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The Secret of a Dog's Sniffer

9 December 2009 (All day)
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B. Craven et al., The Anatomical Record

Top sniffers. Scientists' 3D model of a dog's nasal cavity revealed some secrets of their smelling talents.

After a massive earthquake struck the Sichuan Province of China last year, troops and rescue workers brought in dogs to sift through the rubble. With a scent sensitivity many thousands of times greater than our own, the canines located numerous survivors--even those buried under thick debris for days. Now researchers think that they have figured out a key reason why dogs are such superior sniffers. The study could be useful in building odor-sensitive, artificial-nose machines.

Scientists have known since the 1950s that dogs and other keen-scented mammals such as rats and rabbits have a specialized anatomical structure in their nasal cavity. Called the olfactory recess, it's a large maze of highly convoluted airways that humans and all other primates lack. In dogs, the recess lies right behind the eyes and takes up almost half of the interior of the nose. Scientists knew that something about the structure allows dogs to sniff odors invisible to humans. "But no one had looked at how air and odors actually flow inside" a dog's nose to reach that recess, says Brent Craven, a mechanical engineer at Pennsylvania State University, University Park.

So Craven and his colleagues created a computer model of a canine nose. First, they scanned the nasal airway of a mixed-breed cadaver with high-resolution magnetic resonance imaging. "We then had to make our model simulate how dogs sniff," says Craven. But because that skill had also never been studied (although it has been in humans and rats), the researchers outfitted seven dogs, including a Pomeranian and a Labrador retriever, with a special muzzle. The device measured their rate of sniffing as they smelled things such as spoonfuls of peanut butter and tuna. Despite the wide range of sizes and weights, "all the dogs sniffed at about the same frequency, five times per second," says Craven. That's the same rate at which they pant. But "we knew they were sniffing and not simply breathing," says Craven, because high-speed videos showed "their nostrils flaring, which happens when they're sniffing, not when they're breathing." The team also mapped the flow of air into the dogs' noses as they sniffed, enabling them to calculate the nostrils' aerodynamic reach.

When the computer nose started sniffing, it picked up some surprising results. "First, the simulation showed that when a dog sniffs, each nostril pulls in a separate odor sample," says Craven. Via its olfactory sensors, "the dog can tell which nostril is pulling in the scent," so it knows which direction to go when tracking. Further, the researchers report today in the Journal of the Royal Society Interface, a dog's nose has a unique nasal airflow pattern, which helps transport odor molecules quickly via a single airway to the olfactory recess. Here, the smell is retained in the maze of scent receptors even after the dog exhales; the odor is not expelled as it is with humans. It's "very similar" to a rat's airway, says Craven, who expects that this pattern will be found in any mammal with a keen sense of smell.

"It's a significant study that provides compelling evidence" for why dog noses "make sense of scents in an extremely fine-tuned way," says Marc Bekoff, an ethologist specializing on canids at the University of Colorado, Boulder. That's a necessary step for scientists interested in "artificial nose machines," such as narcotic or explosive detectors, says John Kauer, a neurophysiologist at Tufts University in Boston.