A Dog’s Nose Knows

October 23, 2012

If there's a scent to be found, a dog's nose knows!

The canine species has an incredible sense of smell. Historically, humans teamed up with certain specially bred dogs so they could follow the scent of the targeted animal in hunting forays. Even though people think primarily of modern dogs as being family companions rather than hunting dogs, the canine species has not lost its superb sense of smell.

There are many sniffer dogs that devote their lives to serious work like tracking down escaped criminals, or missing (or drowned) persons for the police force. Dogs can effectively identify bombs, firearms and drugs by sniffing for tiny odour traces at international borders and in airports. They are loyal crime fighting partners, performing tough tasks as only our best friends can! Firefighters even call in dogs for criminal investigations of fires where arson is suspected, because dogs can pick up scents that are left behind. The oil and gas industry employs dogs to identify pipe leaks up to twenty feet underground! And if that isn't enough, dogs have even been known to accurately identify cancer lesions on people. Though the mechanism by which some dogs can identify cancers, including human melanoma (a skin cancer) is still poorly understood, it could be their acute sense of smell working once again for our benefit!

In Canada, trained teams of dogs are used for avalanche victim recovery and are provided by the Canadian Avalanche Rescue Dog Association. Around the world, dogs have been active search and rescue team members during natural disasters (volcanoes and earthquakes), and in man-made disasters like the bombing of the World Trade Center in New York. Sniffing out land mines in far away lands like Afghanistan is yet another duty in the long list of the accomplishments of the sniffer dog.

When veterinary scientists mapped out the function of the canine brain, they found that a very large olfactory lobe had evolved to process a large amount of incoming smell information from the sensitive nose. The lobe is four times larger than in humans, even though our overall brain is much larger! With such a large proportion of their brain devoted only to odour processing, it can be assumed that smelling is a sense of great importance to the canine species. The sensory tissues deep in the nasal cavity also have a very large surface area compared to humans, and the receptors that process the molecules carrying odours are significantly more sensitive than those of humans. Dogs seem to be able to discern a mixture of scents and pull out key traces of compounds of interest to them, and also to follow concentration gradients of the scent molecules. It is estimated that the ability to distinguish between different scents and to pick up scents is about 10,000 times to millions of times better than humans. A typical German shepherd, known to be a top scent tracker has 220 million sensory cells compared with a human's measly 5 million! Cats have about 10 times as many scent receptors as humans, so they also have a distinctly better sense of smell than people, but theirs is not quite as advanced as a dog’s.

Once scent molecules reach the dog, moisture in the nasal cavities (mucus) and a moist nose may help to trap these molecules for processing. The nasal cavities inside the muzzle are formed into complex coiled caverns called nasal labyrinths and these are lined with special sensory cells called olfactory epithelium. A dog’s sniffing behaviour involves taking short deep inhalations. This alters the direction of flow of air in the nose so that it impacts on the main smell sensory tissues. From here, the nerve endings in these specialized receptors transmit information about the odour to the olfactory lobe of the brain via the olfactory nerve. (2012)

It is said that animals can supposedly smell fear, but this does not make sense since fear is an emotion. Research has identified some interesting facts to note. When animals or people are afraid, stressed or excited, changes occur in the body. This may include production of an altered type of sweat that has a different odour. In people, special sweat glands (apocrine) release this high odour sweat. Higher body temperature and perhaps bacterial breakdown of this secretion produces the scent of alert that a dog would possibly sense as a fear state. The skunk family also has a particularly evolved form of apocrine-type gland that releases their notorious skunk odor when they are approached! 

Veterinarians are rarely called upon to assess the sense of smell, except in instances where a prized police or hunting dog loses the ability to follow a scent. Anosmia, or loss of smell is not otherwise problematic for dogs, but it can occur and may follow head trauma (impact) or infection.