- We have deliberately bred some types of dogs to have scent discrimination which is 1,000 to 10,000 times more sensitive than that of humans.
- Pheromones are biological odorants that evolved as a means of communication, usually between animals of the same species.
- Dogs have a second scent-detection organ specifically tuned to recognize and interpret pheromones and other biologically relevant odors.
The scent discrimination ability of dogs appears to be prodigious when compared to that of humans. It has been estimated that the sensitivity of dogs to odors may be 1,000 times, or even 10,000 times, greater than that of people (depending upon the particular scent involved). Their scent detection and discrimination ability is so good that several airports are now using sniffer dogs to detect COVID-19 in arriving passengers.
Although all dogs have fine scent recognition abilities, their talents can be improved through selective breeding. The beagle, the basset, and the bloodhound are good examples of how sensitivity to odors is, at least partly, genetically determined. These dogs have been bred as specialists, with not only a special ability to detect and discriminate scents, but also with a passion to follow, track, and explore odors.
How good is a dog’s nose?
The bony ridge inside of the dog's nose which contains the smell-detecting cells varies in size, depending upon the overall size of the dog's nose. Dogs with longer and wider noses have more of this surface available, while dogs with flat faces and short noses, like pugs or Pekingese, have a smaller surface area in this portion of the nose, and therefore simply don't have the room for as many scent-detecting cells. Typically, the dachshund has around 125 million smell receptor cells, while a fox terrier has 147 million and the German shepherd has about 225 million.
Some dogs, in particular the dogs that we call "scent hounds," have noses that are designed to be very wide and deep in order to pack the largest number of odor analyzing cells into the available space—even if the dog itself is not very large. Thus the very scent-oriented beagle, who normally weighs in at only about 30 pounds, and stands only 13 inches at the shoulder, has the same 225 million scent receptors as the German shepherd dog, who is approximately twice his size at 75 pounds with a height of 24 inches. The bloodhound is the grand champion of scenting, at least in terms of the number of olfactory receptors. These big-nosed dogs check in with around 300 million scent receptors in their noses, the largest number in all of dogdom.
How do these numbers of scent-detecting cells in the dog's nose compare with those in humans? Human beings are not very smell-oriented, and one of the reasons for this is that we have noses that contain a paltry 5 million smell analyzing cells. This means that the average human being has a nose that contains only 2 percent of the number of odor analyzing cells that can be found in the nose of the little beagle.
The dog’s other nose
But that huge number of olfactory receptors in dogs is far from the whole story. It seems that dogs may have a separate, second "nose." Some dog owners have observed an ominous-looking bump on the roof of their dog's mouth, right in the middle and just behind the top two middle teeth. That bump is called the "incisive papilla" (papilla is just the word for bump, and this one is located near the incisor teeth). Some veterinarians report that when dog owners first see this, they often make a panicky veterinary appointment, dreading the worst. However, this bump has a precise function. In the middle of it, there's a hole that is a duct that communicates with the dog's Jacobson organ (also known as the vomeronasal organ). This is a separate olfactory system for analyzing scents.
The particular scents that Jacobson's organ is designed to detect are biological odorants, and most of these are pheromones. Pheromones evolved as a chemical means of communication between animals, usually of the same species. They are secreted from glands found in the genital/anal region and glands between the toes and around the mouth. That means that they are also found in the saliva and urine of dogs.
Some pheromones contain important sexual messages that indicate the reproductive status of the females: for instance, whether they are in the receptive portion of their cycle.
Recent research has shown that there are also pheromones that have a calming effect. When a mother dog gives birth, she releases special pheromones which are meant to comfort her puppies, and nowadays synthetic versions of these are often deliberately used since they appear to relieve anxiety even in adult dogs.
Some pheromones mark territory. When dogs scratch the ground after they have eliminated, they are releasing pheromones from their paw pads which can be detected by other dogs. Territory is also marked in dog urine, which explains why dogs are so obsessed with the act of marking and sniffing the marks of others. These pheromones convey a lot of information about other dogs in the area.
Why does a dog need this extra scenting apparatus?
The particular chemical messages that Jacobson's organ is tuned for are coded in non-volatile molecules. That simply means that they don't vaporize easily, which explains why they are not registered by those many scent-analyzing cells in the dog's nose, which are tuned for volatile substances. So these chemicals must be passed directly through that pore in the roof of the mouth. Generally speaking, these molecules are caught by the moisture on the tongue, and then the dog engages in "tonguing," which involves pushing their tongue against the roof of their mouth to send some pheromones through the duct.
You may also notice some dogs may foam at the mouth and their teeth may chatter after they have sniffed or taken a tentative lick at the urine left by another dog. This is all part of the action required to gather the pheromone molecules and convey them into the opening which leads to Jacobson's organ.
The inside of the vomeronasal organ is lined with receptor cells that collect these chemical messages and then relay them directly to the dog's brain—specifically to the amygdala and hypothalamus. These are important parts of the canine brain that generate emotional and highly motivated behavioral responses.
Do humans have this second nose? Anatomically, Jacobson's organ appears in about one out of four humans; however, it is a dead-end chamber, and there is no evidence that suggests that there are nerve connections between any sensory receptor cells from these sites to the brain. In other words, in people, this is a vestigial leftover from our evolutionary past, much like the fused vertebra at the base of our spine (the coccyx), which is all that remains of our ancestral tails.
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Horowitz, A. (2017). Being a Dog: Following the Dog Into a World of Smell, Scribner: New York.
Salazar, I., Barrios, A. W., and SáNchez-Quinteiro, P. (2016). Revisiting the vomeronasal system from an integrated perspective. The Anatomical Record, 299, 1488–1491. doi: 10.1002/ar.23470