ANCHORAGE, Alaska - They're slow-footed and bullheaded. When threatened, they huddle up. During breeding season, males charge one another and crack heads with such force the sound can sometimes be heard up to a mile away.

But the musk oxen of the Seward Peninsula in northwest Alaska love sex. How else to explain how one of the biggest success stories among large Alaska wildlife - particularly a species that produces just one calf a year. In 1980, just 104 musk oxen roamed the peninsula, according to the Alaska Department of Fish and Game. Today, after 25 years of very restricted hunting, that number is nearly 3,000 animals.

"It's phenomenal," said wildlife biologist Brad Shults of the National Park Service, who is studying them as part of a project to compare the population and ecology of musk oxen on and adjacent to Bering Land Bridge National Preserve to those around Cape Krusenstern National Monument along the Chukchi Sea near Kotzebue. "It was averaging around 16 percent a year, although once we starting clipping the population with hunting, it leveled off."

Ken Adkisson, subsistence program manager for the National Park Service in Nome, agreed.

"Basically, the animals have done very, very well on the Seward Peninsula," he said, particularly herds on the northern portion of the peninsula.

In January, a subsistence registration permit hunt opened near Nome that allowed 34 bulls to be taken and stipulated that horns must be destroyed by Fish and Game. Twenty-nine bulls and five cows were eventually killed in the hunt that lasted nine days in Unit 22C before being shut by emergency order.

Some would like to see hunting expanded.

"The harvest of oxen ... should be increased for the benefit of the herd and range quality," outfitter Brian Simpson wrote in a proposal to expand the number of animals available to nonresidents, which the state board of game will consider at its meeting in November. "Many residents who draw a permit do not hunt ... when they become aware of the logistical difficulties and expense. To date, any resident hunter who truly wishes to hunt this species has had unrestricted opportunity."

Although hunting may have slowed the population increase, it hasn't stopped it.

"We're still experiencing growth," said Fish and Game area biologist Tony Gorn of Nome. "It fits into one of these models of an animal moving into previous unoccupied habitat."

Musk oxen from Greenland first arrived in Alaska in the 1930s, with a small group transplanted to Nunivak Island. In 1970, three dozen of the Nunivak Island animals were transplanted near the Feather River, about 36 miles from Nome. A second transplant followed in 1981, with the release of 35 more animals at the Port Clarence Coast Guard Station, 15 miles west of Teller.

The last Seward Peninsula survey in 2007 found 2,688 of the herbivores that can weigh upwards of 800 pounds - big enough that they seldom flee.

"They just stand there," Norman Manadelook of Teller told National Wildlife magazine in 2002. "Musk ox can be a nuisance. They're not scared of people or of dogs. You practically have to drive them away."

Some locals insist musk oxen are devouring traditional caribou range, harming a species that's an important food staple. Others are concerned they damage grave markers while attempting to scratch themselves.

"The big criticism I've heard is musk oxen stomping areas where people want to pick greens," Gorn said.

But the musk oxen's stubbornness, while frustrating at times, may have a redeeming aspect: Wildlife watchers couldn't ask for a more cooperative animal. Visitors increasingly are venturing out from Nome seeking wild musk oxen, a rare sight in North America.

"It's great. I think it's world class viewing," Adkisson said. "I've had people say that's one of the reasons they've come to Nome."

Already popular with birders seeking rare North American species, Seward Peninsula can offer wildlife fans what Adkisson calls a "double treat" with musk oxen. Most times, the effort doesn't require much more than getting a rental car and asking where herds have been spotted recently.

"The Nome road system offers an amazing opportunity to see musk ox," Gorn said. "It's an experience unmatched anywhere, and it's definitely increasing. In fall, you've got rutting bulls. When the calves are dropping in spring, they're very social. It's pretty neat."

Last summer, he said, passengers aboard cruise ships docked in Nome would tour the outlying roads for wildlife.

"At times, the Nome road system looked like the Denali Park road," he said.

But the permit hunt earlier this year disappointed Gorn.

"It didn't go off as smoothly as we wanted," he said. "It was a real change in opportunity for people who for a long time wanted to hunt musk ox. But people weren't accustomed to hunting herd animals."

Some hunters had trouble distinguishing bulls from cows. And at least four calves were killed by pass-through shots in which bullets went through a bull and killed a calf behind it. Gorn worries that other pass-through deaths may have gone unreported.

"It's an unprecedented opportunity," he said, "but it's going to take a little bit to get everything figured out."

And coexisting with a large horned mammal capable of charging has similarities to coexisting with moose.

Melissa Owens of Nome, who finished 30th in her Iditarod rookie run in 2008, found that out on a training run when a herd of oxen bluff-charged her dog team before she could get them turned around.

Chaos ensued, but no animals were injured.