Wild Ideas: When it comes to dogs, the nose knows

The dog's nose is an amazing olfactory tool, able to detect smells 10,000 to 100,000 times better than ours.Pam Owen
The dog’s nose is an amazing olfactory tool, able to detect smells 10,000 to 100,000 times better than ours.

“The dog’s nose may be his most powerful organ,” veterinarian Randy Kidd wrote in 2004 in an article in Whole Dog Journal.

“It is certainly one of the most dynamic of all animal systems,” Kidd added, “with activities that range from basic smell detection, to sensing fear, to memory, to emotions, to mate- and pack-selection, on to a genetic history carried from one generation to the next.”

The dog’s nose is an amazing olfactory tool, giving dogs a sense of smell 10,000 to 100,000 times as acute as ours, according to an article on the PBS website. Dogs can detect some odors, including the scent of a female in heat, in concentrations as high as one part per trillion, or, as dog-cognition researcher Alexandra Horowitz put it in her book “Inside of a Dog,” “a teaspoon of sugar in a million gallons of water, or two Olympic-sized pools worth.”

It shouldn’t be surprising that dogs have such olfactory acuity, considering they have up to an estimated 300 million olfactory receptors, while we humans have only about six million. About one-third of a dog’s brain is allocated to the chore of olfaction — proportionately 40 times more than the portion in our brains. Among dog breeds, bloodhounds, who lead their species with about 300 million olfactory receptors, win hands down as scent sleuths, according to the book “Understanding Your Dog For Dummies.” Beagles and German shepherds are not far behind, with an estimated 225 million receptors.

In the recent “Nature” series, “Pets: Wild at Heart,” one segment focused on dogs’ olfactory acuity. As the segment shows, the external structure of the dog’s nose also helps in olfaction (scent detection). The nostrils have slits at the side that enable dogs to breathe out the sides while not only avoiding interrupting the flow into the front of the nose but actually enhancing it.

“As the expelled air rotates, it helps draw more scent to the nose,” the narrator explains. “This two-way current helps a dog gather scent almost continuously.” The segment uses Schlieren photography, which captures the flow of fluids or air of varying density around objects, to show the air currents around a dog as it breathes in and out.

Dogs’ nostrils can expand to pick up more scent and contract to prevent sniffing up foreign matter. They can also operate almost independently from each other, each nostril taking in only a narrow field of scent on its side, with a wide separation between the nostrils. This enables dogs to more easily detect the source — and in the case of something moving, the direction — of smells on the basis of the scent trail, which dissipates over time as its molecules disperse.

Tissue just inside a dog’s nostril helps separate air drawn in, sending about 88 percent of the air to the lungs and the rest to a recessed cavity for olfaction. Within the cavity, says the PBS article, the odor-laden air filters through a labyrinth of scroll-like bony structures with receptors that sift out odor molecules on the basis of different chemical properties and communicate with the brain for analysis of them.

When dogs are testing the air for information, they hone in on aldehydes — carbon compounds found in many essential oils, such as that of vanilla, often contributing to the fragrance we find attractive in them. According to the NextGen Dog website, a study conducted at Kolmården Wildlife Park in Sweden early last year found that an aldehyde in blood emits the metallic scent humans typically associate with blood. This aldehyde was, for some wild dog species, just as engaging as the blood odor itself.

By Pam OwenPam Owen

Dogs use a second olfactory organ — the vomeronasal, or “Jacobson’s organ” —to process pheromones, such as those that signal a female dog’s mating readiness and other sex-related details, according to the PBS article. The organ consists of a pair of elongated, fluid-filled sacs that open into either the mouth or the nose, Located above the roof of the mouth and behind the upper incisors, it has its own neural pathway to the region of the hypothalamus associated with sexual and social behaviors, so the information is kept separate from information from other odors.

Kidd writes that the Jacobson’s organ “is apparently able to detect other, normally undetectable, odors” that may enhance a newborn animal’s ability to find its own mother, for example. And he suggests that, since smelling is “hooked into the most primitive areas of an animal’s brain, it might also be linked to sensations created long before the animal was actually born.” Although we humans were long thought to be without this organ, a recent study of 400 human subjects confirmed both its presence and the fact that its nerve connections are capable of sending functional messages to the brain.

Some dogs are able to follow a scent trail that is more than a week old, and, because of this, humans have long used dogs to track game and, more recently, in search and rescue. Kidd notes that dogs have detected drowned people in water more than 80 feet deep. Because of their remarkable olfaction capabilities, dogs are also used by to government law enforcement and security agencies to detect explosives, firearms, drugs, unreported currency, concealed humans and smuggled agriculture products, and to combat terrorism.

One of the newest applications of the canine’s olfactory prowess is in medicine. A growing body of research indicates that dogs can sniff out certain kinds of cancer, including those in the breast, prostate, bladder and lung. According to an article published in April in the British newspaper “The Guardian,” one study showed that dogs were more than 90 percent accurate in detecting prostate cancer among 900 patients, more than a third of whom were known to have the disease (see more on cancer detection in the sidebar). Early work has also begun to use dogs to test the breath of humans “to help diagnose internal diseases before they become evident with other methods,” Kidd writes.

To keep your dog’s nose healthy, Kidd suggests giving it “a daily dose of natural odorants, generated from the fields and woodlands out of doors — the perfect way to build up the reserve of sensory cells and brain connections related to smelling.”

© 2015 Pam Owen

Sniffing out cancer

On its website, Medical Detection Dogs cites sources that suggest the human the normal metabolic processes are altered in disease and this may result in the production of volatile organic compounds, which diffuse into the bloodstream and are then excreted in breath or in urine. The organization says that training a dog to detect the “odor fingerprint” for a particular disease among the thousands of odors normally found in a sample of breath, urine, blood or feces without recourse to the “pure source” is “extremely challenging . . . more complex than training a dog to detect explosives or drugs.” The organization typically trains its dogs for about six months before they are reliable in sniffing out trace amounts of cancer cells in urine samples.

With prostate cancer, dogs have been shown in some studies to be far more accurate than other types of tests, including the PSA (prostate-specific antigen) test, which often produces false positives. A new clinical trial using dogs from the British nonprofit organization Medical Detection Dogs to test their ability to sniff out prostate cancer — the first to be approved by Britain’s National Health Service — is set to begin soon.

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