One of the most fascinating crossroads on earth lies to the northeast of Japan. The ancient Bering land bridge used to span the current Bering Straits, connecting the land masses of Siberia and Alaska into one vast continent and enabling a traffic of plants, animals and even people to exchange across the low-lying ground between Eurasia and North America.

With the ending of the last ice age, approximately 10,000 years ago, sea levels rose again cutting the connection. Now, the narrow Bering Strait linking the Bering Sea to the Chukchi Sea forms an impenetrable barrier to terrestrial species, isolating those on either side of the straits. Only during winter is there any connection now, when a few animals, such as the polar bear and arctic fox, cross between Russia and America on the sea ice.

A relict of the time of the great ice sheets, of the lower sea-levels and hence of the Bering land bridge era, is the extraordinary musk ox, or Ovibos moschatus. Looking like a comically stunted cow wearing massive horns and a fur coat several sizes too large for it, the musk ox is so supremely adapted to cold climates that it cannot survive in warmer southern areas. The extent of its superb insulation would cause it to overheat if it tried.

This shaggy-haired ruminant is known as omingmak among the Inupiaq-speaking Inuit of the Bering land bridge region; their name means "the animal with a skin like a beard," referring to the animal's ground-dragging coat. In Japanese, the musk ox is known as jakou-ushi.

Even the musk oxen's body shape is a clear indication of its superb adaptation to a cold climate. These stocky bodied creatures have short, stout legs, short tails, short necks and large heads, with small ears. Even their muzzles are covered with insulating hair. The bulls stand on average 1 to 1.5 meters tall at the shoulder, they are 2 to 2.5 meters long and they weigh 260 to 400 kg.

The cows are about one-third smaller, but with the same proportions, and like the males they also grow horns, though the bull's horns are larger and heavier than those of the cows. In older males, the horns may reach 60 cm long, but their form, not their length, makes them unusual. They appear to arise from a central parting, spreading sideways from a broad base along the midline of the skull and curve down close to the sides of the head before curving up to sharp points.

The compact form of the musk oxen, and the small size of all protuberances such as ears and tail, means that they have a relatively low surface area for their body volume compared with other animals of their size. A smaller surface area means less precious body heat is lost to the cold air.

The musk oxen's long, dark brown coat reaches almost to the ground, protecting the animal's body like a warm felt cape. The long shaggy outer coat protects against wind, snow and ice, and conceals a thick undercoat of a very special wool known as qiviut. This inner coat provides such excellent insulation that the musk oxen shed it during the relative warmth of the arctic summer.

This excellent insulation layer is shed during late May, but the remaining summer coat is equally protective, against the attack of biting insects. Qiviut ranks among the warmest and lightest of all wool, with the feel of fine cashmere. The shed wool is avidly collected by anyone traveling in musk oxen range during summer.

The musk ox is an animal of the open tundra, where it feeds on typical tundra vegetation including lichens, low-growing grasses, sedges and dwarf mat-forming trees such as arctic willows. Their low-growing food plants are easily covered with snow, so for the short-legged and compact-bodied musk oxen, which do not travel far in winter, tundra areas with shallow snow, or where the wind keeps the ground blown clear of snow, are very important for their winter survival.

They are relatively long-lived creatures, taking three or more years to mature, surviving to be more than 20 years old. Throughout their lives they live in herds for social contact, safety and perhaps to pass on vital knowledge of the fickle tundra environment.

During the late summer and autumn rut, the bulls compete with each other for control of a herd and to gather a harem of cows together. Sparring males are an awesome sight. They mark their jousting ground with the strong musk that gives them their name, then simply charge and ram each other, crashing together head on after a short swift run. The winning bull controls the herd. September is the mating season, and calves are born during late April or May.

During the Pleistocene, from about 1.6 million years ago to about 10,000 years ago, musk ox were found over a massive circumpolar range. As the climate has warmed, and as a result of human activity, their range contracted to just parts of northern Canada and Greenland. In 1929, a group of musk oxen was introduced on to Alaska's Nunivak Island, and they have subsequently been reintroduced to Norway and to Wrangell Island off the Siberian coast.

These rather placid animals are renowned for their method of defending themselves and their young from bears and wolves. When a predator threatens, the adults of the herd form a ring gathering the young on the inside and presenting a ring of horns to the outside. The bulls occasionally, sally out from the ring to press an attack.

Thought effective against wolves and bears I have always wondered against what extinct predators the defense was originally developed. It is certainly an ineffective defense against humans: As the herds stand their ground rather than fleeing, they are easily gunned down. Being so vulnerable, musk oxen disappeared from their last remaining strongholds in northern Alaska during the late 1800s.

Now, a century later, they are beginning to make a comeback. This time with the help of humans.

By the 1970s, the 34 animals translocated from Greenland to Alaska's Nunivak Island had increased sufficiently for the herd to form the origin of subsequent translocations, and 71 animals were moved to the Seward Peninsula. The Seward Peninsula, in northwest Alaska, is one of the most remote national parks in the world. This 1-million-hectare area forms the Bering Land Bridge National Preserve.

So far, the musk oxen population reintroduced there has increased at a rate of 15 to 20 percent each year except during the harshest winters. They have also spread out from the site of the original reintroduction and now seem safely established.

By April 1992, an aerial survey found 706 musk ox on the Seward Peninsula, and at that rate of increase their numbers could have doubled again by now. Unlike caribou, however, musk oxen do not form large herds, nor do they make long distance migrations. Their smaller herds are dependent on a home area that provides sufficient food year-round, and so limited, their numbers level off at a much lower level than caribou.