Despite being the second largest species of deer on Earth, many of us know little more about reindeer than what has been conveyed in Christmas stories. Indeed, in his 2002 book Fauna Britannica, Stefan Buczacki wrote that:
"Outside the month of December, most people in Britain never give the Reindeer a second thought, which is a pity because it is a remarkable beast."
Buzacki's quite right and, from being the only species in which females routinely grow antlers to clicking when they walk, reindeer are a fascinating addition to the deer family.
That which follows is a summary of Reindeer natural history.
The Reindeer at a glance
Reindeer evolved in Europe 5-6 million years ago, later crossing into North America during the early Pleistocene (around 250,000 years ago). Fossil evidence indicates they were present in Britain at least 750,000 years ago, with remains recovered from more than 30 archaeological sites across Britain and 22 in Ireland. Dating analysis suggests reindeer died out in Ireland around 10,250 years ago, though they persisted longer in Britain; the youngest known remains, from a site in Sutherland in the Scottish Highlands, date to approximately 8,300 years ago.
Size: The second largest deer species, after the moose (Alces alces), males stand 100-130 cm (3-4 ft.) at the shoulder and weigh 110-220 kg (242-485 lbs), while females are smaller, standing 95-115 cm at the shoulder and weighing 65-150 kg (143-331 lbs). Domestication appears to have suppressed growth by 20-40% over wild individuals, males being more strongly affected than females.
Appearance: Reindeer have an unspotted grey-brown coat with a lighter ash-grey belly, darker flanks and legs, a white rump, a short tail, and white feet – males also sport a distinctive white mane. Their winter coat is not only lighter in colour than the summer one but considerably thicker, composed of dense underfur beneath broad, hollow outer guard hairs that provide excellent insulation.
Their hooves are broad and well-adapted to challenging terrain. They spread the animal's weight to prevent sinking in boggy or snowy ground, and change with the seasons: foam-like pads expand in summer to provide grip on soft, wet tundra, then contract in winter to expose the hard hoof rim, which aids walking on ice and scraping through snow for food. The feet are also furred, offering additional insulation and grip in icy conditions.
Unusually among deer, both sexes grow seasonal antlers.
Distribution: A small population exists in Britain, introduced from Sweden to the Cairngorms by Mikel Utsi and Ethel Lindgren in 1952. Globally, reindeer range across much of the northern hemisphere -- from Norway through Eurasia to Russia's east coast, and across parts of North America (e.g., Alaska, Canada, Washington, and Idaho) and Greenland -- though their distribution is discontinuous and many populations heavily fragmented. In Europe, the IUCN recognises wild populations only in Norway, Finland, and Russia, and the genetic integrity of these remaining herds has come under scrutiny due to extensive interbreeding with domestic and semi-domestic animals. In North America, the species is commonly known as caribou, while indigenous names include Tuktu (Inuktitut), Tuttu (Inupiaq), Athíko (Woods Cree), Vadzaih (Gwich’in), and Boazu (Northern Sami).
Longevity, Mortality & Predators: Females typically outlive males, surviving perhaps 17 years in good habitat, while males seldom exceed 14 years. The oldest of the Cairngorm herd was 19 years old, while the oldest on record died aged nearly 22 years. Other than humans, who hunt wild individuals and slaughter domestic ones for meat, reindeer have few predators. Globally the grey wolf (Canis lupus) is their primary predator, particularly in North America, and packs can take down deer of all ages. Brown bears (Ursus arctos) and polar bears (Ursus maritimus) occasionally also take deer of all ages – polar bears have been recorded swimming after adult deer and dragging them back to shore to kill and eat them. Bears and wolves will also scavenge carcasses, however, so reindeer in the diet does not necessarily imply predation. Wolverines take calves and very occasionally birthing or infirm individuals, while golden eagles will take calves.
Sexing: The sexes are very similar in appearance, which complicates visual sexing, although males are larger and grow a mane during the breeding season. Uncommonly in deer, both sexes routinely grow antlers, although those of males are consistently larger, irrespective of habitat. The antler cycle of females corresponds closely with snowfall – more snow results in higher percentage of females growing antlers and vice versa. Indeed, in habitats where winters are milder, such as in mountain forests, some females may not grow antlers, and those that do will often grow smaller or asymmetrical ones. A small percentage of females in all populations never grow antlers. Males are called bulls, females cows, and young are calves.
Antlers: Antler buds, which develop into pedicles, are visible in both sexes from around three weeks old. Unusually among cervids, reindeer that have been castrated or had their ovaries removed will still grow, clean, and cast their first set of antlers. While growing, the antlers are covered in velvet -- a furry, nutrient- and oxygen-rich skin that supports the rapidly developing tissue -- which is shed in early September ahead of the rut.
Males begin casting their antlers at the end of the rut: mature bulls may shed as early as November and remain antlerless until February, while immature males and non-pregnant females tend to cast in late winter or early spring, though some young males may hold their antlers until April. Breeding females cast latest of all, typically in May, shortly after giving birth.
The strong correlation between female antler growth and snowfall suggests antler retention is resource-driven. Since breeding females are pregnant during the period of greatest snow cover, keeping their antlers through winter and early spring is thought to help them compete with males for the best foraging patches – as well as serving as a useful tool for scraping away snow. Antler size in females peaks at around three years old, while males reach maximum size and complexity at around five.
Territory/Range: In Britain, reindeer spend most of the year well above the treeline in heather moorland on the slopes of the Cairngorms -- at 400-1,300 m (1,300-4,230 ft.) -- before being brought down to lower pastures in summer to give birth. Outside Scotland, reindeer typically inhabit open tundra, although some populations, particularly in Finland, have adapted to mountain forests. Herds living on tundra may migrate vast distances between inland winter feeding grounds and coastal summer feeding ones – journeys of 5,000 km (3,100 mi.) have been documented, with some herds moving up to 150 km (93 mi.) per day. Excellent swimmers, reindeer will readily cross rivers during migrations, and can sustain a swimming speed of 6.5 km per hour (3.5 knots) – one report of a herd being followed by a speedboat suggests they can swim at speeds of at least 9.6 kmph (5 knots).
Diet: Like all deer, reindeer are ruminants, digesting vegetation through microbial fermentation in a multi-compartment stomach. Their diet consists mainly of soft, easily digested plant material. Lichens -- particularly cup lichens (Cladonia) -- are probably the most significant component in most populations, eaten year-round but especially important in winter. Spring and summer bring fresh grasses, birch shoots, flower buds, and leaves, with fungi taken in autumn. Cowberry, bilberry, and sedges are taken as available. Reindeer can also tolerate plants toxic to most other mammals, including wolf's bane (Aconitum), hellebores (Veratrum), and broomrape (Pedicularis).
Though primarily herbivorous, they will supplement their diet with animal protein – fish, birds' eggs, and chicks taken from nests. There are online reports of reindeer hunting lemmings in North America, though this seems more likely to be opportunistic consumption than active predation.
Reproduction: The rut runs from late September through October, with males calling, parallel walking, and clashing antlers in competition for mates. Bulls attempt to hold females together in a harem, guarding them from rivals. Cows may remain in a harem for much of the breeding season -- partly because it reduces harassment from other males -- but grow more selective around oestrus, calling more frequently and seeking out 'satellite' bulls on the periphery of the herd.
After a gestation of around seven months, a single calf is born -- typically in May or early summer -- weighing 4.5-7 kg (10-15 lb). Twins are very occasionally reported. Calves are born with significant fat deposits around the heart, kidneys, groin, and shoulders, which bolster their ability to thermoregulate. In most populations, the calf is on its feet and following its mother within a couple of hours, and she rejoins the main herd within 20-30 hours of giving birth. Woodland caribou, by contrast, may conceal their calves in vegetation while they graze. The calf feeds almost exclusively on milk for its first month before gradually taking solid food. Reindeer milk is exceptionally rich -- with the highest combined content of proteins, fats, minerals, and carbohydrates of any deer species -- enabling rapid early growth.
The calf remains with its mother through its first winter, reaching independence the following autumn. Sexual maturity varies with habitat quality: females in prime conditions may mature at just six months old, though most do not breed until they are two or three, and males may be unable to hold a harem until they are five or six.
Behaviour & Sociality: Reindeer are gregarious outside the rutting season, forming mixed-sex herds that can number several thousand animals. During the rut, the sexes largely segregate. Females are less aggressive and more sociable than males, and female-only groups tend to be larger than either male-only or mixed-sex ones. Dominant bulls appear to play a stabilising role during the rut, herding females and shaping the group's social dynamics. Studies suggest that, given the choice, females prefer to associate with family members – though during the rut they are often constrained by the dominant bull.
Reindeer are the only mammals known to possess a tapetum -- the layer of reflective cells behind the retina responsible for eyeshine -- that changes colour with the seasons. In summer it is golden, shifting to a deep blue during the dark Arctic winter. This change is caused by rising pressure within the eye compressing the collagen fibres in the tapetum, and makes their vision significantly more sensitive to light, helping them see in near-total darkness. They can also detect ultraviolet light, which is thought to help them locate plants beneath snow, avoid overhead powerlines (which emit UV as corona flashes), and potentially identify wolves, whose hair and urine absorb UV and therefore stand out against a snowy background.
Both sexes produce a distinctive clicking sound as they walk, caused by tendons slipping over sesamoid bones in the ankle. Audible up to 40 m (130 ft.) away, this clicking is thought to help keep herds together when travelling in poor visibility.
Threats: The exact size of the global reindeer population is unknown, though the species is listed as Vulnerable on the IUCN Red List. Estimates vary: a 2023 World Population Review figure put the total at around 9 million, mostly domesticated or semi-domesticated stock, while other sources suggest no more than 6.5 million. The IUCN's most recent assessment (2015) estimated the wild population at 2.89 million mature animals, a figure believed to be declining. Local surveys point to particularly steep falls in parts of North America and Russia, where some populations have been lost entirely.
Climate change is an increasing threat. Warmer winters bring more frequent storms and rain-on-snow events, sealing forage beneath an impenetrable ice crust. Reports from the Russian tundra describe reindeer starving as a result, with some estimates suggesting around 80,000 animals died between 2006 and 2013.
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