RLANDO, Fla. — Under the watchful eye of their trainer, two diminutive beagles are working their way past suitcases randomly laced with beef, pork, apples, citrus and mangoes.
For the dogs and their handlers, who are new at this, the exercise occasionally becomes a confusing jumble of commands, sights, smells, sounds and rewards. But once they have mastered the 10-week training course the dogs will don the distinctive green vests of the Agriculture Department's Beagle Brigade.
The dogs are the department's primary defense against foods from abroad that can unloose disease, pests and blights, threatening agriculture and health. Jay H. Weisz, who directs the department's National Detection Dog Training Center here, said about 65 agricultural canine teams were already at work at airports, postal depots and border crossings, and that number was to double by next year.
Nonthreatening to weary airline passengers and highly motivated by food rewards, beagles and other dogs are in high demand for scent work. Increasingly, they are employed to sniff out explosives, guns, drugs, gold ore, gas pipeline leaks, termites, sea turtle eggs, endangered species, traces of flammable compounds used in arson, brown tree snakes hiding in cargo bound for Hawaii, gypsy moth larvae, estrus in cows and underground water leaks, according to experts in the field.
Dr. Lawrence J. Myers, professor of veterinary medicine at Auburn University and an expert in dogs' remarkable sniffing abilities, is investigating claims by some experts that dogs can even detect melanoma and other cancerous tumors.
Though hounds like beagles are renowned for their sense of smell, as a rule, Dr. Myers said, there is more difference in scenting ability between individual dogs than between breeds of dog. Nearly any dog can be trained to detect specific targets using a system of rewards — food, hard rubber balls and towels are employed by different programs — as long as disease and injury have not impaired its sense of smell and behavioral problems do not hinder its ability to learn, he said.
Trainers look for a physically healthy dog with a strong desire to hunt and retrieve objects and then seek to channel that desire. Many programs prefer traditional police dogs or retrievers and sporting breeds, although mutts and Jack Russell terriers, as well as beagles, are also used.
Despite the growth in number and applications of dogs for detection work, little research has been conducted into their capabilities and olfactory processes. Even less is understood about the complex dance of dog and handler, although that dynamic can have a profound affect on success rates, Dr. Myers said.
The few studies that have been conducted indicate that a well- trained dog and accomplished handler can achieve an accuracy rate of about 95 percent, significantly better than any machine. But, Dr. Myers said, any combination of a bad dog and bad handler can drop that figure to around 60 percent.
For example, dogs can learn to respond to unconscious cues from trainers and end up searching for objects they are not trained to find. They also can stop performing well if not properly stimulated and rewarded for their efforts. Temperature and humidity can affect a dog's ability to smell, as well.
Dr. Gary Settles, professor of mechanical engineering at Penn State, says his research suggests that when a dog inhales, the alar fold, a bulbous obstruction just inside its nostrils, opens to allow air to flow clearly through the upper part of the nose across the mucus-covered scent receptors.
When air is exhaled, the alar fold closes off the top part and directs air down and out through the slits at the side of the dog's nose, Dr. Settles said. The process creates a kind of suction that helps the dog inhale even more odor-laced air while also stirring up particles that might help deliver more scent.
Once inside the nose, chemical vapors — and, perhaps, tiny particles — dissolve in mucus-covered olfactory receptors, which in dogs number around 220 million (roughly 40 times the number found in humans), Dr. Myers said. The chemical interactions are converted to electrical signals that travel along the olfactory nerve to the olfactory bulb and then to nearly all parts of the dog's brain.
Dogs and other animals that rely heavily on the sense of smell can identify odors concentrated in an object or piece of ground as small as a dime, Dr. Settles said, teasing from it all sorts of information.
Scent receptors in insects generally are sensitive to particular pheromones that bring about specific behaviors, said Dr. John Kauer, professor of neuroscience at the Tufts University School of Medicine.
But in dogs and many other animals, individual receptors in the nose can become sensitive to a number of different, unrelated chemicals, Dr. Kauer added. That system creates patterns of odor signals that are then processed in the brain, in ways still not understood, to allow the animal to identify an object and its status and respond accordingly.
Several other biological systems are involved in smell in ways not fully understood. For example, dogs have an organ above the roof of the mouth, behind the incisors. Over the years, many people have suggested that this so-called vomeronasal organ detects pheromones, but researchers say there is no solid evidence of that.
Dr. Kauer is using knowledge of how dogs smell to build an artificial nose for detecting land mines, but so far his best effort is only a tenth as good as a trained dog, he said.
Dr. Myers has shown that scent emanates from an object in a plume that swirls and eddies in a turbulent flow so that there are patches of dense odor and areas of faint odor. In working, a dog quickly scans back and forth with its nose, scanning those densities, until it comes to what it thinks is the source.
That may not always be the object itself, Dr. Myers said. Rather, because of air flow, the source could be in an upper floor of a building or the opposite side of the room. Skilled handlers confirm that their dogs have detected scents emanating from distant sources.
Yet it is also not uncommon for dogs encountering a room full of drugs or a bag loaded with, say, suspect mangoes to fail to alert their handlers. Although the reasons are unclear, the odor in those cases may be so overpowering and evenly distributed around the room that the detector dogs cannot pinpoint the source, Dr. Myers suggested, so they do nothing.
Also, their noses can become saturated with particular odors and desensitized to them, he said. Then, the dog literally needs time to purge the odor-laden mucus from its nose.
Dr. Myers said recent research suggested that two different dogs trained in the same way could respond to entirely different chemical components of an odor and that the components to which they respond could change over time.
The study has clear implications for a dog's success rate because if it is keying on an already faint chemical, when the amount of the substance decreases, the dog may miss detecting it, Dr. Myers said.
That finding and others, researchers say, also point to the danger of using pseudo scents, which are training aids developed to replicate the primary chemical composition of a drug or explosive.
Although the Beagle Brigade trains on actual food, other detection dogs rely on those pseudo scents, but because their chemical composition is limited, it is often difficult for dogs to generalize from those artificial compounds to the actual substances, where chemicals interact and play off each other to form a complex bouquet.
Over the years, researchers have trained rats, ferrets and other animals to detect explosives and drugs with success equal to that of dogs. Used to hunt truffles, pigs are well known for their olfactory acuity.
But people have always returned to dogs for detection work, not only because of their ability to discriminate between odors but also because, in their long association with humans, they have been bred for sociability and trainability.