Smell

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A sense beyond compare

© Diffomédia/Royal Canin

Smell is the dog’s primary sense. It is used for hunting, getting their bearings, communicating with other dogs and conveying their food preferences. Dogs recognise their owner and their home by smell rather than by sight. Smell is also important in the perception and evaluation of food, prevailing over taste. A dog will not eat anything it doesn’t like the smell of.

A dog’s sense of smell is a million times keener than a human’s. Forty times as many brain cells are involved in decoding smells in the dog’s brain than in a human’s. This great sensitivity to smell is also due to the larger surface area of the receptor organ: the olfactory mucosa covers 200cm2 in dogs, compared with just a few cm2 in humans.

Dogs also have a large number of olfactory cells, although the actual number varies depending on the breed (220 million in Labradors and German Shepherds, 70 million in Cocker Spaniels).

Last but not least, the size of the brain processing olfactory stimuli in dogs is around ten times bigger than in humans.

Perception of smell

The mucosa covers the turbinate bones in the nostrils. These bones are scrolled in shape and separated by the sinuses, into which inhaled air rushes, trapping smells. The ethmoid at the back of the nasal cavity is another organ of smell, also made up of sensory cells.

The smells which come into contact with these cells attach themselves to specific receptors, triggering chemical changes which produce a nerve signal that is sent via the olfactory nerve to the area of the brain responsible for processing this information. By 2009, more than 300 types of olfactory receptor had been identified, each dedicated to a specific “odotope” (part of the chemical molecule).

Chemical composition, air humidity and molecular weight determine how well a smell is perceived. A heavy molecule that is mildly water-soluble is easier to perceive. These principles are used in the work of all types of sniffer dog, from search and rescue to drug detection.

The olfactory sensation

Duhayer_Royal Canin

The olfactory sensation can be fairly slow to work in dogs. Not only must the molecules cross the nasal mucosa and attach themselves to the right receptor, a latent period is often needed before the dog really “recognises” the smell. The addition of equivalent stimuli is needed to cross the olfactory threshold and provoke a reaction. In the same phase, the dog’s olfactory acuity will ultimately diminish (generally after 30-45 minutes), in connection with what is called olfactory fatigue. This is when all the specific smell receptors are engaged. When this happens, the dog will have to be rested to give the receptors time to free themselves up.

A dog’s olfactory sensation is optimal after 2-4 minutes’ work; the relaxation phase lasts 3-4 minutes.

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A dog’s olfactory sensation is optimal after 2-4 minutes’ work; the relaxation phase lasts 3-4 minutes

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Lastly, a few useful pieces of information for sniffer dog handlers:

- While females have better olfactory acuity than males, it does vary with the oestrous cycle (oestrogens, like male testosterone, have a positive impact on this).

- The pigmentation of the pituitary mucosa influences olfactory capacity. Dogs with a clear mucosa are less effective.

- A dog’s olfactory acuity can be affected by the biological environment (a sensation of hunger improves olfactory capacity, whereas poor general health or physical tiredness reduces it).

Grossemy
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