Canine olfaction evolved over 40 million years to an exquisite level of sensitivity.
February of 2021 was a snowy month. On the 14th we experienced one of the biggest snow storms in quite a few years. That one dropped about 8 inches of the fluffy white stuff to make the world look like a Currier and Ives print. . We had barely shoveled that one away, when a few days later, another storm struck, leaving five inches more on top of the older snow.
On a snowy walk later that day, my terrier Max, his nose quivering and twitching, excitedly turned from the road to leap up on a pristine snow bank, where about ten feet from the roadway, he buried a good part of his head into the fluffy white stuff. With his tail wagging happily, Max snorted and puffed as he investigated some “dog-intriguing” aroma deeply buried below the surface. I permitted this activity for a while, empathetic with a “canine, co-shut-in” until, impatient to get on, I pulled him away.
I was amazed that Max, on a such a cold day, was able to detect a scent source buried under five inches of fresh snow, and from a good ten feet away. It seemed incomprehensible to me that volatile molecules— enough to alert Max’s nose— might be arising from a cold snow layer buried so deep. But there seems no other explanation.
Though Max’s scenting prowess is “nothing to sniff at” one needn’t search long to learn of many even more amazing examples of a dog’s magnificent scenting ability. Beagles will bay and follow the scent of a cottontail many days old, over snow and even dusty dry ground. St Bernard’s routinely sniff out people buried under tens of feet of avalanche snow in the Alps. “Sniffer dogs” at all our airports are regularly employed to detect the scent of contraband drugs or even explosives hidden in closed and sealed luggage. Then there are examples of dogs being able to detect human diseases or drug use in minor variations in the scent of a subject’s perspiration.
How do they do it?
In both man and dog, olfaction (or the sense of smell) occurs when certain molecules in the air are carried into the back of the nose where they are detected by receptor cells found in the nasal cavity of humans and other mammals.
Researchers have determined that humans may have thousands of receptors far back in our nose. This area is lined with mucous membranes, into which air born molecules settle and are detected by microscopic nerve endings called “cilia” which transmit an impulse directly to the brain. A part of the brain (the olfactory lobe) processes the neural message and recognizes the signal as: “Oh that’s the smell of a Christmas tree”, or a rotten egg, or cut grass...etc.
The olfactory bulb of the brain sits directly behind the nose in mammalian crania and permits odor reception to go directly to the brain—the only sense that does that. Other senses like touch and vision send impulses through the spinal cord first, then to the brain. So smell is immediate and many times more sensitive than other senses such as taste and touch.
Smell has powerful emotional and instinctive components. The olfactory lobe is directly connected to a very primitive part of the brain (Limbic System) which controls instincts, emotions and memory. So smells can almost immediately elicit emotional, sexual, physiological, and instinctive responses. For examples the odor of woodsmoke might brings back the intense memory of a childhood camping trip..on which you fell and were injured. The response to the scent of woodsmoke may elicit a sharp memory of those events, but at the same time elicit anxiety, a rise in heart beat, and higher blood pressure. Or the aroma of a perfume may bring forth the memory of a fond friend. Other smells may cause fright or anxiety that are not consciously controlled.
A dog’s nose works pretty much the same way, but (thankfully) are very different in appearance than ours. Their noses are generally rough surfaced, black, cool to the touch, and have those distinctive slits on the side, and they are almost always moist. These external differences are compounded internally and in functionally as well.
The canine’s sense of smell is well known to be many many times more sensitive than our own. This ability must have evolved over 40 million years ago when the first canids appear in North America—(long before early humans arrived in Africa @ 2 million years ago). Over those many tens of millions of years of evolution they developed a highly sophisticated sense of smell as key to insure their survival.
One reason why a dog’s sense of smell is so much greater than ours is that canine nasal passages contain 300 million olfactory receptors, while we have only about six million (thus dogs have 50-times the reception capacity we have). Furthermore, recent research has indicated that each receptor can detect many different odor molecules. Canid receptors may be able to detect many more than ours, so the number of odors a dog may be able to identify are many, many times greater than the “fifty times greater” number seems to indicate.
But more importantly, the part of the brain in dogs which analyses odor molecules —the olfactory lobe—-is also about 40 times bigger than ours. On those two bases alone their sensitivity is greater by a factor is 2000 alone ( 50 x 40 = 2000). But there is more.
Besides the obvious external differences (our noses are smaller and directed down over our chests, while a dog’s nose is clearly more prominent with its nasal opening directed right out front) the inner workings of the dog nose differs too.
We breathe in and out through the same passages our nostrils. In dogs, inhaled air is directed to the sensitive olfactory lobe by a flap of tissue in its nasal passages, while the exhaled air (with no new scent) is directed out by way of the prominent slits on the sides of a dog’s nose. Thus a dog’s nasal passages have evolved to receive a continual flow of fresh “new “ air over the sensitive olfactory area. Furthermore, dogs can detect which nostril (right or left) is receiving the more concentrated scent. It can then turn its head into that direction to seek confirmation of direction and determine where the scent is more concentrated.
One more recent discovery is that dogs (and other canids, like wolves, coyotes and foxes) can detect infrared radiation, (or heat) with their noses. Snakes are well known to be able to detect this form of radiation. This ability may help explain how a fox or a coyote can detect voles, moles and mice under cover of leaf litter or even under snow..
Furthermore, “smell” for dogs is a much more interesting and pleasurable sensation than it is for humans. Dogs are instinctively attracted to new odors or aromas Veterinary olfactory specialists call this facility “neophilia”, (Greek “ lover of the new”) or being attracted to new odors. . ( See “How powerful is a dog’s nose?”, Phoenix Veterinary Center, 04-23-2020– phoenixvetcenter.com)
The result of all these wonderful adaptations is that dogs sense of smell is enormously more sensitive than ours and even better than advanced instruments designed to detect odors. It is claimed that dogs can detect substances in concentrations as low as one part per trillion. (1: 1,000,000,000,000) or one drop of liquid in “20 Olympic sized pools”. ( See “How powerful is a dog’s nose?”, Phoenix Veterinary Center, 04-23-2020– phoenixvetcenter.com)*
Our dogs live in a world of scents that we can hardly imagine. It provides them today with a complex, rich, multi-sensation of the world around them. Their vision is as acute or better than ours, but besides vision they have a second sense as acute as vision which reveals to them a vast world of information mostly hidden to us. And which provide a huge amount of additional information about their environment.
This ability of scent has been central to their survival, and was a key element in the development of the symbiotic relationship with humans, all of which has also contributed to making them so successful as pets and companions.
My childhood dog Kim, often found the aroma of the cow flop of certain heifers in a neighbor’s pasture so interesting and pleasurable that he would roll himself into the soft brown stuff to create a kind of mushy saddle. This behavior made him “tail wagging” happy to carry this wonderful odor along with him wherever he went that whole day. He was also eager to share these aromas with his human friends. But understandably, when he turned up at the back screen door begging to come in for his supper, he was barred.
Then it was his best friend who was assigned the dirty task of garden hosing the big dog down. His “wet dog shake” had to be carefully avoided on these occasions, or a boy’s carelessness would result in both dog and boy being banished from supper.
[* I calculate the ratio is one drop (20 to a ml) of scent to 25 Olympic pools. ]
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