BLACKFOOT VALLEY, Mont. - Squatting next to his quarry, the dog's eyes locked onto an athletic blond woman who was approaching cautiously.

Her hand reached into her backpack and the Belgian Malinois rose slowly to his feet, his butterscotch eyes burning with intensity. As the woman brought out a strand of knotted rope and leather, he lunged and snapped his powerful jaws around the tanned hide.

He bared his teeth and began to pull, peddling backward. She set her heels and the two whirled around the Western Montana grassland on this crisp autumn morning in a ferocious game of tug of war.

"This," said Megan Parker, "is his paycheck."

The Belgian sheepherder's name is Pepin. He's no hunting dog or sporting breed out for fun. He's a working dog, one of a few dozen highly trained, toy-crazed canines that are changing the way wildlife biologists such as Parker figure out what's lurking in the woods.

These dogs of various breeds don't rely on their eyes, the way puny-nosed humans do to try to make sense of the world. They are trained to use their pronounced noses and superior sense of smell to canvass the landscape for animals, animal scat, rare plants and invasive weeds that too easily elude human discovery.

These elite detection dogs have sniffed out invasive, predatory snails in Hawaii and tree snakes in the jungles of Guam.

They've climbed mountains in Central Asia for telltale signs of snow leopards, hunted for nearly extinct rhinos in Vietnam, padded through Kenya in pursuit of cheetahs and tracked moon bears in China.

Some have taken position on the bow of small boats off New England and used their noses and enthusiastic body movements to point biologists toward the few remaining North Atlantic right whales. Or more precisely, they help steer them to orange, stinky whale waste left behind, which floats for about a half-hour before sinking.

Much of the work involves looking for scat, which can offer a bounty of clues to the health of wildlife populations. Besides analyzing diet and parasites, scientists examine hormones to determine gender, health and stress levels, and extract DNA that confirms the type of species, families and individuals. All that makes possible rigorous population surveys.

Genetic analyses are costly, though, so it pays to use a discriminating nose to determine what samples go to the lab. Detection dogs learn to locate specific targets through extensive training that involves trial and reward: Place a known type of scat in one of a half-dozen jars and teach the dog to associate finding it with a reward. Such exercises get more complicated by setting the scat in a field of mixed smells and rewarding the dog for picking up one specific scent and following it to its source.

Parker, director of the nonprofit Working Dogs for Conservation, relies on Pepin to help her sort through piles of purported snow leopard feces shipped to her from Mongolia. A few weeks ago, she had set out more than 100 samples on cinderblocks in her yard in Bozeman, Mont.; Pepin selected the real thing. His nose knows, she said. "It saves the project a lot of money."

On this fall morning in Western Montana, Pepin headed off to work, bouncing along a dirt road in the back of Parker's Volkswagen hatchback. He couldn't be happier.

Parker and other biologists were working to keep wildlife corridors open for wolverines and other reclusive predators that require room to roam. Now, a wolverine had been spotted on a private 20,000-acre ranch.

When Parker opened the hatchback, Pepin bolted. He tore around the grassland, bouncing with excitement. "He's a psycho-dog," Parker said. In this line of work, that's apparently a compliment.

Trainers have found that working breeds - herders, retrievers and shepherds - often have what it takes for demanding days in the field. It's actually less pedigree and more personality that matters: the intense, type-A, high-energy, obsessive, toy-crazed "play drive."

It's the kind of dog that makes a lousy pet. Without a task, they channel all of their high-strung, neurotic energy into inventing games, like tossing and chewing shoes or rearranging the furniture, one bite at a time.

Dog pounds and shelters are prime recruiting spots. A first test often involves bouncing a ball down the aisle among the cages. Forget the growlers, the lungers, the yappers, the submissive ones that roll over for a belly rub. The pooches with potential cannot keep their eyes off the ball.

"These dogs are so driven, they will work all day to get their ball," said Samuel Wasser, who uses detection dogs as a biologist at the University of Washington's Center for Conservation Biology. "You can throw their ball into their food and they would rather have the ball."

Back on the ranch, Parker engaged in a faux face-to-face wrestling match with Pepin on his hind legs. It was to work off a little steam. Then she instructed him to sit and buckled on a bright orange vest emblazoned with the words "Search Dog."

Pepin went motionless, quietly focused on his master. She put her nose to his and said, "Go to work."

With that, the shepherd unleashed a high-pitched yelp and bounded down a logging road.

Parker trailed after him as he zigzagged across the frozen grassland warming in the early morning sun. His tail whirled, his long tongue dangled from the side of his mouth.

"Check," Parker said, pointing two fingers to the left.

The dog turned, jumped across a log and, with nose in the air, disappeared down a steep embankment into the pine forest. The bell on his collar jingled from the dense woods. A minute later, he burst back into view and exchanged looks with Parker. Nothing.

He lunged in the other direction, eager to continue the search. His nose remained in the air.

Unlike bloodhounds or other trackers, which keep their noses to the ground to smell skin cells that have sloughed off a target, these "air scent" dogs try to catch a whiff of a scent cone that wafts away from its source, much like a trail of cigarette smoke.

In a game of hot-and-cold, the dogs follow the scent back to the source, using their abundance of olfactory sensors and a large portion of their brain - 40 times larger than its equivalent in a human brain - that is hard-wired to identify smells.

Dogs are twice as good at finding scat on roads as humans, and seven times better in vegetation, said Alice Whitelaw, programs director at Working Dogs for Conservation. "We're visual creatures and the dogs make it clear how little we know."

Whitelaw and her German shepherd, Tsavo, recently were looking for a rare flowering lupine on the Oregon prairie when the dog stubbornly ignored a cluster of lupine spotted by the humans. They thought he was making a mistake until a botanist showed up and set them straight. "It happens over and over again," said Debbie Smith, a biologist and dog handler. "The dogs know what they are doing."

Genetic analyses confirmed the detection dogs got it right 98 percent of the time when searching a few years ago for grizzly bear, wolf and cougar scat across 230 square miles in the Centennial Mountains, which straddle the Montana-Idaho state line and connect Yellowstone National Park to the northern Rockies. The evidence was so strong that authorities rejected a planned golf course and subdivision of 1,200 luxury homes that threatened to block a wildlife superhighway and make the grizzly population in Yellowstone more genetically isolated.

"It would have been logistically impossible to trap and collar enough grizzly bears and animals to do the study," said Jon Beckmann, the coordinating scientist with the Wildlife Conservation Society.

Scat-detection dogs are among a number of popular noninvasive techniques used for wildlife surveys. Others include setting bait traps and then using camera traps to snap photos, or placing sticky rub-pads or barbed wire hair-snags to collect bits of fur and skin for DNA analysis. None of these takes as much time, nor stresses the animals as much as if they were trapped, tranquilized and then tracked via radio collars.

Using dogs brings an added advantage: avoiding bias that can skew survey results. "You get trap-loving and trap-shy animals," said Wasser, the University of Washington biologist. Male bears are more willing to take risks than females, especially those with cubs. Dogs find scat from all target animals.

The dogs aren't infallible. Some get distracted. They can err because of poor training or miscues by inexperienced handlers, who can confuse matters by rewarding the wrong behavior.

After all, the dogs are only interested in specific plants or poop for one reason, as Whitelaw explains it, speaking for her three shepherds: "OK, I found it, lady. Now give me my damn ball."

Working his way across the ranch, Pepin showed how temptations abound. He had treed a squirrel. The dog's front legs were bouncing off the trunk, paws clawing the air.

"Hey, you're working," Parker called out. "You can chase squirrels on your off hours."

It was enough to snap him out of play mode. He went back to sweeping the area for fecal matter.

Parker came across droppings that Pepin had seemingly missed. The dung was squished into the ground by bulldozer tracks.

"Check," she called to Pepin. He came back, nostrils twitching. He turned up his nose. Nothing of interest.

Pepin is trained to recognize the scat of wolverines, grizzly bears, black bears, snow leopards and fishers, a forest-dwelling cousin of the weasel and marten. This wasn't any of those.

A couple miles away he picked up the scent of fresh grizzly bear dung. Running back and forth, he found the source and quickly laid down, careful not to touch it so Parker could bag an uncontaminated sample.

The dog stared at Parker.

His butterscotch eyes dilated with anticipation. She reached in her backpack. His tug-toy emerged. Oh boy.