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Super Sniffer
October 12, 2000

Man’s best friend may not always smell so good, but how do dogs smell so well?

The Defense Department wanted to find out, so they asked an aerodynamics expert to investigate just what it is that makes the canine olfactory system work so efficiently. The results could lead to a mechanical sniffer that does the same job as Fido.

Dogs do it

Dogs are renowned for their sense of smell. Estimates on how much better their olfactory system works than ours range from hundreds to as much as one million times as good. That’s because one third of the canine brain is devoted to olfaction and dogs have very sensitive detectors inside their noses for picking up scents. "They use scent the way we read the newspaper," says Gary Settles, professor of mechanical engineering at Penn State University, who studied the canine olfactory system for the Defense Advanced Research Projects Agency (DARPA).

Settles’s Schlieren photography facility helped them see the air that flowed out of dogs’ noses.
If you prefer to see it with RealPlayer, .

courtesy Penn State Gas Dynamics Lab

Canine noses are so sensitive that no mechanical sniffing device exists that can even approach their precision. So it’s only natural that dogs are used for all types of jobs, from rescuing trapped people to drug surveillance to searching for cadavers.

But some working dogs’ jobs can be extremely dangerous. "Dogs are the premier detectors of landmines," explains Settles. "They do an excellent job, but of course they are expensive to train and their lives are threatened in a minefield. So the idea is to come up with an autonomous device that can do this chore as well as a dog can."

Going with the flow

Although research has been done on the interior of dogs’ noses, until now very little was known about the external aerodynamics involved. Settles used several special photographic techniques to get a glimpse of the airflow that’s not normally visible around dogs’ nostrils. He used both pets and trained detection dogs, between one and five years old. The results will be published in an upcoming issue of the Journal of Experimental Biology.

Settles’s lab houses the world’s largest Schlieren photography facility. "The Schlieren technique is most often used for seeing shockwaves about aircraft in high-speed flight," he says. "But we’ve adapted it for other uses, like the warm air flow around humans, and in this case, dogs." The technique uses a 40-inch diameter parabolic mirror (actually a telescope mirror that Settles says would "make a medium-sized astronomy observatory") to pick up small temperature differences in the air and render them visible.

Animation of dog sniffing
When a dog exhales, the nostrils flex and let the air flow out the sides.
image: Penn State Gas Dynamics Lab

His team also used high-speed video—up to one thousand frames per second—to understand the mechanics of dogs’ nostrils, which flair and flex quite rapidly when they sniff. "The complex nostril of the dog functions so as to make it basically a variable geometry flow diverter," says Settles. "The airflow comes straight into the nostrils, but then on exhale, the nostril flares and the slit on the side of the dog’s nose opens, and the airflow is ejected out and down to the sides." This means that the dog isn’t exhaling onto what he’s trying to sniff, allowing him to sniff a weak scent without disturbing it.

The high speed video also showed that while dogs are panting, a large jet of air is expelled from the mouth that covers up scent-bearing currents. So dogs have to stop panting in order to sniff. Also, dogs tend to use short sniffs when tracking a scent that they can get very close to, but will use longer sniffs if the source is out of reach.

Scanning for scents

Drug Sniffing Dogs

Talk about earning your keep. Snag, a U.S. Customs Labrador, has made 118 drug seizures, worth a record $810 million. Meanwhile, Rocky and Barco were responsible for a record 969 seizures in 1988 while they patrolled "Cocaine Alley," the border between Texas and Mexico. Mexican drug lords considered them such a threat that they put a price of $30,000 on their noses.

from Amazing Animal Facts

In another technique, the scientists spread talcum powder and a scent source on a surface and watched what happened when dogs were encouraged to sniff. "We discovered from that that the sniffing process kicks up airborne particles that can then be inhaled into the nose, and this is one possible way that dogs pick up weak scents associated with particles on the surface of the ground," says Settles.

It was also found that the dogs sometimes displayed a behavior that researchers call "scanning." They would lower their noses to the ground near the scent and then move horizontally toward it, pausing when they were directly above it. Then they would scan past the source, and finally return to it. Presumably this allows them to see what they are sniffing before they get there and to get a sense of how the scent is distributed.

The next step is for scientists to apply what they’ve learned to design a sniffing device, especially one that can detect landmines. Part of that research is being done by John Kauer at Tufts University. "We’ve learned some very important things from dogs," says Settles. "We’re applying those to a mechanical sniffer design that tries to mimic what the dog does in order to try and approach the level of capability that the dog has."

Elsewhere on the web:

DOD Official Working Dog Training School

American Kennel Club

Caltech Electronic Nose Project

Canine Olfactory Detection Laboratory

Detecting Illegal Substances, from Chemical & Engineering News

Dogs Against Drugs/Dogs Against Crime

FAA fact sheet on explosives detection canine team program

Landmine statistics

National Narcotic Detector Dog Association

Search and Rescue Association

The Fate of Dog Soldiers

by Jill Max

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