The sense of smell is one of the five senses. The sense of smell is an essential feeling and without it life would be difficult for humans. Can you think of what would happen if humans lost the sense of smell?

What happens when your nose is blocked due to cold? You lose the feeling of smell. Loss of sense of smell also leads to altered taste. Food that used to feel good to you no longer feels as good! So, the feeling of smell is necessary for the sense of taste as well.

So, which animal has the best sense of smell?

I am sure you have heard this phrase before- sniff like a bloodhound. Dogs have been used for many centuries as a hunting aid. Dogs are used to sniff out drugs, explosives and humans. So, what is so special about dogs that make them suitable for these duties?

Dogs have an incredible sense of smell. A dog has over 220 million smells receptors. In contrast, humans only have five million olfactory receptors. The bloodhound has as many as three hundred million receptors for smell.

The dog’s olfactory comprises of two nares, nasal cavity, a specialised layer of olfactory epithelium and a specialised organ- the vomeronasal organ.?

sense of smell

Image from http://www.aces.edu

Air enters through the nares. Odours in the inspired air are ascertained by the olfactory receptors in the specialised olfactory epithelium lining the nasal cavity. Each olfactory receptor stimulates an olfactory nerve. The odours then reach the olfactory lobe by travelling through the olfactory nerve.

The vomeronasal organ is a pair of elongated fluid-filled sac that opens into the mouth or the nose. The olfactory receptors in the vomeronasal organ are different from those in the nasal cavity. The olfactory neurones in the nasal cavity have cilia while those (Neurones) in the vomeronasal cavity usually have microvilli. Also, the neurones in the vomeronasal organ are specialised structures that are necessary for detection of pheromones. The vomeronasal organ is connected to the hypothalamus in the dog’s brain. The hypothalamus regulates many sexual and social behaviours.

So, have you seen a dog sniff? Sniffing is a technique that dogs use to maximise the detection of odours. Sniffing is a series of rapid inhalation and exhalation that is a disruption of the normal breathing pattern. Sniffing creates a pocket within the nasal cavity. This pocket allows accumulation of odour molecules and improves recognition of odours.

Difference between human and canine olfaction

There are many differences between humans and dogs when it comes to the sense of smell. As discussed before, the number of olfactory receptors is huge in dogs when compared to humans. Dogs are also more proficient in detecting pheromones.

Also, dogs have a very well developed olfactory lobe that helps with recognition of smells.

Therefore, your dog is far more proficient in recognising smells than you. However, recent research suggests that human sense of smell is better than we imagined. Although the number of olfactory receptors is lesser than that of a dog, we humans have a greater variety in terms of olfactory receptors. Recent studies suggest that there are over four hundred types of smell receptors in the humans. These (receptor) acting in various permutations and combinations can detect as many as one trillion smells (Nature.?doi:10.1038/nature.2014.14904 ).