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.