Alaska’s Comeback Trail : As caribou and musk ox herds in the state expand, a research station is learning how the beasts brave the Arctic.

There was a time when 600-pound Ice Age relics known as musk oxen roamed with mastodons and hairy mammoths across the northern plains of the continent. But they were intensely hunted for their longhaired hides, and the musk ox became extinct in Alaska.

Their primary defensive posture was to encircle the most vulnerable members of the herd. With their tails in and their heads out, the powerful bulls were able to defend their young and weak against wolves and other predators.

It worked with the wolves, but it set them up like a shooting gallery for the whalers who cruised along their coastal habitats. The last musk ox in Alaska was killed in the 1850s.

Now, thanks to a joint federal-state project, the musk ox is being reintroduced in an area it once called home. And it is doing quite well.

About 5,000 musk oxen now live in the wilds of northern Alaska, said Robert White, director of the Institute of Arctic Biology at the University of Alaska.

White runs the university’s Large Animal Research Station on an old 150-acre homestead on the outskirts of Fairbanks. Until the station was established 10 years ago, scientists were limited to studying the great beasts of the far north in the wild, and that posed real limitations.

But the establishment of the station has enabled scientists to carry out long-term experiments in a tightly controlled environment on the musk ox and the little-understood caribou on a scale they had never reached before.

Current research at the center has forced scientists such as White to change their thinking on some issues. White, for example, no longer believes that hunters alone accounted for the demise of the musk ox.

His studies have shown that the wild musk ox is disinclined to roam in search of food, and it probably will live its entire life in a single valley.

During the winter, when the Arctic tundra is covered with snow, the musk ox survives by finding hillsides where it can eat vegetation exposed by the wind. But during years of exceptionally heavy snowfall, possibly brought on by a warming trend, the snow never leaves and the musk ox doesn’t try to burrow through it to the grass below. So it starves to death, inches from nourishment.

That discovery has led White to believe that changes in the environment, as well as hunting, caused extinction of the musk ox in Alaska.

In addition to 51 musk ox, White and his colleagues at the station are studying 33 caribou and 27 reindeer, a subspecies of caribou.

Caribou, along with the reemerging musk ox, are symbolic of life on the last frontier. Caribou herds, ranging into the tens of thousands, are a natural spectacular seen by few human eyes.

The animals were here long before humans crossed the Bering Land Bridge from Asia and entered the New World at least 12,000 years ago.

White is rewriting the textbooks on how these large creatures survive.

Herds of caribou, sometimes numbering in the thousands, migrate for hundreds of miles each year, traveling from winter feeding grounds to the coastal plains, where they give birth. The caribou senses that, needing about 12 pounds of grass and birch leaves a day to survive, it is necessary to stay on the move.

For decades, these members of the deer family were thought to be simple creatures, governed by simple rules of survival.

But now, scientists are learning that is far from the truth.

“We’ve just found in the last 10 years how complex the caribou is,” White said. “And what’s really neat, we’re also finding out the rules of the game.”

Ten years ago, for example, scientists thought that nearly every caribou cow gave birth to a calf every spring. But research at the center has shown that reproduction depends on a wide range of variables.

Among the new findings concerning caribou:

* Many cows do not become pregnant, because they lack adequate food resources. The ability to become pregnant is controlled by the animal’s fat levels.

* Calves that are larger at birth have a greater chance of surviving, because they are able to accompany their mothers at an earlier stage. Researchers have found that the larger calves are born to females that have strong protein reserves entering the winter before the calving season.

* Biologists had thought that young caribou were left pretty much on their own soon after birth, but researchers at the station have found that many cows nurse their offspring through the winter. The next calving season most likely will find the cows weakened and unable to reproduce.

White and others believe caribou are susceptible to even minor changes in their calving areas. Oil development, for example, could force caribou to calve farther from the coast and much closer to predators such as wolves and bears that live in the hills to the south.

History, however, suggests the caribou is more capable of adapting to change than many had thought. The caribou population in the North Slope area has grown so much since the Prudhoe Bay oil fields were opened nearly 30 years ago that they sometimes have to be chased off runways to enable planes to land.

That high rate of growth often is cited by oil industry leaders as proof that oil and caribou can mix.

But even under the best of circumstances, the life of the caribou is not easy. The caribou’s chief predators are wolves, bears and even eagles.

“I’ve seen a young eagle actually knock down a yearling and kill it,” White said. He suggests that although the Alaskan caribou has thrived in recent decades and now numbers about 700,000, “the population has been stable now for five years, and it might even be falling off.”

Thus the debate over the compatibility of human development and the caribou is not likely to go away soon.

The musk ox, meanwhile, is expected to multiply in northern Alaska, a testament to a human desire to restore what others helped take away.