Our favorite perfume, a holiday meal cooking in the oven, a fragrant bouquet of flowers, even a stinky pile of dog poop—smells can trigger all sorts of good (or bad) memories and feelings in humans, but a dog’s sense of smell is something else entirely.
Hardwired to Smell
Of the five senses, smell is a dog’s predominant sense. While the human nose is equipped with roughly 5 million olfactory receptors, dogs have many millions more—dachshunds have roughly 125 million olfactory receptors, beagles have 225 million, and bloodhounds have up to 300 million.
“Dogs are living in a totally different world than we are, filled with much, much more information than we can possibly process about smell,” Dr. Coren adds. “That’s the way they parse information.”
Since dogs have so many more olfactory receptors, they are able to distinguish individual smells quite easily—their way of seeing the world. Dr. Coren offers this analogy to explain:
“Humans are visual animals and dogs are olfactory animals. For visual animals like ourselves, suppose we have a quilt and throw a book on the quilt and throw a pencil on top of book. We don’t see a mishmash there—we see a pencil, a book, and a quilt. We break it up into individual components visually. But because our sense of smell is poorer than a dog’s, when we walk into a room and someone is cooking chili, we smell that mishmash smell of everything altogether, but dogs break it up; they smell the meat, the tomatoes, the spices, the peppers—each being separate. There is much more for them to process, so they spend more time sniffing things because they are trying to distinguish the components.”
The ability to smell individual components essentially becomes a dog’s way of seeing the world. Although they have limited color vision and can detect motion fairly well, dogs have various vision challenges.
“They have a number of flaws in comparison to primate vision,” Dr. Coren explains. “Dogs are mostly farsighted, they have less binocular vision, less ability to perceive depth, and [their ability to see] things really close to them is very bad because their muzzle gets in the way.”
Of course, dogs have ways to offset their vision shortfalls—the foremost among them being their heightened sense of smell.
With such a superior sense of smell, why do dogs seem to prefer the stinky? From an evolutionary standpoint, rolling in something smelly is pretty straightforward—it was simply a way to mask scent and become better hunters. Modern lap dogs don’t really see it that way—nowadays, it’s more like sensory overload.
“My theory is dogs roll in stinky stuff for the same reason humans wear loud Hawaiian shirts,” Dr. Coren says. “It’s a matter of their dominant sense of smell, whereas for humans, the dominant sense is vision.”
Dogs smell urine and other dogs’ butts for entirely different reasons, though. Since dogs read the world through their noses, Dr. Coren says they write messages to each other in urine—things like age, gender, mood, even health—and those messages linger behind long after the dog has gone.
“The smell goes away for humans because it becomes less strong when it dries,” Dr. Coren adds. “Smell has to be carried to a [human] nose through moisture but it sticks around for quite a long while for dogs.”
But dogs do like to refresh their scent, which is why they will stop at their favorite tree or fire hydrant if you follow the same path on your daily walk.
Taste and Smell
The sense of smell and taste are closely related in humans, but not as much in dogs. Humans have five times more taste receptors than dogs—on average, dogs will have around 1,500 taste buds while humans have roughly 6,000.
“Taste, per se, is not as important [for dogs],” Dr. Coren says.
If your dog is a picky eater and not enticed by the smell of his kibble or snacks, Dr. Coren suggests adding warm water. “Tastes are more dissolved in water and more available to the dog,” he says. “It releases some of the smell, which keeps the dog eating.”
Even though their sense of taste isn’t as refined as a human’s, surely dogs must know poop doesn’t exactly taste good. Dr. Coren says eating poop is simply a matter of accessing undigested nutrients.
“Dogs have a short digestive tract and the richer the diet—like a really high-fat diet—the more likely there will still be undigested nutrients in there,” Dr. Coren explains. “You certainly don’t want them to kiss your face afterwards, but dogs are a waste not, want not type of animal.”
A dog’s nose is so unique, it’s often compared to a human fingerprint—no two are alike. Dr. Coren says prior to microchipping, taking a nose print was a method of identifying dogs. If you want to try taking your dog’s nose print—perhaps framing it next to his picture—Dr. Coren outlines the method in this blog post.
“It’s a great game you can play with the kids,” Dr. Coren adds. “It’s going to take several tries—cooperating with this sort of thing is not in the dog’s nature.”
We already know dogs can learn all sorts of information by sniffing other dogs’ butts, but does the same principle apply to sniffing human crotches?
Dogs have apocrine (sweat) glands—which release pheromones that convey useful information like health and mood—over their entire body, but they are more heavily concentrated in the crotch and anal regions. Human apocrine glands are found in only certain areas of the body with the highest concentrations in the underarm and crotch. Dr. Coren says dogs sniff these areas in humans for the same reasons they sniff other dogs’ butts, and they can pick up certain useful information.
“Dogs—especially male dogs—are very responsive to the sexual state of other animals,” Dr. Coren explains.
Dogs can pick up on ovulation in humans—which is why sniffing female human crotches may increase at certain times of the month—as well as in other animals. According to Dr. Coren, dogs are even used to help dairy farmers capitalize on a very short short fertility window. Dogs are also trained to sniff out certain types of cancer, when a person will go into diabetic shock, and even certain strains of infectious bacteria.
The Bottom Line
Humans primary sense is vision, but dogs “see” through their nose. With their acute sense of smell, they distinguish individual components of smell to understand the world around them. Although often silly and sometimes stinky to us, these sniffing moves are perfectly normal to our furry friends.
Top image via flickr