Dog vs. Human: Who Smells Better?

22-02 four-dog-snouts

Photo above from: https://www.rover.com/blog/can-dogs-smell-cancer/

22-01 human nose                     22-03 another Human Nose

Nose on left from: http://health.howstuffworks.com/mental-health/human-nature/perception/question139.htm

 Nose on right from: http://www.geeky-gadgets.com/brilliant-trio-create-breath-powered-charger-05-10-2011/

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;”22-08 coffee-cup

Photo from http://nowiknow.com/quizzes/coffee-versus-tea-who-drinks-what-and-where/coffee-cup-01/

[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.)

   22-09 swimming pool - Copy     22-09 swimming pool - Copy

Photos from http://www.salbcn.com/barcelona-sports-tours/swimming/calella-olympic-pool

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.

22-11 wet nose   Photo from http://www.akc.org/akc-dog-lovers/german-shepherd-dog-owners/

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.22-12 nose vents
22-13 nose 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.

22-14 vomeronasal organ

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).

17-01 Sarah & Harley pee

But what do they really smell for when they search for the perfect place to pee?

 

The Pug Smells!

Do you know how a pug smells? If you’re expecting a discussion of the odor of a pug, that’s very simple: It depends on what the little critter has gotten into and how long ago he had his last bath. But today’s topic is the sense of smell.

Previously, I’ve focused on some health effects of the pug’s squished snout. That short snout also affects how well the pug can smell. That’s because the ability to detect odors is determined largely by the number of scent receptors in the nasal passages, and the number of receptors is largely determined by the size and length of the nose and snout; it’s a matter of the amount of space available for the receptors.

Obviously, a dog like the Basset hound has a bigger and longer nose than either the Boston terrier or the Pekingese, all shown below.

21-00 basset

Basset photo above is from http://www.thehappywoofer.com/basset_hound.php.

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21-02 Boston terrier

Boston terrier photo above is from  http://www.pets4homes.co.uk/pet-advice/health-issues-more-commonly-seen-in-the-boston-terrier.html.

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21-03 Pekingese

Pekingese photo above is from  http://www.yourpurebredpuppy.com/training/pekingese.html.

Dogs with larger noses and longer snouts have more nasal space and thus, room for more scent receptors. Therefore, such dogs are better able to distinguish smells than are dogs with smaller noses and shorter snouts – such as, of course, pugs.

21-04 Bloodhound

For example, scientists have determined that bloodhounds, being large, with long noses, have about 300 million scent receptors. (Bloodhound photo above is from https://en.wikipedia.org/wiki/Bloodhound)

21-05 Benji

On the other hand, dachshunds, being small, but with long noses, have about 125 million scent receptors. (Dachshund in photo above is our pugs’ friend Benji, who is able to detect odors that they miss.)

Based on the facts above, I’d “guesstimate” that small, flat-nosed dogs like our pug Sarah may have as few as 100 million scent receptors. Here’s Sarah again (below), with a frontal view of her squash-nosed self:

21-06 Sarah cuddle bed crop 2

Lest you grieve overly much for Sarah and her brachycephalic kin, bear in mind that in a sniffing contest between a pug and a human, the pug always wins. Humans have only 5 to 6 million scent receptors, compared to my “back of the envelope” estimate of 100 million for pugs. Sarah may not be able to track an escaped convict through the Everglades, but she always knows when my pocket contains an empty plastic bag that once held dog cookies.

And whenever a morsel of food rolls under the stove or anywhere else that her truncated snout can’t reach, our pug Sarah knows it’s there. She places herself in front of the offending appliance or furniture and “points” her unpointed nose in its direction until she can get a human (usually me) to fish out the crumb for her.

21-07 Sarah smells

For more information about the dog’s sense of smell, read the chapter titled “Sniff,” pp. 67-88 in Inside of a Dog: What Dogs See, Smell, and Know by Alexandra Horowitz. (2009). Scribner.