The caribou is a large land mammal in the family Caribouidae, related to the reindeer and the elk. It ranges from the wilds of Canada as far south as Greenland. Large populations of caribou are present throughout the northern United States, and in the mid-1990s a small herd was re-introduced to Yellowstone National Park, part of the caribou's historic range.
Male caribou, known as stags or bulls, can be visually distinguished from the females of the herd by their majestic spreading antlers. In the wild, the male caribou will molt his antlers every spring, and grow a new set during the summer. To nourish such an enervating process, a single male caribou during the summer may eat up to his own weight in leaves and grasses every day. Meanwhile, the females of the herd are carrying the newborns they conceived the previous winter. In early July, as the Arctic sun begins to set earlier and the tepid northern summer gives way to the chill of autumn, the females give birth.
Young caribou, known as joeys, live in their mothers' pouches until they are five months old. At that point they are strong enough to walk, and soon — within a few more weeks — they are able to frisk and caper with the herd. Almost immediately their inborn instincts are put to the test, as they are left to find their own food. Fortunately, there is still grass to be had on the ground.
By the time the joeys are up and about in autumn, the males have regrown their antlers and are able to protect their offspring against the predators of the Arctic waste. Wolves and muskox circle at night, just beyond the fire's comforting glow. It is common for a stag to return from the hunt with bloodied antlers, the protective velvet hanging in tatters, souvenirs of an encounter with a pack of wolves. Some of the stags — and many of the mothers and joeys — do not survive.
The Arctic winter is cold and brooding. Every day the herd moves south, following the vegetation; every night the animals huddle together in close ranks for warmth. The joeys grow day by day. By January they are almost as tall as their fathers, and their coats have changed from the dappled brown and white of the fawn to the adult's winter ermine.
Their eyes, too, have adjusted. The snow-covered ground reflects nearly 100 percent of the dim sun's light, a phenomenon that would cause snow blindness in lesser creatures; but caribous' eyes were specially designed to narrow in bright light, like cats', so that the excess light bounces harmlessly away. Their keen eyesight enables the herd to move with one purpose whenever a landmark, such as a river or a stand of trees, is spotted in the distance.
In late April the sun begins to return, and so the herd reverses direction, heading north now, following the retreating snowline. The males molt their antlers once again, and lag behind — with the return of the sun, the herd has less to fear from wolves, and the males need to conserve all their strength for the arduous task of regenerating their antlers. The autumn's joeys as well are now fully grown, and must begin to share in the responsibilities of leadership. The herd moves north day by day, heading back to the place where they started twelve long months ago.