Sensory Abilities: Olfaction

The dog’s sense of smell has attracted a great deal of enthusiastic attention from both applied and scientific quarters but has only slowly received appropriate experimental study. Historically, almost supernatural capabilities were attributed to a dog’s nose, often resulting in the promulgation of some rather fantastic and insupportable claims about canine olfactory abilities. In addition, many equally incredible theories have been posited regarding the way in which a dog’s olfactory apparatus works. These theories have ranged from the absurd to the occult. For example, one fanciful account hypothesized that irradiated energy emanating from living cells was absorbed by various materials stepped upon, and then re-emitted and detected by the dog’s nose. Other discarded theories posited the notion that electrical waves or vibrations were responsible for the extraordinary feats of canine olfaction. One speculative adherent of the wave theory actually proposed that a pendulum be employed as an instrument for measuring the dog’s olfactory acuity. Over the years, many important advances have been made in the study of olfaction, largely supplanting theories like the foregoing with more scientifically grounded alternatives. Currently, the science of smell is making important strides toward a more complete understanding of the intricate biochemical and neurological substrates of olfaction.

Mechanics of Smell

The sense of smell enables dogs to analyze the environment for significant chemical signs or disturbances. During the process of smelling, a sample of air containing the odor is sniffed and directed deep into the posterior portion of the left and right nasal cavities. Once in the nasal cavity, the odor accumulates on a mucous layer containing millions of odor-sensitive cilia. The cilia are hairlike dendritic elaborations of the olfactory receptor neuron. Each olfactory receptor has 10 or more immotile cilia that collect odorant molecules. The convoluted epithelial membrane containing these olfactory receptors is supported by a complex structure of turbinate bones. This arrangement allows for maximal contact between the collected odor and the olfactory mucosa. In addition, the cilia themselves add considerably to the overall membrane surface area exposed to odorant molecules. A dog’s olfactory neuroepithelium contains as many as 250 million receptor cells. When stretched out, the surface area of the olfactory epithelium has been estimated to range (depending on the breed) from 20 to 200 square centimeters. In comparison, the human olfactory neuroepithelium covers a mere 2 to 4 square centimeters and contains only about 5 million receptor cells per nasal cavity.

Olfactory Transduction

Sensory data from these millions of receptor cells is conducted through the cribriform plate into the nearby olfactory bulbs within the cranium. Once in the olfactory bulbs, the axons converge upon the glomeruli. The glomeruli are spherical structures that integrate and organize olfactory input. There are far fewer glomeruli than olfactory receptor axons, requiring that many thousands of axons share individual glomeruli. Within each glomerulus, olfactory axons form synapses with second-order olfactory neurons called mitral cells. From the glomeruli, the information is passed onto other parts of the brain for higher processing, associative identification, and interpretation (see). As one might expect, the olfactory bulbs in dogs are considerably larger than those in humans.

The manner in which olfaction occurs is not fully understood, but important advances in the study of olfactory reception have been made by Axel and his associates at Columbia University. By using molecular genetics and sophisticated biochemical procedures, they have been able to isolate a large gene family dedicated to the synthesis of olfactory receptor. The researchers have found that the olfactory neuroepithelium contains neurons possessing about 1000 different receptors, coded by an incredible 1% of the mammalian genome. In rats, one in every 100 genes is involved in the reception of odors, making olfactory receptor genes the largest family of genes currently known to exist. Each receptor protein is highly selective and will bind only to a select group of odorants. In combination, these diverse receptors yield an extraordinary diversity of smells. Whereas humans are believed to discriminate around 10,000 separate odors, dogs are probably able to detect a far larger number. These findings are extraordinary when one considers a comparison with all of the rich diversity of human color vision that is provided by only three kinds of photoreceptors differentially sensitive to three overlapping bands of visible light. Given that 1000 different olfactory receptors appear to exist, the potential number of smells available to the mammalian nose is staggering.

Each olfactory neuron expresses a receptor specialized for the detection of a specific type of odor molecule. All of these many receptor proteins are coupled to G proteins concentrated on the distal portion of the cilia. The receptor protein in conjunction with the G protein activates a cascade of biochemical events mediated by a second messenger within the cell, resulting in the depolarization of the olfactory neuron and the production of an action potential or signal. These olfactory receptor neurons are distributed randomly in several specialized zones on the olfactory epithelium. The axons of olfactory neurons with the same receptor converge on the same glomeruli in the olfactory bulb. This is an extraordinary finding, since these receptor neurons are destroyed and shed after a functional life of 6 to 8 weeks. Olfactory neurons are being constantly replaced by underlying stem cells that subsequently send axons to the same locality in the olfactory bulb. How this is accomplished is not known.

Olfactory Acuity

Numerous studies have demonstrated that a dog’s sense of smell is extremely sensitive. W. Neuhaus employed an olfactometer for mixing and delivering odorant samples at very low concentrations. His method involved evaporating the sample into a controlled airstream and directing it out through three separate ports, two of which contained air without any odorant. The dog was trained to choose between the three by pressing its nose against a box located behind the port associated with the sample. The concentration of the odorant sample was progressively lowered until the dog could no longer select the correct port. Surprisingly, he found that some substances were not detected by dogs at concentrations much lower than that detectable by humans. In most instances, however, the dog’s ability was much superior. For example, he estimated that a dog’s ability to detect butyric acid (a component in sweat that smells like dirty socks) is from 1 million to 100 million times better than a human’s ability. These results (if true) mean that dogs may be able to detect 1 milligram of butyric acid in 100 million cubic meters of air. Pearsall and Verbruggen illustrate the extent of these incredible findings with a striking analogy:

Comparison with our own nose is difficult, but an example may help: One of the substances released by human perspiration is butyric acid. If one gram of this chemical (a small drop in the bottom of a teaspoon) were to be spread throughout a ten-story building, a person could smell it at the window only at the moment of release. If this same amount were spread over the entire city of Philadelphia, a dog could smell it anywhere, even up to altitude of 300 feet. (1982:5)

Butyric acid is a prominent feature of the scent picture utilized by dogs while tracking. Wright makes a number of probing observations and calculations based on assumptions drawn from Neuhaus’s findings and the abilities of tracker dogs:

There are several sources of skin secretions: sweat glands, “odour glands”, fat glands, and various others. The sole of the foot has only sweat glands, but they are present in large numbers: up to 1000 per square centimetre. Therefore the sweat glands are likely to be the most important. Over a period of 24 hours, the human body secretes about 800 c.c. of sweat, and from the two million or so sweat glands on the sole of each foot, about 2 per cent of the daily production, or about 16 c.c, would be released. Human sweat has about 0.156 per cent acid of which about one-quarter is aliphatic. If only 1/1000 of this penetrates steadily through the sole and the seams of the shoe, it can be calculated that of an acid such as butyric acid, at least 2.5 X 1011 molecules would be left behind in each footprint. This is well over a million times the threshold amount for the dog, and could still give a detectable smell when dispersed in 28 cubic meters of air. (1964:76)

These numbers are staggering, especially if one considers that some bloodhounds can follow trails several days old over rough terrain and then pick out the tracked person from a lineup of 10 people.

Ashton and colleagues (1957) also found that a dog’s ability to detect various odorants was not equally proficient for all sample substances. An apparent factor is related to the size of the molecule involved. Fatty acids differing by only a single atom of carbon resuited in significantly different olfactory thresholds. The more carbon atoms the molecule possessed, the lower was the dog’s olfactory threshold for its detection. A possible explanation for this finding is that organic molecules with long carbon chains possibly trigger action potentials in a correspondingly greater number of olfactory receptor neurons than do molecules composed of fewer carbon atoms. The lower thresholds may be obtained because more receptor neurons are fired by molecules possessing a greater number of carbon atoms than those possessing only a few.

More recent and carefully controlled studies have compared the dog’s olfactory ability with that of humans. These experiments have found somewhat less dramatic differences between humans and dogs — at least with respect to the substances investigated. Krestel and colleagues (1984) compared the dog’s ability to detect amyl acetate with that of human subjects. Using a conditioned suppression technique, they determined that dogs can detect the substance at a concentration 2.6 log units lower (i.e., about 400 times better) than human test subjects. Marshall and Moulton (1981) determined that dogs could detect alpha-ionone at concentrations 3 to 4 log units lower than that detectable by humans (i.e., approximately 1000 to 10,000 times better).

Certainly not all dogs possess such incredible olfactory sensitivity. To obtain a general indication of the dog’s olfactory sensitivity, Myers (1991) developed a home test for evaluating a dog’s sense of smell. Eugenol, a pure olfactory stimulant, is used in a series of progressively dilute solutions and is systematically presented to a dog. The dog’s reaction to each sample is noted. The odorant evokes a range of unconditioned reactions in dogs, including moving the head, licking the nose, or sniffing. This method of evaluating olfactory function has at least two potential shortcomings. First, the dog’s reaction to the substance must be interpreted by the owner, who may or may not evaluate its reaction correctly. Secondly, the owner may inadvertently (unconsciously) provide the dog with cues to help it perform better. These problems suggest that the method is best suited for determining gross functions rather than subtle olfactory thresholds.

Biological and Social Functions of Smell

Besides the obvious usefulness of an acute sense of smell for the detection of prey animals, many social functions are coordinated by olfaction. Most dogs engage in scent marking and scent-mark investigation. Dun-bar and Carmichael (1981) studied the urinary elimination patterns of laboratory beagles, finding that male dogs spend significantly more time investigating and marking samples of urine belonging to strange males than samples belonging to themselves or other males with whom they are familiar. Their study suggests that dogs are not responding to the smell of urine per se but to some specific pheromonal identifier within the context of urine that excites interest and triggers a marking response. Supporting the view that an olfactory signal triggers the marking response, Shafik (1994) has demonstrated in dogs an olfactory micturition reflex between the nasal mucosa and the urethral sphincters. Electrostimulation of the nasal mucosa appears to relax urethral sphincter muscles in dogs. The author speculates that this reflex induces elimination in the absence of a full bladder, thus contributing to the tendency of dogs to eliminate repeatedly in response to specific odorants rather than in response to signals from pressure receptors in the bladder wall. Other studies have shown that the frequency of sniffing and urinary marking is significantly reduced in animals that have been castrated or rendered anosmia Among rats, testosterone has been proven to play a significant role as a hormonal enhancer of olfactory acuity. Perhaps the decline of sniffing and marking in castrated males is due to relevant pheromones failing to reach thresholds detectable by altered dogs.

Defecation may also serve some olfactory-signaling purpose, although few dogs examine the fecal droppings of conspecifics with the same degree of interest that they exhibit toward urinary scent marks. Some co-prophagic dogs may be very interested in the feces of other dogs, but not apparently for the signaling value of the excrement, but rather for its potential nutritional content. The anal glands secrete a strong-smelling substance into the fecal bolus just before it is excreted. The function of these anal secretions is not known, although dogs will copiously and violently express them when aroused with intense fear. Perhaps anal fluids contain chemical signals that dogs use alone or in conjunction with other chemosensory and physical cues to express or communicate some, as yet unknown, psychosocial intention or meaning. Among wolves, the alphas are most likely to deposit anal gland secretions into their feces and to concentrate their depositions in one area. Houpt (1991) has speculated that dogs scratch the ground after defecating in order to spread the fecal scent around, but this is an unlikely explanation for such behavior. Most dogs rarely (if ever) disturb their excrement (or urine marks) as the result of such scratching activity. Peters and Mech (1975) observed that wolves actually step away from deposited scats and urine marks before scratching. Some species do scatter their feces around after elimination (e.g., the hippopotamus), ostensibly to mark or maintain territory, but dogs do not engage in this sort of ritual. Two potentially significant outcomes of scratching after eliminating is the deposition of identifying pheromonal scents from the paws, augmented by impressive (perhaps, even provocative) visual signs of general size, weight, and vigor impressed into the earth with claws like a signature. This latter observation is consistent with the finding that only high-ranking wolves scratch after eliminating. The sense of smell aids dogs in identifying the sexual status and receptivity of potential mates (Beach et al., 1983). Females begin depositing pheromonal clues about their pending status long before they are actually prepared to accept the males’ advances. Such advanced invitation is widely advertised through increased urine marking on the female’s part. Upon detecting this evidence of incipient estrus on their excursions, male dogs may become highly aroused and motivated by the female’s sexual status but will be roundly rejected if they locate her. Desmond Morris speculates that the reason for these mixed signals is simple — the female secretes these early olfactory signals (pheromones) to maximize the probability of finding a mate:

This may seem like a pointless period of teasing the male. If she will not accept him, why send out all those appealing scent signals? The answer is that it is important for her to ensure that all potential mates are well aware of her condition, so that when the crucial moment comes she will not find herself mateless. Ovulation occurs spontaneously on the second day of the estrus period proper. A day or two after that the bitch is ready to be fertilized. If males are absent then she will have to wait another six months for her next chance. (1986:92-93)

The dog is highly attracted to the vaginal secretions of the estrous female, which he persistently licks, hounding her until at last she consents to his courtship efforts.

Another important function of olfaction is kinship recognition. Hepper observed that puppies recognize littermates and prefer contact with them over nonlittermates, and he speculated that some combination of olfactory and visual information mediates such kin recognition and contact preference: “Pups oriented to the cage by visual cues and then used olfactory cues for ‘close-up’ recognition” (1986:289). Mekosh-Rosenbaum and coworkers (1994) demonstrated that puppies do use olfactory cues to identify littermates. Puppies at various ages were exposed to the bedding of both kin and nonkin conspecifics. Young puppies (20 to 24 days old) spent significantly more time investigating and making contact with kin bedding than with nonkin bedding. After 31 days of age, however, puppies began spending approximately equal time investigating kin and nonkin bedding. Interestingly, between days 52 and 56, male puppies were significantly more attracted to nonkin bedding. They speculate that this shift coincides with a “weakening of the mother-litter bond, leading toward the pups’ ultimate independence” (1994:498).

This change coincides with the optimum time for placing a puppy in its permanent human home at around 7 weeks of age.

The canid habit of rubbing on strong-smelling substances is as common as it is intriguing. The habit appears in puppies as young as 3 months of age. Dogs scented in such a manner are immensely interesting to other dogs, attracting the active attention of conspecifics with whom they happen to come into contact. To the chagrin of the owner, the behavior is often exhibited immediately following a bath. The most commonly posited theory for the habit is olfactory camouflage. By rubbing in the strongest ambient smell, a predator might enjoy some slight advantage while stalking its prey. Although this theory seems plausible enough, it has been rejected by some authorities, based on the hunting techniques of the wolf. A second theory suggests that the habit provides a kind of scent identity shared by the pack — with any strong odor being a sufficient stimulus to excite socially infectious and ecstatic rubbing — regardless of the source. Captive wolves have been observed rubbing in the same scented spot until the whole pack is scented with the odor (E. Klinghammer, personal communication). While the object of such behavior is typically carrion or dung, any strongly odiferous substance will attract the response from wolves — even expensive perfume!. Fox has suggested that dogs may be motivated by “an aesthetic appreciation of odors” (1972:222) or, perhaps, such behavior may serve to enhance social recognition and contact (1971a). Kleiman (1966) suggested that the typical physical movement associated with the pattern is intended to impart the animal’s scent to the object rubbed upon — not necessarily to receive odor from it. Morris (1986) rejected this suggestion, arguing that if the canid’s intention was to mask the odor it would deposit an equally intense smell (feces or urine) — not simply rub on it. He speculated that a possible purpose for the habit is to obtain and share information about the surrounding environment with other pack members via various scents the scouting wolf has rolled upon. Although pack members show great interest in the returning scout and appear to delight in the smells that he has collected, whether this exchange ever results in the initiation of a hunting sortie has not been determined. To my knowledge, there has not been a controlled scientific investigation of this interesting phenomenon.

Ability to Detect and Discriminate Human Odors

Besides playing an important role in the social identification of conspecifics, the sense of smell is also used by dogs to identify people. Furthermore, the manner in which dogs smell and where they smell may be significant. Millot and colleagues (1987) reported that during spontaneous interaction between dogs and children, dogs more commonly sniffed the face during appeasing and friendly interaction, directed smelling to arms and legs during competitive encounters, and directed olfactory interest to the child’s chest and legs when he or she was not behaving in any special way toward the dog. Smell may give observant dogs many clues about the emotional status of their owner or guest. Dogs appear to react differentially to the smells of people according to their emotional states and health. Owners have frequently commented on such abilities being exhibited by their dogs. Reportedly, Montaguer has found that dogs exhibit a repulsion toward the odor of psychotic children. According to LeGuerer, Montaguer performed a series of experiments with childlike dummies, with one dummy wearing undergarments saturated with the smell of a psychotic child while the other one wore undergarments imbued with the odor of a normal child. The dog actively avoided coming into contact with the dummy wearing underwear having the odor of the psychotic child. Edney, who has studied a group of dogs believed to possess the ability to anticipate epileptic seizures in their owners, has speculated that affected dogs may be responding to “distinctive odors generated in the aura phase of epilepsy” (1993:337). Strong and associates (1999) have recently confirmed that seizure-alert dogs can be specifically trained to detect signs of impending seizure. The dogs included in the study were able to warn their owners of impending seizure from 15 to 45 minutes prior to the seizures onset. An apparent beneficial by-product of such dogs was a significant reduction of seizure activity in their owners. Another interesting area involves dogs belonging to diabetics. Lim and colleagues (1992) found 15 cases in which some dogs appear to detect and react to hypoglycemic episodes in their owners. In another study, Smith and Sines (1960) found that rats could be trained to discriminate reliably between sweat samples taken from schizophrenics and sweat samples taken from non-schizophrenic controls. Perhaps, in the future, dogs will serve chemosensory diagnostic functions as yet not fully exposed — for example, the early detection of various mental and physical disease conditions. In addition, a dog’s nose might be usefully employed for the detection of environmental pollutants at concentrations below the threshold of currently available mechanical means.

The dog’s ability to detect and identify human scent is extraordinary. For example, King and coworkers (1964) found that dogs could detect the presence of a single fingerprint placed on a glass slide that was up to 6 weeks old (indoor samples). In their experiment, each discrimination trial involved four blank slides and one fingerprinted slide. Correct choices required that the dog sit in front of the fingerprinted slide. They compared the dogs’ accuracy of detection along two separate dimensions of scent viability: age of scent and the effect of outdoor weathering. Toward this end, some of the slides were carefully preserved indoors while others were exposed to outdoor conditions for varying lengths of time before testing began. The dogs could easily detect indoor fingerprint samples after 3 weeks but were successful only 50% of the time after 6 weeks. They failed to detect outdoor samples reliably after 2 to 3 weeks. Fingerprints on slides covered by a film of water could not be detected.

Kami us (1955) evaluated the dog’s ability to discriminate between the scent of different people, including family members and twins. He demonstrated that dogs could easily and reliably make such discriminations, even between family members — unless they happened to be identical twins. He used freshly laundered handkerchiefs that had been scented from the armpits by the test subjects. The dogs were trained to sniff the hand of the subject and then to select the handkerchief that had been handled by that person. In the case of identical twins, the dogs appeared to treat the handkerchiefs scented by them as identical. This outcome suggests that the preferred scent cues were not incidental olfactory stimuli like clothing, diet, or emotional states. The really interesting result of Kalmus’s study, however, occurred during tracking tests. On the whole, dogs that were given the scent of one twin would readily follow the other, unless both twins laid the track side by side and then split off in opposite directions. Under such conditions, one of the dogs studied consistently tracked the twin who had actually provided the sample scent, suggesting that under certain conditions dogs might rely on other secondary olfactory markers (perhaps incidental and transient) to differentiate the human scent.

A more recent study by Hepper (1988) found that both genetic and environmental factors affect a dog’s ability to discriminate between twins. The experiments used a matching-to-sample method. Twins were instructed to wear two T-shirts over a 48-hour period. The dog was presented with one of the T-shirts to sample for several seconds. Meanwhile, the matching T-shirt along with the other twin’s T-shirt had been crumpled and placed into a small plastic trough standing 10 feet away from the dog and handler. The dog was sent to retrieve one of the two T-shirts. Hepper found that dogs could accurately discriminate between twins so long as they differed in one of two directions: genetic relatedness or environment factors (e.g., diet). The dogs were unable to discriminate between infant identical twins if they had been fed the same diet.

A few years ago, Brisbin and Austad (1991, 1993) evoked a controversy by suggesting that dogs could not reliably match scents collected from different parts of the body to the correct human donor, thus contradicting Kalmus’s previous finding that scent samples taken from the armpit could be accurately matched with scents taken from the hand. Their study aimed at determining the extent to which dogs could generalize scent discrimination training and matching abilities to scents collected from different parts of the body. The study was limited to three dogs — all previously trained to discriminate scent articles (AKC Utility Test) from scent collected from the hands only. None of the dogs had any previous experience involving the discrimination of scent from other parts of the body or law-enforcement experience. The researchers found that when the dogs were prompted to discriminate the scent sample taken from their owner’s elbow from the scent samples collected from the hands of a stranger, they were only successful 57.9% of the time (results not rising above statistical chance).

In reply to Brisbin and Austad, Sommerville and colleagues (1993) criticized their study, arguing that the resulting findings suffered from an inherent ambiguity stemming from the way in which the dogs were trained and tested. For one thing, the dogs involved were trained to discriminate only scent collected from the hands and were naive with regard to the discrimination of scents obtained from other parts of the body. Ostensibly, the dogs had learned the scent signature of hands but not a reliable specifying signature of a person’s identity per se. According to Sommerville and associates, the results reported by Brisbin and Austad were inconclusive, measuring an artifact resulting from inadequate preparatory training rather than a lack of ability to generalize or match scent accurately. The researchers subsequently carried out a much more extensive study of their own to test this general hypothesis. In contrast to the negative findings of Brisbin and Austad, they demonstrated that, if properly selected and trained, dogs can reliably discriminate and match body scents collected from different parts of the body to the correct donor:

Our results show that dogs can efficiently match objects bearing the scents of individual humans whom they do not know even when the scented objects have been in contact with different parts of the body and collected with no particular precautions to avoid environmental contamination. … Our results suggest that if dogs are selected well, sympathetically trained and entirely dedicated to scent discrimination in a well-managed unit they are likely to maintain a dependably high performance over long periods. (1994:1446-1448)

The significance of the Brisbin-Sommerville controversy is to underscore the importance of careful selection, extensive training, and testing/certification of dogs used by law enforcement for tracking and identifying suspects.

Localizing the Origin and Direction of Odors

The primary function of olfaction in dogs is to detect and locate odors emanating from the surrounding environment. Von Bekesy (1964) performed a series of experiments to determine whether olfactory localization occurred in a manner analogous to directional hearing. He discovered that in the process of sniffing there exists a small time delay between the odorant entering one nostril before reaching the other, unless the source of the odorant is located directly in front. A difference of as little as 0.3 millisecond between nostrils was found sufficient to calculate the odorant’s general direction of origin. He also found that differential olfactory analysis of the relative concentration of the left sample as compared with the right one provided additional information about the odorant’s location. From this information, a gradient is formed from which a dog can calculate the approximate direction of the origin of the odorant by the differences of concentration entering the respective nostrils.

Schwenk (1994), who has studied the chemosensory locating abilities of snakes, has shown that a snake’s tongue serves a similar direction-finding function as that performed by the separated nostrils of mammals. Scent gathered by one fork of a snake’s flicking tongue is slightly more or less concentrated than scent gathered by the other. By comparing these differences via the vomeronasal organ (VNO), a snake is able to trail and locate prey animals wounded with venom. If the forked portion of the tongue is severed, a snake is unable to trail. Further, if one side of the VNO is blocked, a snake tends to trail in the direction of the unblocked side, consequently moving about in a wide circular path.

Few trails in nature are found at their source but are crossed and detected at some arbitrary point along their length. Determining which way to go once a trail is located is a challenge to the olfactory abilities of predators. This is also a problem of considerable importance for dogs trained to track people. McCartney (1968) discussed early directional tracking experiments carried out by Belleville, a Berlin police officer, who found that trained tracker dogs, when led to the midpoint of a track and started at right angles to it, chose the correct direction in only 47% of the trials. He concluded that the correct determination of track direction by the dogs was probably based on little more than chance. Other disappointing reports confirmed that the directional choice appeared to be based on statistical chance. Similar results were found by MacKenzie and Schultz (1987), who tested 22 dogs trained to track but not trained to determine the direction of the track. Although six of the dogs exhibited perfect scores, the overall statistical picture for the group of dogs as a whole was not much better than random chance.

In contrast to these earlier difficulties with directional tracking, Steen and Wilsson (1990) found that professional tracker dogs can reliably choose the right direction on the basis of olfactory information alone. The researchers laid 50-m tracks on grass and on an asphalt airstrip. After 20 to 30 minutes, the dogs were brought to the track, faced in a perpendicular direction relative to the track, and unleashed. The dogs reliably determined the correct direction of the track.

Thessen and colleagues (1993) have confirmed the earlier findings of Steen and Wilsson with dogs previously trained for directional tracking. They tested German shepherd tracking dogs on fresh tracks 20 minutes old on grass and 3 minutes old on concrete (10 trials per dog on each surface). The dogs were equipped with a remote microphone and transmitter that recorded sniffing sounds. Other movements were recorded by a video recorder. The researchers found that directional tracking involves three distinct phases. (1) A searching phase during which the dogs moved and sniffed rapidly. The dogs sniffed at a rate of approximately 6 times per second during all phases. (2) A deciding phase characterized by slower movements and longer sniffing periods and with the dog’s nose placed closer to the ground. The deciding phase lasted 3 to 5 seconds and involved the dogs sniffing at two to five footprints before choosing a direction. (3) A tracking phase involved more active movement and sniffing, similar to those observed during the searching phase.

Steen and Wilsson (1990) have hypothesized that a dog’s ability to determine the direction of the track depends on a comparison of olfactory concentrations emanating from consecutive steps, thereby forming an olfactory intensity gradient. If this is true, it empirically confirms the incredible power of the dog’s nose:

If we assume that each footprint smelled the same at the moment it was set, and that the scent evaporated at a constant rate, we can get an idea of the dogs’ sense of smell. We walked at a rate of one step per second and tracks were 30 min (1800 s) old when the dogs were tested. The smell from one print should therefore theoretically be 1/1800 stronger than that of the foregoing. This indicates that the dogs were highly sensitive to an odour difference of this magnitude. (Steen and Wilsson, 1990:534)

As extraordinary as these numbers seem at first glance, most trails in nature are far older and demand even greater sensitivity for determining their directionality than required by the experimental arrangements producing the above estimates.

William Carr and associates at Beaver College (Glenside, PA) have studied various factors believed to influence the acquisition of directional tracking. Of particular interest is testing the intensity-gradient hypothesis proposed by Steen and Wilsson. The intensity-gradient hypothesis presumes that dogs can detect a difference of polarity/intensity existing between successive steps made by a track layer, possibly because the scent associated with the preceding step has undergone perceptible diminishment relative to the scent adhering to the succeeding step. To test this hypothesis, they have performed a number of directional tracking experiments comparing the dog’s performance on normal and polarity-enhanced tracks.

In one experiment, two previously trained dogs were tested. One of them was tested on a normal track laid at a rate of 1 step per second. The second dog received identical testing but on a polarity-enhanced track laid at a rate of 1 step per 10 seconds. This was accomplished by the track layer resting upon a walker and holding the trailing step up for 10 seconds before stepping down again. The operative assumption here is that an increased delay between successive steps would make it easier for dogs to detect differences between them, perhaps as the result of scent dissipation or some unknown qualitative change in the scent picture. As expected from previous experiments, the first dog responded correctly on 14 of 20 trials, whereas the second dog, working on the polarity-enhanced track, responded correctly in 17 of 20 trials — a 21% improvement over this dog’s previous score on a normal 1 step/second track. In another experiment, the interval between critical steps at the choice point was increased to 80 seconds between steps. This arrangement resulted in correct directional choices in 80% to 90% of trials. These experiments appear to confirm the earlier findings of Steen and Wilsson regarding a trained dog’s ability to determine the direction of track above chance levels of significance, as well as provide significant evidence supporting the intensity-gradient theory of directional tracking.

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