way. To get to their favorite huckleberry patch, they don't follow the trail to the tree with the broken limb, and then turn left at the big mossy rock.

"No, they have an olfactory map."

Take the scent of the trail to the smell of the anthill, then follow the smell of water to the perfume of huckleberries.

It is difficult, Stevenson said, for humans to imagine such a way of knowing, but for bears it's essential.

Each spring, when they emerge from the den, they are literally starving. There's no time to wander around and look for food, to look for tracks in the snow and to follow them, perhaps, to a protein meal.

"They have to smell food over huge distances, and then go straight to it," Stevenson said. "If they can't, they die."

An odd hobby

Stevenson was a neurosurgeon from 1965 until 1993, a pioneer of micro-neuro surgery. These days, he lives near Yellowstone National Park and is affiliated with the University of Wyoming.

Of late, he's been combining his lifelong career with his new neighbors the bears, and hopes to create a first-ever neuron-anatomy atlas of bear physiology, using brain anatomy to show how the big bruins work.

The biggest trouble, he said, is finding bear bodies to study. Part of his presentation includes a clip from a National Geographic documentary, in which he and his colleagues are seen sedating a bear and sliding her gently into the cavernous den of a modern MRI machine.

"You have to be very careful," he said, which seems a bit self-evident when you're talking about a bear in a hospital room.

The job is somewhat less nerve-wracking if the bear is dead. Occasionally, Stevenson said, wildlife officials will give him a call when a bear is killed. Then he rushes to Bozeman, where the state of Montana's wildlife laboratory is located - and the bear's head is drained of blood, pumped full of formaldehyde, stabilized and prepped for transport.

Then Stevenson puts it in his plastic cooler, hits the local grocery store for a bag or two of ice and heads for Missoula, where technicians at Community Medical Center work after-hours to make MRI images of bear brains.

It is, he admits, a decidedly odd hobby.

But it is paying off in terms of understanding how grizzly bears think and operate. Stevenson now knows, for instance, that the percentage of a bear's brain devoted to scent is at least five times greater than the percentage of human brain allocated to olfactory systems.

In other words, humans smell in black and white, while bears enjoy the full kaleidoscope.

"A polar bear will walk 100 miles in a straight line to reach a female ready to breed," he said. "That's what the bear's nose can do. They smell a million times better than we do."

A human brain weighs in at about 1,500 grams, huge compared to a 450-gram bear brain. And yet our olfactory bulb is the size of a pencil eraser. The bear's is the size of your thumb. That's a lot of smell power for such a small brain.

And even before the brain, he said, the bear's body is built to sniff.

The black pad on the bear's snout, like a dog's nose, is wired with hundreds upon hundreds of tiny muscles. Bears can manipulate their nostrils the way dexterous people control their nimble fingers.

The smells then travel up two 9-inch channels, with hundreds of times the surface area of a human's nose, to a spot where 10 million nerve strands and a billion receptor cells fire electrical signals directly into the brain, through countless tiny pathways and onto the brain's cribiform plate.

The large hippocampus "remembers" the scent, adding it to the mental map.

Just imagine the blinding brain punch a blast of pepper spray must deliver to that system.

"It's not just heat and discomfort," Stevenson said. "It actually scrambles the brain."

Inside a bear's head

Stevenson's research, like his lectures, rambles wildly, like a big-bottomed bear across a broad landscape. But stick with him, because he's on the scent, and is headed somewhere particular.

He moves on to bear sight. ("They definitely see in color, but not the way we do. We're totally visual. There's no way bears see as well as you and me.") And bear hearing. ("I think their hearing is quite good. It's nothing compared to that nose, though.") At some point, he said, he'd like to "stain" bear brain cells, so he can track sight and sound directly.

He travels to museums and to the Smithsonian, spinning tales of tracking bear skulls and bear brains from coast to coast. He dives into the cerebellum, the homunculus, the arcane reaches of creased and folded tissue.

He rumbles into the vomer-nasal passages, that curious place between smell and taste, then takes off into the frontal sinus. He stops just long enough to hand you a business card - it says only "Bear Brain Anatomy" - and then launches into the neurophysiology of mammalian auditory pathways.

"This is a whole new way of knowing the species," Stevenson says as he tucks his brain back into its clear plastic jug of formaldehyde. He stacks the picnic plates, and tucks the skulls back in their packing boxes.

"For me, this is absolutely fascinating - like getting inside a bear's head and seeing with his eyes, smelling with his nose. It gives people an idea of how they see us, which is not something people think about very much."

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