Humans and dogs experience the world through a very different combination of senses. To most humans, sight is the most important sense, followed by touch, sound, and smell. For dogs, the order is smell, sight, sound, and then touch, with a dog’s sense of smell being by far the most important.
The easy way to remember it is: Nose, eyes, ears, in that order.
When you approach a dog, the dog has already investigated you by scent, from as far as fifty yards away. To a dog, our scent and energy are our “names,” and the dog will have figured out quite a lot about you just through smell alone. This is also why your dog will sometimes start barking at something long before you can see or hear it. It’s not psychic powers. It’s sense of smell.
This nose-eyes-ears priority of senses to a dog follows the pattern in their development as a puppy. Dogs are born blind and deaf and rely on their sense of smell in order to find their mothers and nurse, so their first association with survival is scent. This connection lasts for the rest of a dog’s life, which is why they are constantly sniffing, especially in strange or new environments.
When you’re out on the walk, you might be able to detect the scent of the grass nearby, but to a dog that grass really is a record of what’s been going on – and the information in it is more than just which other dogs have peed there.
Living things constantly give off aromas. We don’t do it just by sweating. Every time we exhale, we send scent information out into the world. We also constantly shed skin cells and hair, which get carried off into the wind and eventually settle on the ground. Where our shoes or feet touch the ground, they also leave scent information.
A dog can smell all of these molecules and learn a lot from them: who or what has been there, how long ago they were there, how close they are now, and which way they went. It’s a world that’s mostly invisible to us, but it is the place your dog lives every day.