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.
The Secret of a Dog's Sniffer
By Virginia Morell 9 December 2009
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.