Photo above from: https://www.rover.com/blog/can-dogs-smell-cancer/
The post from March 9, 2016 (The Pug Smells!) focused on how dogs with small, short snouts, such as pugs and their brachycephalic brethren, cannot detect scents as well as dogs with large, long snouts, such as Basset hounds and bloodhounds. You may recall that pugs’ snouts simply don’t have enough space to accommodate as many scent receptors as those of bigger dogs. (With the sense of smell, size evidently does matter.)
Nevertheless, in comparison to my own ability to detect scents, even pugs excel. Their noses easily detect odors (particularly of edible items) that I don’t even notice. In fact, scientists have learned that a dog’s sense of smell is 10,000 to 100,000 times as acute as that of humans.
A couple of scientists put this difference into perspective:
Working from the lower estimate of a dog’s sense of smell being “only” 10,000 times better than that of humans, the former director of Florida State University’s Sensory Research Institute, James Walker, explained, “If you make the analogy to vision, what you and I can see at a third of a mile, a dog could see more than 3,000 miles away and still see as well.” (cited in Dogs’ Dazzling Sense of Smell by Peter Tyson, NOVA scienceNOW, posted 10/04/12 at http://www.pbs.org/wgbh/nova/nature/dogs-sense-of-smell.html.)
Another analogy is provided by Alexandra Horowitz, an animal behaviourist who studies dog cognition at Barnard College in New York City: “We might notice if our coffee’s been sweetened with a teaspoon of sugar;”
[but] “a dog can detect a teaspoon of sugar diluted in a million gallons of water: two Olympic-sized pools full.” (Inside of a Dog: What Dogs See, Smell, and Know. 2009. Scribner. p. 72.)
The difference in smelling ability between humans and dogs is partly due to the number of scent receptors: about 5 or 6 million for a human vs. 100 million or more for a dog. But a dog has other anatomic and physiological advantages as well. These advantages are explained by Tyson (cited above) and by Horowitz (cited above, pp. 67-74.) I’ll try to summarize these advantages here:
First, unlike a human’s nose, a dog’s nose stays wet.
Airborne odor molecules stick to the wet nose, and they’re easily licked into the mouth. From there, they’re carried on through the rest of the dog’s olfactory apparatus.
But before the scent molecules move much past the nostrils, the dog can sniff and snort and move the scent-laden air around. He can also widen or narrow his nostrils to help direct the air flow. A dog’s nostrils, unlike those of humans, are open at the sides; they look somewhat like vents.
(Brown dog nose from https://en.wikipedia.org/wiki/Dog_anatomy and black dog nose from Black Dog’s Black Nose on the End of His Black Snout, by Quin, Liam R.E.: Mars and Reuben (2007) at http://www.holoweb.net/~liam/pictures/2007-04-cat-and-dog/pages/img_1758/1024×683.html)
The anatomy of a dog’s nostrils actually helps make it possible for him to know which nostril an odor enters from! So, from the very beginning, a dog has a great clue to which direction he should go to find the source of an incoming odor.
Moreover, as the air enters a dog’s nose, it doesn’t just go straight through the throat to the lungs, as ours does. Instead, it splits along two separate pathways: Part of it goes to the lungs for breathing, and part goes to the dog’s olfactory area, which is loaded with scent receptors. (You can find Brent Craven’s excellent copyrighted diagram of the two air flow patterns at http://www.pbs.org/wgbh/nova/nature/dogs-sense-of-smell.html.)
When the odor-laden air gets to the back of a dog’s nasal passage, it encounters the vomeronasal (or Jacobson’s) organ. This organ is especially sensitive to pheromones released by other animals, often in their urine. Pheromones are hormone-like chemical substances that convey information about the sex and sexual readiness of the dog who released them.
Diagram from http://www.boonescreekbeagles.com/senses.html
(Obviously this diagram is not of a pug!)
A dog has still more advantages in the sense of smell. With vision and hearing, the input must be processed through intermediary organs (in the eyes and ears) before it goes to the brain. With the sense of smell, however, a dog’s nose receptors have a direct route to the specific area of the brain where olfactory data are interpreted. As Alexandra Horowitz says, “[The dog’s] nose is also the fastest route by which information can get to the brain.” (Inside of a Dog: What Dogs See, Smell, and Know. 2009. Scribner. p. 72.).
And – if that weren’t enough – the olfactory section of the brain is “approximately 40 times larger in dogs than in humans, relative to total brain size.” (from The Dog’s Amazing Nose! at http://www.balancebehaviour.org/blah-1/ ).
So it’s no wonder that our pugs astound me with their ability to out-smell me every day (not that I want to be able to smell everything that they can).
But what do they really smell for when they search for the perfect place to pee?