You smell an apple pie baking in the oven. Much later, you close your eyes and remember the wonderful aroma. Your sense of smell and your memory of fragrances are great, but your dog’s are better. His sense of smell is 10,000 to 100,000 times more sensitive and his aroma memory bank is amazing. A quick comparison of noses sheds light on why a dog’s sense of smell is so superior to ours.
Breathing and smelling are two separate functions. The human nose provides only one air passage for both. When you inhale, air enters both nostrils and passes over bony scroll-shaped plates called turbinates. They house 5 million scent receptor cells that cover an area equivalent to the size of a postage stamp. The receptors recognize odor molecules by their shape and send nerve signals to the brain for analysis so you know what you sniffed. When you exhale, air follows the same route in reverse which actually forces incoming odors out.
A dog’s nose is more complex. His two nostrils breathe independently and can determine into which nostril an odor arrives. In the wild, this tells a dog if an enemy is on his right or his left. A dog exhales through the slits in the side of his nose so new aromas can enter the nostrils.
A fold of tissue just inside the dog’s nose forms two pathways. About 88 percent of incoming air is used for respiration and goes to the lungs. The remaining 12 percent, is meant for smelling.
The actual number of scent receptors varies with the breed of dog. A Dachshund has 125 million. A German Shepherd has a better sense of smell with 225 million scent receptors. The Bloodhound has a very sensitive nose with 300 million nerve endings that sense aromas.
A dog’s aroma memory bank and his keen sense of smell can identify many things including drugs, explosives, and his owner. Gregory Bern and a team of researchers from Emory University wanted to know which odors got the greatest reaction in the part of the dog’s brain that analyzes smells. The researchers used functional Magnetic Resonance Imaging to test the brain activity of 12 dogs of different breeds. Each dog was exposed to scent samples from the dog himself, an unfamiliar dog, a dog that lived in the same household, a human the dog had never met, and a human that lived in the dog’s household. The investigators anticipated the dogs would be highly tuned to the smell of other dogs. Instead the dogs reacted most strongly to the human they knew.
Gregory Bern’s study tells us that the scent of a loved human lingers in a dog’s brain much like the scent of a loved one’s perfume lingers in the memory of a human. A whiff of your scent on a piece of clothing or furniture apparently keeps kind thoughts of you in your dog’s memory while you are away. So if you come home to find your dog cuddled next to your old shoe, he is probably using your scent to conjure up fond thoughts of you.