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The Science of Scent: Part One

Updated: Mar 11, 2023

As the guardian of two scent hounds, I have developed a love and, indeed, an absolute fascination for nosework. Scent detection and mantrailing have played a huge part in building the confidence and resilience of my nervous boy, as well as helping to further cement our relationship, while teaching us how to work as a team. We both enjoy it hugely, and we get to learn lots of new skills, as well as meeting new people and dogs in a controlled, managed way. Watching Jack work and gain confidence never fails to impress and amaze me!

In this two-part blog, I take a look at the workings of the nose, I explore some fascinating facts, along with some of the many benefits of nosework, and I discuss how you can introduce scentwork at home, through enrichment activities.

Let’s start with a look at the science behind the canine nose!

The Canine Nose Versus the Human Nose

The average dog has around two hundred and twenty million olfactory receptors in the nose, although it can be as many as three hundred million in dogs bred specifically for scentwork. Those tend to have longer ears which help to capture the scent and move it towards the nose. In comparison, the human has an average of only six million scent receptors. According to Dr Michael T. Nappier DVM, DABVP of the Virginia-Maryland College of Veterinary Medicine, their excellent sense of smell enables a dog to detect the equivalent of half a teaspoon of sugar in an Olympic-sized swimming pool!

The canine frontal cortex is smaller than our own, therefore it has different emotional capabilities; it is this part of the brain which is responsible for conscious scent perception. When it comes to canine scenting ability, their brains are forty times more capable than our own, which equates to a sense of smell 10,000 to 100,000 times more powerful. To help this to compute fully, James Walker, former director of the Sensory Research Institute at Florida State University, describes this in terms of sight: “If you make the analogy of vision, what you and I can see at a third of a mile, a dog could see more than 3,000 miles away and still see as well.” Phenomenal stuff!

The wind musicians amongst you will be thoroughly impressed that dogs can circular breathe. They draw in and process scent at the same time as they breathe in and out, creating a constant stream of air, effortlessly. They can also use their nostrils independently, creating a 3D scent picture that allows them to pinpoint exactly where a smell originates from. Mind-blowing, really!

Why Is Their Sense of Smell So Important?

The dog’s senses are the link between the world and his brain: the eyes, ears, sense of touch and taste are all direct routes to it. The brain is the memory storage area, leading to the ability to learn to search for a specific item, and to remember previous lessons on scent and technique. The dog’s sense of smell is the key way in which he views the world, in contrast with us humans, who see the world through our eyes, primarily. To emphasise the importance of this sense, it is worth noting that when puppies are born, they have a heat-seeking capability through their noses, which helps them to orientate towards their mum and the rest of the litter. They are born unable to see, and their hearing is not yet developed, therefore, the nose is of vital importance, particularly as it takes a few days for the eyes and ear canals to open.

The dog’s sense of smell is so refined that he can distinguish gender, health, illness, what the weather is, what time of day it is, what was eaten the day before, the need to toilet, and our own unique and distinct scent. They communicate socially by scent, not just through body language such as calming signals, appeasement gestures and displacement behaviours.

The Science Bit!

Dogs detect scent through the tiny particles that surround an item as it moves through the air, via the process of diffusion. The way in which scent diffuses varies widely, with gas and vapour having the strongest scent, and solids having the weakest, because the particles are more tightly knitted together. When a dog is searching for an item, they follow the increasing level of scent until they reach the item. The smellier an object is, the more easily the dog will find it. The odour forms a scent pool, which is the density of the scent as it diffuses away from an item.

With no movement in the air, the scent will be diffused evenly around the object, forming a pool effect. If there is air movement, the scent particles will form a cone shape in the direction that the air is moving. The dog then follows the pool or cone shape of the scent density to reach the search item or object. The scent particles pass through softer materials quicker than solid ones, so will result in a different scent picture for the dog. With larger obstacles, scent tends to bounce off them and back again, resulting in a mix of high-density scent particles.

Air temperature also affects how the dog detects scent. When it is warmer, the scent particles enlarge and rise, so they may be detected above the dog’s head. When it is colder, the scent particles will shrink and remain closer to the ground. When a dog is scenting, the outer parts of the nostrils move to allow for expansion and contraction, preventing the entry of foreign bodies. He will often close his mouth when he finds a trace of a scent, to allow more air to pass into the nasal cavity.

When a dog is sniffing, he inhales the scent into his nasal cavities, where the particles are caught in mucus and then processed by the sensory cells. The soft tissue inside the nasal area accommodates both air and scent, but can still differentiate between the two. Scent and air are separated in the nasal cavity immediately after inhalation. Air is filtered to the lungs and circulatory system to carry oxygen to the cells, while scent travels upwards to the olfactory receptors and olfactory bulb at the front of the brain. The smaller quantity of air arrives here, where turbinates, which are small, bony structures, “strain” the scent, which then triggers the dog’s olfactory receptors to connect with the brain and process the scent. Exhalation occurs through the slits at the side of the dog’s nose to prevent scent from being expelled from the nostrils; the dog can draw in fresh scents continuously and breathe out old ones. They can also determine which scents entered through which nostril!

Approximately a third of the brain is dedicated to the ability to detect scent. Scent receptors extend on cilia (hair-like receptor cells) from each of the sensory cells into the nasal cavity; the scent receptors trap the smells, and the messages are delivered via sensory axons (transmission lines) to the olfactory bulb. Once here, scents are carried to the frontal cortex as well as to other regions of the brain, including the centres for emotions, memory and pleasure. These are all interconnected, which helps the dog to translate the meaning of the smells. The scent arrives at the hippocampus for scent recognition, whilst the hypothalamus and amygdala deal with the emotional and motivational aspects. The olfactory cortex then distinguishes whether the scent is known or a new one, and then distributes it to the relevant area of the brain.

The Jacobson’s organ, also known as the vomeronasal organ, forms the secondary olfactory system and has a primary function of detecting pheromones and the opportunity to breed. It is found inside the nasal cavity and opens into the top part of the mouth and at the base of the nasal passage. The Jacobson’s organ does not detect ordinary odours, but responds to substances with large molecules, which are often odourless. The sensory cells communicate with the accessory bulbs and the reproductive and emotional part of the brain. The pheromone processing and interpretation is kept separate from that of basic odours, as the Jacobson’s organ has its own set of nerves leading to the pheromone-analysing part of the brain. The canine nose and sense of smell are hugely expanded by the Jacobson’s organ, and certain scents may well be linked with memories and emotions.

I hope you have enjoyed reading about the science of canine olfaction. Join me in part two of this blog to explore some of the many benefits of scentwork, and how to introduce it at home through enrichment activities. You can read it here:

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