Last April, in a narrow mountain valley in northwestern Colorado, Cristina Eisenberg was searching for scat. The diminutive, dark-haired biologist and two members of her field crew had set up a kilometer-long transect through elk habitat, and the trio was walking slowly along the line. It was a raw day, cold and windy with spells of freezing rain, and the biologists had been moving through meadows for hours, looking for elk poop, deer poop, coyote poop, mountain lion poop. This was old-fashioned wildlife biology -- hardly glamorous work -- but in it lay the story of the landscape, of the pursuers and the pursued, and Eisenberg was absorbed in the tale.

Then, on the edge of an aspen grove, one of the biologists saw something unusual: a scat roughly as long and wide as a banana, tapered at the ends, perhaps two months old. When Eisenberg examined it, she saw that it contained hair from deer or elk and shards of bone, some almost as long as a fingernail. It smelled distinctively earthy, like a shady forest floor.

In the course of her research, Eisenberg had seen and handled thousands of scats just like this one, but not here, not in Colorado. Everything about it -- the size, the shape, the smell, the contents -- indicated a creature that had been extirpated from the state more than 70 years ago. Everything about it said wolf.

Within an hour and a half, the crew found a similar scat, some 500 yards away. Later that day, in another aspen grove about five miles away, they found two more. Less than a week later, Eisenberg's lead tracker, Dan Hansche, found a wolf-like scat with a similar, smaller scat laid on top -- suggesting, Eisenberg says, that an adult wolf had been teaching its pup to mark territory.

As the weather warmed last summer, the field crew found 11 more wolf-like scats, and Hansche documented a set of tracks with wolf characteristics. Then, at dawn on July 27, Eisenberg and another biologist were driving down a winding valley road, deep in a discussion about statistics, when Eisenberg spotted a black shape running across a bright green alfalfa field, perhaps 100 yards away. The stance, the gait and the set of the ears all suggested the wolves Eisenberg had spent so much time observing near her home in northwestern Montana.

This past November, during another trip to her Colorado study area, she found another set of wolf-like tracks, fresh prints that extended at least a quarter-mile up a snowy ranch road. All in all, Eisenberg and her crew found some 18 separate signs of wolf activity during visits over a seven-month period. This animal -- or animals -- was no mere passerby.

Officially, wild wolves do not live in Colorado. The nearest established population is in Wyoming, where gray wolves were introduced to Yellowstone National Park in 1995. But rumors of wolf sightings abound in Colorado, and in recent years, at least two wolves have died in the state. In 2004, a young radio-collared female wolf from Yellowstone was killed on Interstate 70 near Idaho Springs, about 30 miles west of Denver. In the winter of 2009, another young female collared wolf traveled a 1,000-mile-long route from the Yellowstone region to the Meeker, Colo., area, roughly 20 miles from where Eisenberg and her crew work. That wolf's death, in April, is still under investigation by the U.S. Fish and Wildlife Service, and neither state nor federal officials will comment on the matter.

Like most scientists, Eisenberg and her colleagues are cautious. For months, even among themselves, they half-jokingly spoke of "visitors from the North," reluctant to name a species as controversial as the gray wolf. They emphasize that DNA testing, now under way at a lab at the University of California, Los Angeles, is needed to back up their identification of the animal or animals that produced the scat and tracks. But whatever the animal is, it appears to be eating what wild wolves eat, and traveling over the landscape the way wild wolves do.

When wolves arrive in an ecosystem, everything changes: the ecology, the politics, relationships both animal and human. "We know more about wolves, and the management of wolves, than we do about many other forms of wildlife," says Douglas Smith, leader of the Yellowstone wolf project. "But we rarely get to put it into practice, because people freak out, flat-out freak out, when a wolf shows up."

Wolves herald a grand experiment -- and in Colorado, that experiment may already be under way.