Dog sense of smell

Scent hounds, especially the bloodhound, are bred for their keen sense of smell.

The dog sense of smell is the most powerful sense of this species, the olfactory system of canines being much more complex and developed than that of humans. Dogs have roughly forty times more smell-sensitive receptors than humans, ranging from about 125 million to nearly 300 million in some dog breeds, such as bloodhounds. These receptors are spread over an area about the size of a pocket handkerchief (compared to 5 million over an area the size of a postage stamp for humans). Dogs' sense of smell also includes the use of the vomeronasal organ, which is used primarily for social interactions.

The dog has mobile nostrils that help it determine the direction of the scent. Unlike humans, dogs do not need to fill up their lungs as they continuously bring odors into their noses in bursts of 3-7 sniffs. The dog noses have a bony structure inside that humans do not have, which allows the air that has been sniffed to pass over a bony shelf to which odor molecules adhere. The air above this shelf is not washed out when the dog breathes normally, so the scent molecules accumulate in the nasal chambers and the scent builds with intensity, allowing the dog to detect the faintest of odors and can even detect emotions.

One study into the learning ability of dogs compared to wolves indicated that dogs have a better sense of smell than wolves when locating hidden food, but there has yet been no experimental data to support this view.

A typical dog's nose

The wet nose, or rhinarium, is essential for determining the direction of the air current containing the smell. Cold receptors in the skin are sensitive to the cooling of the skin by evaporation of the moisture by air currents.

Function and mechanism

Characteristics

A great part of a dog's brain is dedicated to acquiring and interpreting odors. A dog utilises its nose as a guide in sniffing in order to follow a trail. The animal performs a series of short inhalations and expirations, permitting it to bring the odor-bearing molecules in contact with its olfactory mucosa. These molecules are dissolved and then absorbed by cells of the olfactory epithelium before they reach neurons, which transmit information to the brain. Dogs with elongated noses perform better than dogs with a flat noses. Additionally, larger dogs have a better sense of smell than smaller dogs.

The external surface of a dog's nose contains a number of pressure and temperature sensors. Forced and rapid evaporation of the wet skin due to wind chills the surface of the nose, this cold is perceived by thermoreceptors. These receptors thus give the animal the direction of the wind.

Odor is the only active sense which a dog possesses from birth, this allows young puppies to find their mother's nipples. It is also the last sense which remains during old age after the other senses are progressively diminished.

Memorization ability

The brain of a dog is able to memorize more 100,000 different odors and its ability to smell is 1,000 to 10,000 times stronger than that of a person. The predominance of this sense allows a dog to associate odors with situations and contexts, in a manner quite similar to the visual memory of humans.

Accuracy

The accuracy of a dog's nose is very high. For example, a dog is able to differentiate odors of members of the same family or the scents of two identical twins provided that they, each on their own, traced their one scent. Dogs are equally able to recognize the odor of a hand on an object after two seconds of contact, and are in fact able to detect the trace scent of fingers on glass six weeks after contact, even if this glass has been handled by different people.

Accuracy is variable among dog breeds: labrador retrievers have 220 million sensor cells, German shepherds have 200 million and fox terriers have 147 million.

Use in communication

The specific odors which dogs use to communicate arise from pheromones. These are chemical substances participating in communication between individuals belonging to the same species. Different hormones are excreted when a dog is angry, nervous or confident, and certain chemical signs serve to identify the sex and age of the individual, and if it is a female, to identify if she is in her estrous cycle ("in heat"), if she is pregnant, or if she has just given birth. A great portion of these chemical pheromones are dissolved in a dog's urine.

Canines possess the ability to discern odors one by one after they have been mixed. The animal can thus find its food, locate its territory or any other person, animal or object.

Problems and conditions

Anosmia is a great handicap for dogs. The loss of smell is due to the absence or the destruction of certain olfactory receptors. This issue can arise due to genetics, medical treatment, infection or brain damage. Loss of smell is a rare problem, but generally more frequent in albino dogs.

Use of ability by humans

Humans learned to use dogs' exceptional sense of smell, primarily for hunting but also, more recently, for diverse types of searches.

Rescue and detection of substances

Police use dogs for various purposes purposes including searching for people (the most well known being search dogs for avalanches), for explosives, for narcotics as well as for flammable materials.

Detection of disease

Some studies have found that certain dogs are able to detect diseases among human beings, notably cancer. The British medical journal The Lancet, suggested for the first time in 1989 the use of dogs to detect cancer. However studies should be carried out to determine the nature of the molecule produced by cancerous cells which could pave the way towards new tests for early screening of cancer.

According to these British findings and experts of the NGO Medical Detection Dogs in Gambia, certain dogs should be equally able to detect malaria in infants who do not yet present with symptoms.

Due to the specific odor produced by an epileptic person, dogs are able to detect an epileptic crisis before it is triggered. The sense of smell, very efficient in dogs, is used everyday in a number of areas of research, concerning the detection of illicit substances, the search for dangerous materials as well as finding insects such as bed bugs.

Hunting dogs

Employed for various purposes in hound-hunting, the hunting dog is above all used for its tracking abilities, that is finding and sniffing out prey.

Truffle dogs

This type of dog has been trained and raised to find truffles buried in the ground.

See also

• Scent hound
• Sighthound
• Dog anatomy