Mammals
Reindeer
Rangifer tarandus · also called caribou, reindeer, wild reindeer
Reindeer and caribou are one animal with two names — a circumpolar deer built entirely around surviving the Arctic. It grows antlers on both sexes, sees a band of light no other large mammal can, walks with an audible click, and, in North America, makes the longest overland journey of any animal on Earth.
Overview
Rangifer tarandus rings the northern hemisphere across tundra, boreal forest, and mountains. In Eurasia it is the reindeer, existing as both wild and herded animals; in North America the wild form is the caribou, never domesticated. It is a herd animal, in some regions gathering into aggregations of hundreds of thousands.
Arctic engineering
- Winter food: where other deer would starve, reindeer dig feeding craters in the snow to reach “reindeer lichen,” which their specialised gut microbes can break down.
- UV vision: reindeer eyes pass ultraviolet light to the retina. Snow reflects UV; lichen, wolf fur, and urine absorb it, so those things stand out as dark marks on a glowing field. The eye even shifts colour — gold in summer, blue in winter — to gather more light in the polar night.
- Clicking step: a tendon snapping over a foot bone clicks with every stride, helping a herd stay together in whiteout conditions.
Antlers on females
Reindeer are the only deer in which females routinely grow antlers. Males shed theirs in early winter; females keep theirs through winter, using them to defend feeding craters — a pregnant female can hold her patch of lichen against a larger, now-antlerless male.
The great migration
Some North American caribou herds cover more than 4,800 kilometres a year between calving grounds and winter range — the longest documented land migration of any mammal.
Status
Wild Rangifer is Vulnerable, down roughly 40% in recent decades, with some herds collapsing by over 90%. A warming Arctic (shifting vegetation, rain-on-snow icing that locks away the lichen), insect harassment, and industrial fragmentation of migration routes are the main causes. Domesticated reindeer, herded across northern Eurasia, still number in the millions.
Reindeer: common questions
Are reindeer and caribou the same animal?
Yes. Rangifer tarandus is one species with a circumpolar range and several subspecies. In Europe and Asia the animal is called reindeer, and there both wild and domesticated (herded) populations exist. In North America the wild animal is called caribou, and it has never been domesticated there. Domesticated reindeer tend to be shorter-legged, stockier, and more variable in colour than wild caribou because of centuries of selective herding, but they are the same species and can interbreed.
Do reindeer really see ultraviolet light?
Yes, and it appears to be an adaptation to the Arctic. Most mammals filter out UV to keep their vision sharp, but reindeer let it through to the retina. Snow reflects UV strongly and evenly, while several things reindeer care about absorb it and therefore show up as dark shapes against the bright background — lichens (their winter food), the fur of wolves, and patches of urine that can signal danger or another animal. In the low, blue-shifted light of the Arctic winter, this extra channel of information is valuable. The structure of a reindeer's eye also physically changes between summer and winter, shifting the reflective layer at the back from gold to blue to boost sensitivity in the long polar night.
Why do female reindeer have antlers?
Reindeer are the only deer in which antlers are normal in both sexes. Males grow large antlers and shed them in early winter after the rut. Females grow smaller antlers and keep them through the winter and into spring. The leading explanation is competition for food — in deep snow, reindeer dig feeding craters down to the lichen, and an antlered animal can defend its crater from others. A pregnant female with antlers can hold a feeding site against a larger but now antlerless male, which matters for the calf she is carrying. (This also means the reindeer pulling a sleigh on 25 December, still bearing antlers, would all be female.)
Why are wild caribou declining so fast?
Wild Rangifer has dropped by roughly 40% globally in a few decades, and some herds have crashed by over 90%. The drivers combine — a warming Arctic changes plant communities and creates rain-on-snow icing events that seal the lichen away under an ice crust that reindeer cannot break, insect harassment worsens, and industrial development (roads, mines, oil and gas, logging) fragments the vast ranges the animals need and gives wolves and bears easier access to the herds along cleared corridors. Overhunting has hit some populations. Domesticated reindeer, by contrast, remain numerous.