Were people to guess the terrestrial animal with the longest annual migration, how many would opt for the correct answer of caribou?
Found in Arctic, subarctic, tundra, mountain and boreal regions, the caribou’s circumpolar distribution sees its range span from northern Europe to Russia and Siberia, before traversing North America.
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Where do caribou live?
As the sole deer representative to have been successfully semi-domesticated, with many of the north European populations effectively herded as livestock by the Sámi people for thousands of years, it is only across Alaska and Canada that caribou largely roam wild and free.
Alaskan and Canadian caribou can be subdivided into two distinct lineages – boreal (woodland)and migratory tundra (or barren-ground) caribou. Boreal caribou tend to inhabit mature, lichen-rich forests, occur in small, isolated groups and are fairly sedentary.
By contrast, the caribou of the open tundra tend to be smaller and lighter-coloured, with shorter legs and less substantial antlers. These barren-ground caribou can be further divided into geographically distinct herds, all largely confined to specific wintering quarters, migratory routes and calving grounds.
One of the largest and healthiest populations is the Porcupine herd, named after the river that runs across much of its range.
The Porcupine herd is considered to have the longest round-trip migration of roughly 1,350km, with around 200,000 caribou crossing borders between their wintering quarters in Yukon and calving grounds in northern Alaska.
Why do caribou migrate?
The primary motives for caribou migration are to find seasonally available food, to better protect their young from an array of predators, and to escape biting insects. The precise timings and routes of this monumental migration are primarily driven by environmental cues (which vary from year to year) and the collective social memory of the herd, acquired from previous years’ movements.
Leaving their wintering grounds in spring, the herds head north to access the highly nutritious sedges, grasses and shrubs that emerge as the tundra’s snow melts. Following routes used for generations, this massed movement has been likened to a ‘river of caribou’ in some years, while in others it appears piecemeal.
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Certainly, in the case of the Porcupine herd, the heavily pregnant cows will set their internal satnav for the coastal plain adjacent to the Beaufort Sea, a flat, barren expanse that makes it easier for them to keep an eye out for predators such as golden eagles, brown bears and wolves, while the coastal breeze helps keep the insect count down.
Upon arrival, the females will aim to calve synchronously in either May or June – a strategy designed to effectively ‘swamp’ any predators while the young are at their most vulnerable.
Nevertheless, in such a remote location, life is still tough for a newborn calf. A 1983 study, for example, found that 25 per cent would die within the first month of life, primarily due to poor nutrition or predation.
Come the advent of summer, hordes of pestilential mosquitoes and warble flies reach such an unbearable level that the caribou are forced to move to windy coasts, ridges and snow patches.
Packing tightly together, these aggregations are thought to provide much-needed respite from the relentless insect harassment.
But when the temperature drops and days shorten in September, it necessitates another movement. In the case of the Porcupine herd, this will see it drifting back across the border into Yukon, before heading south to more sheltered conditions.
The caribou is undoubtedly a keystone species across much of the planet’s northerly latitudes. But a 2024 report found that migratory populations have fallen by 65 per cent, due to human influences such as climate change and the opening up of mines and roads. These all combine to negatively impact this world-beating migration.
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Where to see the caribou migration
Caribou are functionally extinct in the contiguous United States. The South Selkirk herd had dwindled to just three animals by 2018, when they were translocated to a more suitable habitat in Canada.
Kobuk Valley National Park, Alaska, USA
The Western Arctic herd uses the park as part of its migration route, with Onion Portage on the Kobuk River a vital crossing point for the caribou for thousands of years.
Denali National Park, Alaska, USA
This is the only barren ground in North America where caribou are not hunted. With a population of around 3,000, animals from the herd are often seen along the 148km Denali Park Road.
Old Crow, Yukon, Canada
While the precise movement of the Porcupine herd varies each year, the caribou invariably pass by the remote community of Old Crow on their way to calving grounds in the Arctic National Wildlife Refuge in Alaska.
Schmok Lake, northern Manitoba, Canada
A key area for the passage of the Qamanirjuaq herd in April, moving between their overwintering areas and summer calving grounds near Rankin Inlet in Nunavut.
Bathurst Inlet, Nunavut, Canada
The Bathurst herd ranges across the Northwest Territories and western Nunavut, but in late April the herd migrates north to calving grounds near Bathurst Inlet.
