[Seany (OP)]

I've heard that they can trace smells up to tens of kilometres.. How is this possible?

[another_someone]

Why not - there are people who can smell a factory chimney many tens of kilometers away.  Anything that carries on the wind, can carry a long way - and if the smell is powerful, or your sense of smell particularly acute to that smell (and depending on what other smells might confuse your sense of smell), there is no reason not to smell it.

All that you are smelling is some chemical that is allowed to drift on the air - whether that be the odour of some sulphurous compound from a factory chimney, the exhaust from a motor car, the traces of slat spray from the sea, or whiffs of the scent of a human being.

[Seany (OP)]

But.. A person was camping and they put a banana under their pillow and fell asleep.

They woke up and a bear was hovering over him, trying to get the banana under the pillow. Luckily the man survived..

However, the bear came from 20-30km away.

Us humans can't even smell the banana 10 metres away. What makes the bear so unique?

[another_someone]

If you were staving, and in clear air, I suspect you would smell a banana from a good few metres away (although I doubt we would do it kilometres away).

There are two issues here.

Firstly, I would expect that a bear has more olfactory nerve endings, and more of his brain dedicated to processing olfactory signals, than we do.

But secondly, do we actually know that the bear could smell the banana 20-30Km away.  Even if we were to know that the bear came from that distance away, can we really know what it was that attracted him to come that distance - maybe it was another thing altogether that initially attracted him, but once he got close, he switched his attention to the banana?

[Seany (OP)]

Hmm.. Good point. But I'm pretty sure than a bear can smell a piece of meat cooking from at least 10km away. And I'd be pretty sure that it would have been its initial attention.

[JimBob]

The bears ability to smallis perhaps the best developement of the sense of smell in the animal world.

From  http://www.bowhunting.net/bearhunting.net/bear2.html#smell : Smell

Whether low to the ground or held high in the wind, the nose of a bear is its key to its surroundings. "Smell," writes Herrero, "is the fundamental and most important sense a bear has. A bear's nose is its window into the world just as our eyes are."

 The keen sense of smell--the olfactory awareness--of bears is excellent. No animal has more acuteness of smell; it allows the location of mates, the avoidance of humans and other bears, the identification of cubs and the location of food sources. ". . . the nose provides the leading sense in the search for nourishment," notes Paul Schullery in The Bears of Yellowstone. The nose of the bear is somewhat "pig-like," with a pad extending a short distance in front of the snout.

A bear has been known to detect a human scent more than fourteen hours after the person passed along a trail. "The olfactory sense of the bears ranks among the keenest in the animal world," according to George Laycock in The Wild Bears. "A black bear in northern California was once seen to travel upwind three miles in a straight line to reach the carcass of a dead deer."

The sense of smell of polar bears may be the finest--able to detect a seal several miles away--and, as Domico relates, ". . . male polar bears march in a straight line, over the tops of pressure ridges of uplifted ice . . . up to 40 miles to reach a prey animal they had detected."

An old, and much related, Indian saying may best describe the olfactory awareness of bears. "A pine needle fell in the forest. The eagle saw it. The deer heard it. The bear smelled it."
 
Bear olfactory sense has been an evolutionary trait that has developed for survival. It includes a larger brain mass to process stimulants to its sense of smell (olfactory lobe)and the development of a much larger area of he nose that processes small, an area about 500 times as effective at catching stray odors as a Blood Hound. Blood Hounds (or St. Hubert's Hound)can detect scents as small as that given off by one or two skin cells given of by humans.

Just think, we go around shedding skin at a rate of thousands of cells per minute. Is it anywonder your mother needs to dust.