Dogs may not have the best eyesight, but with a sense of smell up to 100,000 better than a human’s sense, who needs to see anything? Dogs have such a powerful sense of smell because their olfactory membrane is much larger than ours; the olfactory membrane is located in the snout and can be the size of a handkerchief and include over 225 million olfactory receptors. Humans? We have a postage-stamp-sized olfactory membrane with a comparatively pitiful 5 million receptors. Which is why dogs have a rather impolite of greeting each other: sniffing each other’s butts. Much of the way dogs communicate with each other and learn about the world is through their sense of smell, so they’re learning a whole lot more about that other dog by getting all up in its poop chute than you can imagine.
Dogs can learn a lot about from another dog through the different smells of that dog’s derriere; scent molecules and pheromones are like a biography. A dog’s butt includes two pouches on each side, also known as the anal sacs. The anal sacs are home to chemical-releasing glands that emit scent molecules and pheromones that can inform the inquisitive pooch about their new friend’s sex, health, diet, reproductive status, and even its emotions. While humans would rather shake hands and say “hi, I’m…”, dogs are happy as can be introducing themselves by allowing another dog to take a whiff.
Here’s a great video from the American Chemical Society, part of their Reactions series, that gives an in-depth explanation of the types of chemicals that dogs can smell on a new friend’s derriere, as well as how dogs can pick out subtle scents even when they’re hiding underneath some of a dog butt’s less delicate smells.