Published online 17 December 2006 | Nature | doi:10.1038/news061211-18

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Human reputation for poor sense of smell is down to lack of practice.

A dog tracks a pheasant (top) in much the same way that a volunteer student tracks some chocolate (bottom). Jess Porter, UC Berkeley

If you think only hounds can track a scent trail, think again: people can follow their noses too, a new study says. And they do so in a way very similar to dogs, suggesting we're not so bad at detecting smells — we're just out of practice.

Scientists have found that humans have far fewer genes that encode smell receptors than do other animals such as rats and dogs. This seemed to suggest that we're not as talented at discerning scents as other beasts, perhaps because we lost our sense of smell when we began to walk upright, and lifted our noses far away from the aroma-rich earth.

A team of neuroscientists and engineers, led by Noam Sobel of the University of California, Berkeley, and the Weizmann Institute of Science in Rehovot, Israel, decided to test this conventional wisdom.

The team first laid down a 10-metre-long trail of chocolate essential oil in a grass field (the scent was detectable but not strong or overpowering). Then they enlisted 32 Berkeley undergraduates, blindfolded them, blocked their ears and set them loose in the field to try to track the scent. Each student got three chances to track the scent in ten minutes; two-thirds of the subjects finished the task. And when four students practiced the task over three days, they got better at it.

Next, the team tested how the students were following the trails. They counted how many whiffs of air each student took while tracking the scent trail, and tested the effect of blocking one nostril at a time. The scientists found that humans act much like dogs do while tracking a scent, sniffing repeatedly to trace the smell's source. They didn't do so well with one blocked nostril, suggesting that the stereo effect of two nostrils helps people to locate odours in space.

The study proves that humans aren't so bad at smelling after all, says neuroscientist Gordon Shepherd of Yale University in New Haven, Connecticut.

Smart scents

Shepherd has argued that although we have fewer odour receptors than other animals, we may compensate for this with an improved ability to analyse scent information with our large brains1. We may just seem worse at tracking scents because we don't practice this skill from birth, the way that dogs do, he argues.

"We have a much better sense of smell than rats and dogs because of our greater brainpower," says Shepherd. "This shows that in a few training sets, humans can achieve something that other animals spend their life being trained to do."

But although the human tracking power seems to have been previously underappreciated, this doesn't mean that people can also smell as wide a range of substances, at as low a level, as animals can.

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Dogs are still better at picking up the whiff of a particular person from a discarded item of clothing, or alerting officials to traces of drugs, explosives and other contraband, says neuroscientist Jess Porter of the University of California, Berkeley, and first author on the paper, which is published online by Nature Neuroscience2.

"Our work certainly proves that if you can smell something, you can improve your ability to track it. So if you walk by a bakery on your way to work and you can smell it, you can probably close your eyes and find your way to the bakery by your nose," says Porter. "That's more likely than sniffing out a bomb."

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