Dancing Across the Top of the World

Text by Liz McKenzie and Photos by Richard Nelson

Dancing Across the Top of the World

For many thousands of years, caribou have been awakening the quiet world of the north as they've journeyed across the landscape during their annual migrations. In fact, caribou bones in northern Alaska have been dated to at least 28,000 years ago, during the late Pleistocene, when they shared the tundra with long-vanished mammoths and mastodons.

More than anything else, what distinguishes this member of the deer family from all other mammals of the north is their nearly continuous movement.

For example, moose tend to stay in one area to feed—some even spending their whole lives within a five mile area—but caribou are constantly on the go, dancing across the land, from wintering grounds, to calving grounds, to their summer range, and back again.

Caribou don't travel in a straight line either, so they cover many more miles than the distance between summer and winter ranges would indicate. In fact, satellite tracking has shown that caribou in the Porcupine herd travel more than 3,000 miles during their annual migration from the boreal forest in Canada to the North Slope of Alaska.

It's a sure thing that caribou will migrate, but their exact route is another question entirely. Trying to guess what mountain pass, what valley, what river a caribou herd might cross is anything but an exact science…hunters know well that expecting a caribou herd to take the same route twice is a recipe for a hungry winter. This scattered and varied travel also helps protect the slow-growing and fragile tundra plants—on which caribou depend—from overgrazing.

While their northern cousin, the moose, keeps mostly to itself, caribou live in herds throughout the year, especially during the spring and fall migrations, and the early summer calving season. Migrating caribou often travel en mass—sometimes in spectacular groups of 100,000 or more—that looks as if the entire landscape is moving.

They often encounter challenging terrain, yet they travel over it with grace and power—trotting up a mountain slope as if it were flat tundra, plunging into frigid water, swimming in rivers and lakes almost as easily as they move across the land. They also cover great distances in snow, walking in long lines that create packed trails like spider webs woven across the vast white land.

Caribou are brilliantly designed for their northern world—dense warm fur, phenomenal endurance, a genius for navigation, ability to find food in a frugal environment, and a constant yearning for travel. Caribou are good-sized animals. In Alaska, adult cows weigh 175-225 pounds, and the bulls average 350-400 pounds, with records of up to 700 pounds.

Caribou are also beautiful animals, especially in the fall when their newly grown winter coat is rich brown, with a white cloak wrapped around the neck, and a long white mane. The bull's crowning flourish is a magnificent rack of antlers, sculpted bone crescents measuring up to four feet tall. Caribou are unique among members of the North American deer family, because the cows also have antlers, although they are smaller than the bull's huge adornments.

Above all, caribou are a highly successful animal that thrives in one of the earth's most difficult environments. There are about five million caribou (and closely related reindeer) living today in North America and northern Eurasia. This includes roughly a million in Alaska and 2.4 million in Canada. 

Winter in the Far North

Caribou thrive where the winter brings endless months of driving blizzards and brutal cold—temperatures sometimes plunging to minus 60 or 70 F. How do they survive in such extreme conditions, in a land that seems devoid of food?

The answer lies in the amazing ways in which their bodies and behaviors have adapted to their environment.

Perhaps most importantly, caribou fur is the warmest of any land mammal. The outer layer of a caribou's coat consists of long, guard hairs, and each one is hollow, to trap a tiny pocket of air inside, providing excellent insulation. This cloak of hollow fur explains why caribou float so high when they swim across rivers and lakes.

Beneath the guard hairs is a layer of short, dense under fur that fills every minute space between the guard hairs, adding even more warmth. A less well-equipped animal would suffer a fatal loss of body heat in the bitter cold and gales of winter in the far north, but caribou keep relatively warm, thanks to their highly perfected winter coat. Most caribou also spend the frigid months in the sheltering boreal forest or along its edges, where temperatures may be extremely cold but they can escape the gales that sweep across the Arctic tundra and the exposed mountain slopes.

Cold temperatures are not the only challenge for an animal in the Arctic winter. Deep snow makes travel difficult and food much harder to find. Here again caribou are beautifully equipped with splayed-out hooves that act like snowshoes and keep them from sinking into the deep powder. And they make excellent shovels to dig for food buried under the snow.

In the winter caribou eat small dried plants left from the previous summer, as well as twigs and sticks; but their favorite and most nutritious food is lichen. When Athabaskan Indian or Inupiaq Eskimos come across a place with hundreds or thousands of small craters in the snow, they know that caribou have been here…digging for their food.

The constant searching and excavating takes an enormous amount of energy, so caribou use their excellent sense of smell to determine if there are in fact plants below. They will even poke their nose down into the snow to detect plants buried deep underneath. The fur on a caribou's nose is short so it's less likely to become caked with ice and snow.

Luckily the hair on their legs is also short, so ice and snow won't cling to them as they walk in the deep snow and dig for food. After a long winter, broad tracts of boreal forest and tundra meadows can be covered with craters made by the caribou. Interestingly, a mother caribou will defend a crater that her calf has dug, even using her antlers to drive an intruder away. 

Spring Migration

As early as late winter caribou cows begin to drift north while they keep pawing the snow for food. Deep inside, they seem to feel an urge to start moving toward their calving grounds.

Cows and their female yearling calves are first to set off on the spring migration. Male calves lag behind and learn from the older bulls about the timing of their migration.

The journey to the calving grounds is anything but easy. There are many challenges—deep snow, high steep mountain ranges, sharp-walled canyons, rushing icy rivers and predators like wolves and grizzly bears. Caribou have an uncanny ability to find their way and a powerful determination to keep heading northward.

Caribou are good swimmers, paddling with their splayed hooves and buoyant coat making them float high in the water. They are experts in navigating swift waters, letting the water raft them down as they work their way to the other side, rather than fighting the current to cross in a straight line.

A curious phenomenon happens during this spring migration—the cows maintain a certain distance from each other and when the lead cow stops, the others stop to feed too, as if the leader has signaled to the others that it's time to rest and feed.

Moving ever northwards, the herd itself becomes like a river, now a long curving line, now braided, now fanning out over the flat areas.

Nothing can keep the cows from the calving grounds except late, deep snow or a river too swift and choked with ice—in this situation, cows may be forced to give birth before reaching the calving grounds, but this is hardly ideal. 

The Calving Grounds

Biologists have learned that within large regions like Alaska or northern Canada, caribou tend to live in separate herds or populations. In Alaska there are 32 of these groups, each with its own name—such as the Teshekpuk, Denali, Nelchina, and Forty-mile herds.

Each year, a caribou herd returns to its traditional calving grounds, as it has done for many thousands of years. For example, the Porcupine herd travels from Canada, crossing the Porcupine River, eventually reaching their calving grounds in the Arctic National Wildlife Refuge in Alaska. The Western Arctic Herd—the largest in Alaska with about 340,000 animals in 2010—uses an area within the National Petroleum Reserve-Alaska.

The calving grounds are critical for the long-term survival of caribou, and their migration and biological processes are timed to coincide with arrival when conditions are ideal for the birth and survival of newborns.

Predators are one important factor in the timing and place of caribou calving. For example, when females from the Porcupine Herd are giving birth on the coastal plain of the Arctic National Refuge, wolves are far away in their mountain dens, also giving birth and tending their new pups.

However, some wolves follow caribou out onto the tundra, along with grizzly bears newly emerged from their winter dens and golden eagles that migrate north at this time. Caribou have adapted by giving birth all at once—a phenomenon called "predator swamping"—which means only a small number of calves are taken before they quickly become fast enough to escape. This gives each individual calf the best chance of survival.

Also, the far northern calving grounds are cooler and windier than areas to the south. This is important because tormenting insects like mosquitoes and warble flies need warmer temperatures. Anyone who has visited the arctic on a calm, mild day can attest to the misery inflicted by millions of mosquitos. With luck, the cows have a chance to give birth and the new calves can gain strength and speed before the insects become prolific and nasty.

At this time too, there is a sudden outburst of fresh, green tundra plants. This new growth is the most nutritious of all—perfect for a new mother nursing her calf. She feeds almost continuously on lichens, sedges, mushrooms, tender willow leaves and flowering plants. Cotton grass is a common plant on the tundra and at this time the nutrition-packed flower buds are especially important for caribou.

And perhaps most interesting of all, these calving grounds have been nourished for millennia by the cows themselves. About a week after the calves are born, the cow's antlers loosen and fall off. Her antlers are rich in calcium from the plants that she ate after leaving the calving grounds the previous spring.

The cow carries this vital micronutrient in her antlers and delivers it to the coastal tundra every spring. Then lemmings and other small rodents chew on the antlers, and their droppings in turn deliver the calcium into the soil where plant roots draw it up. Finally, when a mother caribou eats these plants, calcium brought north by countless generations of her ancestors enters her body and helps to nourish her new calf through her milk.

So not only does a caribou mother carry her developing young to the calving grounds, she also brings vital nutrients that will contribute to this process of enriching the tundra soil. It's a marvelous solution to the problem of getting essential nutrients to the calving grounds where they're needed most. And this has been going on for tens of thousands of years.

Caribou calving grounds can be huge and vary with the size of the herd, from 2,000 to 12,000 square miles—as large as the U.S. state of Maryland.

Interestingly, when the cows reach their calving area, the lead cow moves far enough into it to make room for the cows that arrive last, while all of the pregnant mothers maintain a certain distance between each other. Spreading out like this protects individual newborn calves, because if they were all born close together it would be easier for wolves or grizzly bears to kill many calves at the same time.

Caribou cows are in labor for a few hours at most and are back on their feet licking and nuzzling their single, unbelievably cute newborn. The calves are tawny colored with dark eye smudges, and they start nursing shortly after being born.

Cows are particularly skittish and wary at this time, because their calves are vulnerable to predators. Luckily a caribou calf can run within hours of birth and it can swim after just a few days.

Caribou calves seem to love to frolic and dash about. Within the first week a calf can run as fast as its mother. And this is important because the herd will be on the move as soon as the calves have the speed and stamina to keep up.

Once calving is complete, the entire herd—now including the bulls and young males—migrates to summer grazing areas. 

The Challenges of Summer: Insects and Predators

For many animals summer is a time of relative ease…the snow is gone, temperatures are warm, and food is plentiful. While caribou do find rich grazing and the calves grow bigger, summer is not without serious challenges.

On a warm, windless tundra day, hoards of mosquitos torment the caribou, swarming around their eyes, noses, ears and mouths, constantly whining and worse, sucking blood. Caribou are shedding their winter coats, making it easier for the bugs to get down through their fur and bite.

Other insects can add to the misery of arctic summer. Warble flies lay their eggs in a caribou's fur and when they hatch the larvae burrow down through the animal's skin. Even worse, botflies crawl up into the caribou's noses to deposit their larvae, which grow in the animal's throat.

The swarming bugs don't kill the caribou but make them miserable. The animals are constantly moving to escape, using tremendous amounts of energy, and they don't get to feed as much, so they lose weight.

To escape, the caribou gather on snow patches or leftover ice along the rivers, they climb to the windy mountaintops, or they move out to the cooler, breezy coast…even onto the sea ice itself. Wherever they are, chilly or windy weather keeps the insects from flying, so the caribou can rest, feed, and gain back some lost weight.

By late summer, caribou start shedding their buff colored fur, replacing it with a very dark coat, so during the transition they have a mottled and somewhat ragged appearance.

The bulls' antlers are fully grown and females are just starting to grow their new antlers for the year. Calves have doubled their weight and are now grazing on their own, having weaned off the rich mother's milk.

During the spring migration toward the calving grounds, cows and bulls move separately and at different times. But in midsummer they all come together in huge numbers for what is known as the "post-calving aggregation". They are moving very quickly now, driven by a special urgency.

The aggregation has two benefits. Large groups offer protection to the individuals from predators, especially to those closer to the inside of the group. The other benefit is that tightly bunched, constantly moving herds give caribou some relief from insects.

These congregations can be very large, numbering in the tens of thousands, sometimes even 100,000 strong. In fact, there have been documented aggregations of more than 300,000 caribou.

In such a vast herd a cow and calf can easily become separated. A mother and calf unable to find each other in the crowd will drop to the back of the herd, running back and forth along the edge of the herd, calling for each other—and with luck they will be reunited. This can also happen when caribou swim across rivers, especially where the current is strong or calves have trouble climbing steep banks. If a calf is unable to find its mother, it is doomed, as no other cow will nurse it…and the lost one becomes easy prey.

Mating and Moving into Winter

By early fall the bull's spectacular antlers begin to lose their soft velvet covering. It falls off in strips, sometimes hanging from the tines. The tissue underneath is bright bloody red but changes eventually to a beautiful mahogany color.

When the rutting (or mating) season begins, the bulls look spectacular—their winter coats are fully grown, their antlers are grand, and their necks have thickened. They are in the best condition of the year.

The prime bulls have skirmishes, pushing and shoving, clashing their antlers, to establish their dominance. They also grunt and snort and chase after the females. The bulls' behavior triggers estrus—the female's ability to breed successfully. Timing is very important, because the breeding period only lasts about two weeks, so the females become pregnant simultaneously and then give birth around the same time next summer. This also ensures that the herd's movements and calving are predetermined for the coming year.

The bulls will mate with multiple females, but the cows only breed once in the season.

Caribou bulls lose their antlers soon after the rut, while the females' smaller but elegantly shaped antlers are still developing, and covered with fuzzy brown velvet. Even this year's calves are growing their first little set of antlers.

With the earliest significant snowfall, the fall migration begins, although it is anything but steady or predictable. If the snow stops or the temperature warms and the snow disappears, caribou may slow down for a while, but eventually they reach their wintering grounds where the land is now clothed in white.

Lichen, a plant high in carbohydrates, is the most important winter food for caribou, comprising about three fourths of their diet. These ground-hugging plants are easily trampled in summer when they are dry and brittle, but once buried in snow they become more flexible and can withstand grazing.

And so, the winter snows and winds, the darkness, the continual pawing for food buried under the drifts begins in earnest for these amazingly well adapted animals the caribou…who dance over the northern edge of the world, in an ancient and never ending circle.