Why Do Dogs Sniff Each Other's Butts?
When dogs sniff each other's butts, they aren’t just huffing stale fart fumes, but “speaking with chemicals,” as the American Chemical Society puts it. A dog’s sense of smell is 10,000 to 100,000 times better than ours thanks to its large olfactory membrane, a lump of tissue tucked up in its snout that can be as large as a handkerchief and hold more than 225 million olfactory receptors. (Your olfactory membrane is puny by comparison—just the size of a postage stamp and home to only 5 million olfactory receptors.) Much dog-to-dog communication is done through smell.
When one dog greets another with a nose in the derrière, it’s basically getting a brief biography of its new friend, written in scent molecules and pheromones. To the sides of a dog’s butt are pouches called anal sacs; these contain glands which secrete chemicals that can tell a sniffer a lot of information about their owner, like gender, reproductive status, and clues about its health, diet, and even emotional state. It may seem like a strange way to go about it, but butt sniffing is just the canine version of shaking hands and introducing yourself.
The video below, from the American Chemical Society’s Reactions series, explains just what chemicals pooches use to make small talk and how that information doesn’t get overwhelmed by the less savory odors emanating from Fido’s backdoor.