“We might notice if our coffee’s been sweetened with a teaspoon of sugar; a dog can detect a teaspoon of sugar in a million gallons of water: two Olympic-sized pools full.”

Alexandra Horowitz: The Inside of a Dog: What Dogs See, Smell and Know

Dogs have the most amazing sense of smell. In The Man Who Mistook His Wife for a Hat, neurologist Dr. Oliver Sacks writes of a medical student who took recreational drugs and dreamt he was a dog.

The student woke with dog-like senses including a heightened sense of smell and could identify patients by scent, smell their emotions and pinpoint places by odour. Inanimate objects took on emotions and he describes “the happy smell of water...the brave smell of a stone”. 

This medical student offers us a window into our dog’s world, where scent is colourful, meaningful, evokes strong emotions and relays vital information for survival. In fact a dog’s sense of smell is so acute that it is thought he can pinpoint individual odours. Chicken broth, for example, would smell of meat and carrots and onions and stock: each ingredient having its own specific identity.

As humans relying heavily on sight it is hard to imagine scent influencing every activity, so let’s examine how a dog’s olfactory system differs from ours and why they can perform remarkable feats using smell.

About one eighth of a dog’s brain is dedicated to scent. The olfactory bulb, where information about odours is processed, is about the size of a plum in dogs, whereas in humans it is the size of a raisin. Dogs can detect odour molecules 10,000 to 100,000 times lower can we can.

The olfactory epithelium contains neurons and supporting cells, and a dog has between 220 million and 2 billion olfactory neurons while we have a mere 12 to 14 million. Pigment in the olfactory areas plays a part in a dog’s scenting skills too: dogs with richer brown pigment have superior scenting ability to albino dogs who lack skin and hair pigment.

The greasy smears left on the windows by your dog’s cold wet nose are due to the secretion of a thin layer of mucus covering the nose. This moisture helps to trap scent molecules more efficiently, and dog may lick a dry nose to aid scenting. Dogs wiggle their nose when scenting, opening and closing the side slits.

They have sweat glands on their nose and these, combined with the respiratory turbinates (bony scrolls) within the nasal chambers that warm or cool air passing through, help maintain body temperature.

Sniffing causes air to be taken into the nostrils and over the bony subethmoidal shelf as it travels to the scent receptors. Scent molecules dissolve and stick to protein receptor cells in the olfactory epithelium that are sensitive to particular chemicals, and this determines their ability to detect certain scents. Each neuron expresses one type of receptor, and neurons sharing identical receptors send their messages to the same part of the brain.

Scent molecules travel to the olfactory bulb, to the primary olfactory cortex and to the thalamus, which transmits them to the orbitofrontal cortex for processing. Signals also travel to the amygdala, hippocampus and other areas of the brain processing emotion.

It is thought that scent memory lasts for life and so, for example, when my Golden Retriever met her daughter for the first time in five years there was instant recognition and none of the usual sniffing rituals that unfamiliar dogs display.

Along with most reptiles, amphibians and mammals, dogs have evolved a second olfactory system called the vomeronasal (or Jacobson’s) organ, and you may be familiar with this when you see your dog licking the air or mouth-smacking.

This action is called the flehmen response and allows scent particles to be captured by the tongue, opening tiny ducts in the roof of the mouth (behind the front teeth) to allow species-specific chemicals such as sex pheromones to access the vomeronasal organ. (Non-pheromone scents such as food are recognised by the olfactory epithelium.)

Search dogs are seen to lick a scent article as if to get extra information. The vomeronasal organ has its own nerves leading to the part of the brain devoted to interpreting its specific signals, and even allows dogs to smell under water. Thanks to the ability to draw in scent through the two tiny holes in the roof of their mouths without inhaling, dogs are able to locate scent through their open mouth when diving in water.

As we have learned, a dog’s nose is his most important sense and we can even influence his mood and wellbeing with scent however not all scents are mood-enhancing.

We frequently fill our home with aromas that can negatively impact our dogs. Air fresheners, aerosol sprays and scented candles are examples of indoor pollutants that may disturb their ability to discriminate scents. Synthetic perfumes can contain toxic chemicals linked to hormone disruption, and some chemically scented flea or herbal collars are thought to adversely affect a dog’s ability to smell. 

Limit your dog’s exposure to strongly perfumed washing powders and conditioners when laundering dog bedding or dog coats by moving to natural chemical-free products. Moving away from toxic scents there are many others, both man-made and natural, that can positively influence your dog’s mood.

Odours that have a soothing effect are useful for behaviour modification. For instance one product on the market diffuses comforting man-made pheromones and another uses natural oils that enhance the production of the calming neurotransmitter GABA. 

Some essential oils are considered toxic however others can positively affect mood and a study in the Applied Animal Behaviour Science Journal (2005) found that both lavender and chamomile reduced barking and movement in shelter dogs compared to those breathing fresh air, while peppermint and rosemary increased activity and noise(1). Aside from scents that calm, nose work activities can play a crucial part in wellbeing.

Foraging behaviour fires the seeking circuit, a reward system in the brain that drives dogs to search for food. Foraging can be a welcome distraction from unwanted behaviour such as humping or barking, or useful for calming a puppy’s ‘mad five minutes’. 

Foraging is stimulating for dogs with sensory impairment such as deafness or blindness, and finding treats hidden in a snuffle mat is a handy distraction while grooming. Hunting tit-bits thrown on the ground outdoors can defuse potentially challenging situations such as meeting a horse or a jogger when on a walk. (Tip: when asking your dog to find dropped treats, tap your foot on the ground near the treats rather than point with your hand, which brings your dog’s focus downwards.)

Foraging can also provide gentle weight-bearing exercise and mental stimulation for dogs who are on restricted exercise due to medical needs.Another group who can benefit greatly from nose work are senior dogs.

Copyright 2018, British Association of Homeopathic Veterinary Surgeons