We've all seen dogs sniffing each other's butts. Big dogs, little dogs, fluffy dogs, sleek dogs. They. All. Do. It. Little dogs seem especially delighted to sniff the nose-height butt of a larger dog. It's so convenient! Meanwhile, the humans who come along with the dogs for walkies are left to stand awkwardly by, laughing in embarrassment and saying things like, "Yeah, he does that."
But there's no need to be embarrassed, humans. This butt-sniffing routine is no different from you using your eyeballs to note someone's hair and clothing. Dogs can even determine the mood of another dog through butt sniffing, the same way we might notice another human's mood by seeing a smile or a frown. Let's find out how they do it.
Dogs are far, far better at smelling things than people. Where humans have about 5 million olfactory receptors in their noses, dogs have 150 million to 300 million, depending on the breed. Bloodhounds, for example, are some of the best canine sniffers. About a third of a dog's brain is devoted to processing these scent signals, while a human only uses 1/20 of their brain for smells.
Dogs also have a special area in their snout called a Jacobson's organ, or more officially, the vomeronasal organ. It has openings in the roof of a dog's mouth and a direct connection to the dog's brain. But it doesn't pick up smells the way we think of them, like lavender or gasoline wafting on the breeze. This organ does something more like chemical analysis on the molecules that enter it. This is how dogs can read gender, mood and health of another dog.
Near the base of their tails, dogs have anal sacs. They are horribly smelly to humans, so you probably assume that's what dogs are going for with the butt sniff. Especially beagles, who seem to have never met a foul smell they didn't want to roll in.
Butt no! The smells we smell are not what dogs are after. Those glands are apocrine glands, and humans have them too. Dogs don't even really take note of the poopy smell; that's just another smell. They're looking for the information contained in the pheromones manufactured by those glands.
These chemicals are processed by the Jacobson's organ — which, remember, doesn't process smells as we know them. This chemical cocktail is what dogs "read" to learn another dog's gender, mating status, mood, health, what they've eaten lately and probably much more. There are apocrine glands all over a dog's body, but those two butt sacs are the easiest to access.