On the whole, we humans don't rely on smell to experience the world. Instead, we emphasize our sense of sight. If we smell something, that information serves as a cue to look for the source, not to interpret the smell itself.
But dogs are different. Smell is the primary way they experience the world, and sight is of secondary importance.
"They might look at someone with their eyes; as you approach, they look at you," dog cognition researcher Alexandra Horowitz told Business Insider. "But then once they've noticed that there's something with their eyes, they use smell to tell that it's you. So they sort of reverse that very familiar use of ours."
Stop to smell the roses — in stereo
Dogs' noses are optimized for smelling.
It all starts with that wet, spongy nose, as the video above explains in detail. It can capture many smells that are carried by the breeze. On top of that, dogs can smell in stereo, with each nostril able to smell different scents. This helps them determine which direction a smell is coming from and a host of other information. And the cool factor doesn't end there. Dogs' noses are designed so that inhaling and exhaling occur through separate passages. Dogs exhale through slits on the sides of their noses, creating little currents of air that, as they inhale, allow them to take in even more scent molecules.
This is clearly a super-sniffer device, and that's before even explaining what's happening inside.
Once a scent travels into their nostrils, a fold of tissue directs the scents into two different passages. One passage is for oxygen and the second passage is for the scents. This second passage is filled with olfactory receptor cells, about 300 million of them. For comparison, we have a paltry 5 million.
Being able to take in all these smells wouldn't mean much without a way to process them, let alone remember them. For these reasons, the olfactory bulb of dogs' brains that carries out this action takes up many times more relative space in the brain. The olfactory bulb connects to a few different parts of the brain, including the regions responsible for behavior, memory, emotions and taste. All these regions are also connected, and together they form a complex web that ultimately helps dogs determine what they're smelling and where it's coming from. It also helps to form associations with those smells.
That's not all, though. Thanks to the vomeronasal organ located just above the mouth, dogs are able to detect hormones that all animals release, including humans. These hormones help them identify potential mates and to differentiate between friendly and threatening animals. When it comes to humans, this ability to pick up hormones helps them to identify our emotional states, and it can even tell them when someone is pregnant or ill.
A smell to remember
The associations between smells as well as dogs' ability to remember them is what helps them not only track smells but aids them in identifying others.
"We basically have a cloud of smell around us. That's interesting, because it means a dog can smell you before you're really there," Horowitz said. "If you're around the corner, your cloud of smell is coming around ahead of you."
Sure, maybe your dog remembers roughly what time you get home, but it can also smell you, the car and whatever else it needs to identify you before you're even within eyesight.
Smelling is also how dogs are able to communicate outside, too. As we've previously reported, a walk isn't just a walk for your dog; it's a way to know how other dogs in the neighborhood are doing, and if there are any new dogs around. The scents tell them whether or not the dog is healthy, what it's eaten, and if the dog is male or female.
A cold nose for heat
Interestingly, dog noses aren't just for sniffing out dogs and people. A new study finds that they may also be able to sense weak radiating heat. The cold, wet tip of a dog's nose — called the rhinarium — makes it particularly sensitive to heat released by thermal radiation. This ability would help carnivores find warm-blooded prey.
Other animals, such as raccoons and moles, also have rhinarium which they use for tactile sensitivity. But because dogs' noses are cold, their tactile abilities aren't as great, leading researchers to believe the nose has greater abilities beyond just touch and smell. Their results were published in Scientific Reports.
So the next time your dog sniffs the air or a favored spot or really wants to smell your shoes, just let the dog do his thing. He's simply trying to drink in all the information he can about the world around him.
Editor's note: This story has been updated with new information since it was published in April 2018.