Lead by a Nose
Rightly or wrongly, most of us consider sight and hearing to be our primary senses. Smell is something we take for granted, at least until a bad head cold clogs our sinuses and remind us just how much important information our noses routinely deliver. Think of a world without the scent of lilacs in the spring, the enticing aroma of dinner cooking, or - on the other end of the scale - essence of wet dog and the penetrating pungency of dead skunk on the highway. The nose is a kind of early warning system. If we smell smoke, we know to check for something burning on the stove, and an exploratory sniff tells us when the baby needs changing. Smell also plays a significant role (probably greater than we realize) in our recongition of familiar people and places.
As has already been discussed in this series, dogs experience the world around them differently than we do. There version is no more than adequate, and although their hearing is keen, their sense of smell is so overwhelmingly powerful that it simply transcends all the other senses combined. It is so powerful, in fact, that it almost defies comprehension; we struggle to come up with metaphores to describe it.
A few facts and figures might help put things into perspective. Dogs have up to 200 million or more scent receptors in their noses, while people have a mere 10 million or so. The surface area of the olfactory membrane (the part of the nose concerned with smell) is around 10 cm squared - in man and considerably larger in dogs (170 cm squared, more or less, depending on the breed and the shape of the muzzle).
About one-eighth of the canine brain is devoted to olfaction, a much larger portion than in humans, we can scarcely imagine the intense bombardment of complex sensory data the dog's brain recieves from his nose. The dog can successfully detect chemical compounds in concentrations as low as a few parts per million, billion, or even trillion, and pick them out while performing under the most adverse contaminated conditions. It has been suggested that his sense of smell is a million times more acute than a human's. Contrary to popular belief, man-made analytical instruments now rival the sensitivity of the dog's nose, but for the most part, such machines' detecting capacity cannot yet compete with the dog in terms of accuracy and discrimination.
TAKE A BIG WHIFFSmell is one of the chemical senses (taste is the other) that requires actual inhalation or ingestion of chemical substances, which then attach themselves to receptor cells in the body. Odour molecules must be small, and volatile, in order to reach and enter the nose during breathing or sniffing. They are directed into the nose by the nostrils, and enter the bone-and-cartilage-encased nasal cavity, which runs the length of the muzzle and opens into the throat behind the soft palate. As they travel through the nose, odour molecules encounter the olfactory membrane, an epithelial layer that lines part of the nasal cavity and consists of three layers of cells: supporting cells (which produce mucus); olfactory receptor cells; and basal cells, which are the source of new receptors. (Olfactory receptor cells are the only neurons in the mammalian nervous system that are regularly replaced; they have a lifespan of four to eight weeks.)
The olfactory system of dogs and other animals include the vomeronasal organ, a mysterious structure also identified in humans, but thought, until very recently, to be vestigial and non-functional in our species. This is a pouch lined with receptor cells and located above the roof of the mouth behind the incisors, which ducts leading to both the mouth and the nose. It is connected directly to the mid-brain via an accessory olfactory bulb.
The vomeronasal organ has been dubbed "the erotic nose," since at least some species (perhaps including dogs) it apparently detects, not odour molecules, but pheromones. Pheromones are body scents, chemicals that, when released by one individual of a species, elict a specific (often sexual) response in another of the same species.
THE LONG AND SHORT OF ITThe annatomy of the dog's nose, especially its bony framework, greatly influences his smelling ability. As might be expected, long-nosed dogs are somewhat better at scenting than flat-faced dogs - probably because they have space for more olfactory receptors - but the noses of all dogs are built to enhance performance.
The nostrils are highly mobile, which may help the dog to dtermine the direction of the scent, and when he sniffs, as opposed to when he breathes normally, incoming odour-laden air is directed onto the olfactory membrane over a structure (not seen in man) call the subethmoidal shelf. This shelf serves to accumulate and hold odour molecules in the nose as air is expired. The nasal mucus that traps these molecules is also produced in much larger quantities in the dog's nose than it is in man's.
Testing has shown that, even within the long-nosed group, there are significant breed differences in the dog's ability to smell, and perhaps even greater differences among individuals within the breeds.
A very long time ago, man noticed his canine companions' incredible talent, began selecting for it, and has continued to do so. Bloodhounds, Beagles and other scent hounds, as well as German Shepherd Dogs, have more sensitive noses than do many other breeds. DOgs purposely bred to carry out specific tasts, such as drug or land-mine detection, are particularly keen-nosed. Everything is relative, of course. A pekingese might not rival a Bloodhound as a tracker, but his sniffing capcity is still profound, at least from our nasally challenged human point of view.
THE NOSE KNOWSIf the mechanics of scent are difficult to define, then their application by the dog is downright mind-boggling. To say that he puts his remarkable sensory apparatus to good use is the understatement of the year.
He uses his sense of smell to find and choose food, to recognize his friends (and enemies), to locate bitches in heat - in general, he gets through his day largely by following his nose.
Meanwhile, we humans have arrived on the scene, and realizing we might just be on to a very good thing, have helped him take the development of his rare skill a step or two further, through specialized training. Sometimes our 'help' may be more of a hindrance, mostly due to our incomplete unerstanding and appreciation of the workings of the canine nose. Many a talnted tracking dog has been inadvertently steered in the wrong direction for example, by a handler who thought he knew better than the dog where the trail should lead. The nose knows, and arguing against the dog's instinct in cases like these is usually ill-advised.
DOgs can differentiate odour secretions from several parts of a person's body. They also seem to be able to identify different human emotional states.
They can locate, and stay focused on, a faint target scent in an environment flooded with a myriad of intensely stronger smells. Search and rescue dogs track lost children over difficult terain, and locate victims of natural (and man-made) disasters, burried deep under snow or rubble. Other dogs are specially trained in the gruesome task of seeking out cadavers and body parts.
Still other specialize in identifying drugs, gas leaks, fire accelerants, explosives - even cancerous tumours. Over the past few years, researchers studying malignant melanoma have found several biomarkers of the disease in urine and blood. Meanwhile, in 1989, a letter in a medical journal described a patient whose dog persistently explored a spot on the patient's leg that turned out to be melanoma. From such evidence, it seemed reasonable to assume that dogs could be trained to detect melanoma by smelling the skin surface for these chemical markers, thus aiding early diagnoses, and preliminary experimental results suggest that they can.
Yet another frontier may be opening up for the dog's nose to explore. Scientists in several fields continue to work to find synthetic replacements for the dog - robot noses, so to speak - but so far their success has been limited. Dogs won't be losing their scenting jobs any time soon.
When we greet a stranger at our front door, we may look quizzically into their eyes to try to determine whether he's friend or foe, while our dog - always favouring the direct approach - is likely to smell his crotch. We must blush with embarrassment, but it's certain the dog has learned far more about our visitorin that brief moment than we will know after an hour's conversation. Dogs ahve no false modesty, which is part of what makes them so endearing.
DOGS IN CANADA
October 2002
