Dogs have a remarkable ability to detect and discriminate scents. This allows us to use dogs for many tasks, such as detecting drugs, explosives, landmines, agricultural products, and even more esoteric things, such as bedbugs or termites. Recently, dogs have been used to identify criminals via a process called a "scent lineup." The rationale is that whenever a crime is committed, evidence is always left behind. Of course some criminals may be cautious enough to avoid leaving behind obvious clues, such as fingerprints, hair, or other forms of trace evidence. However, despite the best efforts of the most meticulously careful criminal, one type of invisible trace evidence is almost always deposited at the scene of crime and on the items they used as part of their nefarious activities. No matter how hard they try, offenders cannot avoid leaving behind their unique human scent. The principle behind a scent lineup is that a dog is given a chance to sniff the scent taken from an item at the crime scene and then to determine if that scent matches the scent of an item handled by a suspect. Some courts are hesitant to accept evidence from canine identifications. For this reason formal procedures for scent lineups this have been worked out by the Dutch and Polish national police forces and evidence from this kind of test has been accepted in many courts in North America. However, in all of those courts where scent lineup evidence is accepted, a decision has been made to only accept the evidence if the dog making the identification is a Bloodhound. The courts seem to be acting on the belief that the Bloodhound's nose is more sensitive than that of other dogs. That raises the question as to whether some breeds of dogs really do have better scent discrimination ability than others.
Although all dogs have fine scent recognition abilities, their talents can be improved through selective breeding. The Beagle, 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.
The bony ridge inside of the dog's nose that 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 smaller surface area in this portion of the nose, and therefore simply don't have the room for as many scent detecting cells. For example, 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 twice his size at 75 pounds with a height of 24 inches. Consistent with the suggestion of legal authorities that there is something special about the bloodhound, he in fact 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 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. That means that the average human being has a nose which contains only two percent of the number of odor analyzing cells that can be found in the nose of the little Beagle.
There is one additional quirk associated with a dog's scent detecting ability. For reasons that are not completely clear, male dogs seem to have better scent discrimination than female dogs. Some behavioral scientists have suggested that this is not because the male dog's nose is more sensitive, but rather because he is simply a more interested and focused on smells than the female dog is.
Stanley Coren is the author of many books including: Born to Bark, The Modern Dog, Why Do Dogs Have Wet Noses? The Pawprints of History, How Dogs Think, How To Speak Dog, Why We Love the Dogs We Do, What Do Dogs Know? The Intelligence of Dogs, Why Does My Dog Act That Way? Understanding Dogs for Dummies, Sleep Thieves, The Left-hander Syndrome
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