PUEBLO, Colo. (AP) - For a hint of what lures Mark and Carol Rickman to Yellowstone National Park at least three times a year, just listen to the greeting on their answering machine. Callers first hear an eerie, almost otherworldly chorus of faint howls. Then a human voice comes on to say "The 'pack' is out but someone will return the call as soon as they get back to 'the den."'

You might say the Rickmans have gone ape over wolves, so much so that they spend six to eight weeks a year in Yellowstone, watching them, recording their movements, their interactions and their daily lives. The couple is enthralled by the wolves' role in the ecosystem, by their haunting howl on a winter's night and by their behavior.

"They're such neat animals, and very human in many ways," Carol says. "They're intelligent, organized, family-oriented. You've never seen anything as cute as a wolf playing with her pups."

Mark, an anesthesiologist, and Carol, a medical technologist, work at Pueblo hospitals most of the year. But every chance they get, the Rickmans drive the 500-plus miles to Yellowstone to watch North America's largest wild canines.

Carol is a Pueblo native, and Mark grew up in Littleton, so they share Western roots. As children, they both loved dogs, foxes, coyotes and wolves. But they were especially fascinated by wolves. "I'd always thought they were beautiful animals, and the more I learned about them, the more I wanted to know," Carol says.

Mark had visited Yellowstone many times, and after the couple married, he took Carol to the park for her first visit in 1993. "I immediately fell in love with it," she says.

The Rickmans would go during the winter to cross-country ski, and in summer to hike. They were intrigued when they heard wolves were being reintroduced to the park, and experienced their first sighting Oct. 1, 1997.

"We saw Wolf 39," Carol says. "The first time we saw her, we watched her sleep for two hours. We were so excited, we didn't care."

"It was mesmerizing," Mark says.

They were hooked.

They even bought a cabin just outside the northeast entrance to the park, a cabin often used by other wolf-watchers.

"It's sort of a wolf-watcher's flophouse," Carol says, stroking her 14-year-old husky mix, Chinook, who accompanies the Rickmans on trips. "I think we've only spent about two nights there alone."

It's no surprise that the Rickmans are avid fans of the wolf-reintroduction program, which has been so successful that the animals might be taken off the endangered list. The couple was recently interviewed for a program on National Public Radio about the restoration of the predators to the park.

Since the reintroduction of wolves, the elk population has come under control, allowing grasses and shoots of small trees to thrive, scientists say. In turn, beavers and other small mammals and rodents are thriving and providing food for birds of prey.

"Wolves are so much better at environmental control than people," Carol says.

"It's all about balance, and they're an essential part of the picture," Mark adds.

Each time the Rickmans go to Yellowstone, they hook up with Rick McIntyre, a park employee, author and wolf behavior expert. He trusts them to track and record the daily activities of wolves in the park, and he uses their findings to better understand the behavior of the wolf packs there.

But the Rickmans are not the only individuals who do this. People from all over the world travel to Yellowstone to help McIntyre watch the wolves, for their own enjoyment and for scientific research.

"You wouldn't stand out in the cold from dawn till dusk if you weren't committed to helping," Mark says.

Carol, a volunteer at the Pueblo Zoo and one of its representatives to the Wolf Forum of the Southern Rockies, says the couple's commitment to the wolves will last indefinitely.

"We just persecuted them for such a long time that it's just wonderful to see them being brought back to the ecosystem," she says. "I just never want to take for granted the fact that they're there."

On the Net:

Wolf restoration site: http://www.nps.gov/yell/nature/animals/wolf/wolfrest.html

AP-WS-04-14-04 1704EDT