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Why are dogs such top smellers?

What makes dogs' noses and their sense of smell so powerful?

The millions of receptors inside a dog's nose are one of the reasons they have such a powerful sense of smell

The millions of receptors inside a dog's nose are one of the reasons they have such a powerful sense of smell (Source: Mike Segar, Reuters)


Thanks to an impressive network of mucus-coated tubes, Rover's nose is top dog when it comes to sniffing out drugs, explosives, noxious chemicals and even disease.

"All dogs have a great sense of smell, but those with a long snout and big muzzle, like bloodhounds, beagles, labradors and German short-haired pointers are the best," says Geoff O'Neil, a senior quarantine dog handler and trainer at the Australian Quarantine and Inspection Service (AQIS).

What makes dogs' noses so powerful is that they are able to draw in large volumes of air, heat and moisten it, and expose the odour molecules to the sensory cells that make up over 50 per cent of their internal nasal area.

"Those sensory cells then transmit signals to the brain, 10 per cent of which deals only with olfaction — processing and considering smells," says O'Neil.

By comparison, less than one per cent of the human brain is devoted to interpreting smells.

The US Defense Advanced Research Projects Agency, which is trying to develop a more sensitive artificial nose, recently collaborated with Penn State University to make a fluid dynamics model of a dog's nose.

They found that the snout's geometry ensures that a smooth flow of air is delivered to a network of tiny tubes containing millions of receptors that bind to the scent molecules in the air.

The number and variety of receptors allows dogs to detect odours at concentrations as low as a few parts per trillion.

"If you were to spread out the receptor cells of a human nose, they would probably just cover a postage stamp; a dog's by comparison would cover a tea towel," says O'Neil.

"In practice, this means that while we smell a complete odour — such as spaghetti bolognese cooking, a dog can detect a vast array of individual ingredients, down to individual bay leaves.

"That's why trying to disguise drug shipments with coffee doesn't work — all that does is draw human attention."

AQIS dogs undergo basic training where they learn to detect 10 specific odours, like apples, eggs and seeds, and ignore 13 common background odours, like bread, perfume and chocolate, to help them distinguish exactly what they're looking for.

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Sickly scents

"Dogs' only limitation is their human handler's ability to work with them effectively," says O'Neil.

Greg Horrocks, a senior inspector at Queensland Primary Industries and Fisheries agrees.

"Dogs are being used to detect cancer and other diseases like diabetes and infections, and are able to do so because those diseases emit chemicals that smell different to normal human smells," he says.

"Even untrained dogs that have a very close rapport with their owner are able to pick up isolated chemical changes that occur in seizure-prone patients, and warn them prior to the seizure taking hold."

The human sense of smell just isn't powerful enough to detect most of the subtle chemical changes that make a sick person smell differently from a healthy one.

Horrocks trains the world's only organochlorine-detecting dogs. Their job involves detecting residues from organochlorine pesticides, which were widely used in the 1950s and finally outlawed in the mid-1980s because of their long half-life in soils and high toxicity.

"These chemicals can remain in the soil for generations, passing into animals that eat vegetation growing in an affected area," he says.

"Australia's agricultural export industry largely depends on supplying chemical-free beef, so we work with farmers to identify contaminated areas, where concentrations can be as low as a few parts per billion."

Horrocks says an electronic nose could never replace a dog, because dogs "can map out a large area and identify contaminated soil with almost 100 per cent accuracy" whereas artificial sensors need to be within close proximity to the contamination.

So dogs' role on our biosecurity frontline seems assured, at least for now.

They are currently being used to sniff out imported red fire ants in Queensland, a slightly ironic situation according to O'Neil, because insects have the best sense of smell of all.

"If we could harness that, nothing would go undetected for long," he says.

Geoff O'Neil and Greg Horrocks were interviewed by Rachel Sullivan.

Tags: animal-behaviour, chemistry, mammals, dog

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Published 02 July 2009