Student participates in study on human smelling ability. (Jessica Porter/Associated Press)

'Tis the season for the nose

It's an old conversation starter but let's have a go at it: If you had to give up one sense, what would it be? Solid arguments can be made for each sense, but given the choice in this parlour game most people prefer to keep sight, touch and hearing, with taste and smell at the bottom of the list.

We are well into the festive season, with its usual bombardment of our olfactory receptors — nutmeg, apples, cinnamon, oranges, Christmas evergreens, baked turkey and stuffing, scented candles, gingerbread, wines, rum and eggnog. If there is a season for the nose, it is now.

The perception is that humans aren't very good smellers. There is the argument that since humans began to walk upright their noses moved farther from the ground and they began to lose their ability to follow a scent.

Now a study at the University of California Berkeley reports that if humans keep their noses to the ground they can indeed follow a scent much like a dog (though not as effectively as a bloodhound, the sad-faced dog that can detect a scent four days old with a nose said to be a million times more sensitive than a human's).

The neuroscience department at Berkeley used students to crawl on all fours — wearing a cushioned bodysuit and padded gloves with ears plugged and eyes blindfolded — to sniff out a 10-metre trail of lightly scented chocolate.

Why two nostrils?

The object was to determine why mammals have two nostrils, and whether it might have something to do with mammals having two eyes and two ears. The study showed that if both nostrils are used, 66 per cent of the students managed to follow the scented trail, but with one nostril taped shut only 36 per cent succeeded.

Be that as it may, the sense of smell is as vital as any of the senses, as was shown last year when researchers at Amersham Hospital in Buckinghamshire, England, conducted an experiment to see whether dogs could sniff out cancers. Six dogs were used to find cancer in the urine samples of seven patients, only one of whom had bladder cancer.

The dogs were trained to lie down beside the dish containing the urine of the cancer patient. Some were better at it than others, but the overall success rate was 41 per cent. By chance alone, the rate would be about14 per cent (one-seventh of the seven samples).

"The 41 per cent, as far as I'm concerned, was a remarkable result," said Dr. Carolyn Willis, a research dermatologist involved in the study. "And it was highly statistically significant."

The significance, she said, is that it showed the dogs weren't guessing. Perhaps equally significant, some of the dogs kept lying down beside a dish of supposedly non-cancerous urine, which caused some of the researchers to suspect the experiment was a failure, only to discover that the urine came from a patient with kidney cancer who also had bladder cancer.

The dog-sniffing experiment wasn't the first to suggest cancers can be detected by smell, Willis said, citing Hippocrates who said in the sixth century that certain diseases have certain smells – diabetes has a fruity smell, liver disease a musty smell. Tests continue in California with dogs on the scent of lung cancer, and in England with dogs trained to sniff out prostate cancer.

Honeybee receives sugar water reward for sniffing out explosives. (Los Alamos National Laboratory/Associated Press)

Dogs aren't the only keen sniffers. A male silkworm can smell a potential mate 11 kilometres away, but can't smell anything else. An eel can detect a thimbleful of artificial scent in a large lake.

Now there are bomb-sniffing bees. And humans really aren't that bad when it comes to the sense of smell.

Evolution and the nose

Gordon M. Shepherd of the department of neurobiology at the Yale University school of medicine challenges the general belief that humans have a poor sense of smell, even though some of this belief is based on scientific evidence. He has written a paper published by the International Society for Computational Biology titled The Human Sense of Smell: Are We Better than We Think?

He cites evolutionary changes that brought about the gradual ascendance of vision and reduction of smell, evidenced by the progressive diminution of the snout as the eyes moved to the middle of the face. Concurrently, as humans began to walk upright their noses moved away from the ground.

Shepherd compares mice to humans, observing that mice have some 1,300 olfactory receptor genes, whereas humans have only 350. The big "however" here is that humans have a much more highly developed brain, with a capacity for language, all of which enhances the human olfactory system, adding a keen sense of discrimination in the way humans smell. He says that in tests of odor detection, humans outperform sensitive measuring instruments such as the gas chromatograph.

"These results indicate that humans are not poor smellers (a condition technically called microsmats), but rather are relatively good, perhaps even excellent smellers (macrosmats)," Shepherd says.

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