For dog owners, the scene is common. While walking their dog, they see another person approaching with a dog of their own. As they get close, the people may wave and say hello, while the dogs sniff butts in a greeting that would make Emily Post roll over in her grave. To the humans, this exchange can be embarrassing or gross, but it’s just another example of animal behaviors developed over generations of evolution and domestication.
Dog owners wonder why dogs sniff each other’s butts just like they wonder why their dog digs at the floor before laying down or sprints around after a bath. Dogs sniff butts for the same reason people make eye contact when greeting someone: It fills in the gaps to tell them more about the dog they’re meeting. This dog behavior is a form of chemical communication that provides context clues about whether they might be a potential friend - or in need of help.
Read on to find out the story behind your dog’s obsession with behinds - and why they might get a little too close and personal with each other's privates.
Dogs have a second olfactory system inside their snouts called the Jacobson’s organ. This organ is used exclusively for detecting specific scents and pheromones involved in chemical communication. Those loaded aromas are undetectable to human noses, especially if they're masked by the smell of feces or urine. Nerves go straight from the Jacobson’s organ to a dog’s brain, making it easier to pinpoint information from pheromones and ignore interference from other ambient smells.
When you see a dog flare its nostrils, you're watching it engage the Jacobson’s organ to go into a super-smelling mode and take a deep whiff of something nearby.
Dogs' sense of smell is so strong that it's their most reliable sense for interacting with the world around them. Humans have about 5 million receptors in their noses for smelling. That may sound like a lot, but in this category, dogs dwarf humans, with about 150 million olfactory receptors between their regular nasal organ and Jacobson’s organ.
Dogs also devote about one-third of their brain mass to the detection and interpretation of different aromas. That mental processing power helps dogs sift through each subtle scent and translate what it means.
It’s not completely accurate to say dogs smell each other’s butts. While the nose may be pointed in the direction of the butt, dogs are specifically smelling a pair of glands found in the anal sac: the apocrine gland and sebaceous gland. These glands produce a series of chemical compounds that form a dog’s aroma.
Apocrine glands are found all over a dog’s body, but the largest and densest cluster of apocrine glands is in the anal sac. The apocrine gland is responsible for the “dog smell” that people can smell, while both glands generate the undetectable aromas that only dogs are searching for.
The scents made by the apocrine and sebaceous glands contain a lot of information about a dog’s general well-being. A dog can tell another dog’s gender, breeding status, diet, overall health, and general emotional state with one big inhale.
The glands are an evolutionary form of identification, like an aromatic fingerprint. Since some aspects of a dog’s aroma never change, this helps dogs recognize one another after time apart. Every dog’s scent is unique, so a quick sniff to the posterior can confirm whether another pup is a long-lost litter mate or a buddy from an old obedience class.