Canine Sense of Smell
March 31, 2021
Smells are detected by a specialised lining in the back of the nasal cavity, just in front of the brain, called the olfactory epithelium. Some idea of the importance of their sense of smell can be gained from the surface area of this epithelium, which is 18-150 cm2 in dogs (depending on breed), 21 cm2 in cats, but only 3-4 cm2 in man.
The olfactory epithelium of the dog is a relatively simple structure compared to, for example, the retina of the eye. It is covered by a layer of mucus, into which the airborne molecules which cause sensation of odour must dissolve before they can be detected by the special chemical receptors. The receptors are mostly located on projections called cilia, which lie in the mucus. In the dog and cat, these cilia are both longer and more numerous than in many other species, presumably to enhance the sensitivity of their sense of smell.
Each receptor is a nerve cell, which transmits information to other nerve cells in the olfactory part of the brain. It is difficult to classify the receptors into types, the way that has been possible for the taste receptors found on the tongue. The distinct odour of particular substances is therefore thought to rely on comparisons between the firing patterns of several different types of receptor.
When dogs deliberately sniff objects, the duration of the sniffs is adjusted to allow optimum exposure of the olfactory epithelium to the stimulus. If a stimulus is weak, dogs tend to increase the rate of sniffing, rather than the duration of each sniff.
Dogs not only have a well-developed olfactory sense, but are also readily trained and this combination of skills has been exploited by man for thousands of years for the hunting, tracking and retrieving of game.
The tracking skills of dogs depend on their ability to detect the volatile fatty acids found in human sweat, in concentrations which are hundreds or thousands of times lower than our own threshold for smell. Dogs can also detect whole human odours, even those derived from fi ngerprints, both when fresh and following a week or more of weathering. Perhaps most impressive is the dog's abilities to detect features specific to the individual in all the body odours (palm, armpit, sole) from a single person, even though these regional odours smell quite different to us.
The ability of trained dogs to distinguish between different people has been investigated using pairs of identical twins. In tracking experiments, when direct comparisons could be made, the twins could be reliably discriminated, but in successive choices the same dogs indicated that the twins were much more similar to each other than to other humans. In other words, tracking dogs can tell people apart by their smell, about as well as we can by sight.