The Svalbard reindeer is a diminutive Arctic deer found only on the Svalbard archipelago, far north of mainland Norway. It is the smallest subspecies of reindeer on the planet, with adult males typically weighing 65 to 90 kilograms (143 to 198 pounds) and females 53 to 70 kilograms (117 to 154 pounds), roughly half the size of their mainland cousins. These reindeer have thrived in Svalbard’s extreme environment for thousands of years, having been isolated there since the last Ice Age.

Compared to other reindeer, Svalbard reindeer have shorter legs and a smaller, rounded head. This compact build, along with short ears and a short tail, reduces the surface area that can lose heat, a classic cold-climate adaptation. They also grow an extremely thick winter coat that turns a pale grayish-white. The fur is so dense and fluffy that even a starving Svalbard reindeer can look fat in winter because the puffy insulation layer hides a bony frame beneath.

Inside, these reindeer have more tricks to handle the cold. They pack on unusual amounts of body fat during the good times, essentially carrying an internal fuel tank, and their metabolism can shift into low gear in winter. Amazingly, a Svalbard reindeer does not become truly cold-stressed until about −50 °C (−58 °F); by comparison, regular Norwegian reindeer start feeling the chill at around −30 °C. Slowing their metabolism and burning that stored fat lets them stay warm and energized through Svalbard’s long, frigid winters.

In the brief Arctic summer, when the sun never sets, these reindeer take full advantage of the 24-hour buffet. They roam the green tundra valleys munching grasses, sedges, leaves, moss, lichens, and tiny flowering plants, basically any vegetation they can find. The summer plants are nutritious, and the reindeer eat nearly nonstop to build up fat reserves before winter hits. By autumn, a healthy Svalbard reindeer carries a thick layer of stored energy.

When winter descends, food almost disappears. Svalbard reindeer do not migrate south like many other reindeer; there is nowhere to go on their island home, so they stay put. Instead, they switch to survival mode. The fat stored in summer becomes their main energy source. Over winter, a reindeer might lose nearly half its body weight living off its fat and muscle stores. They move as little as possible to save energy, often sheltering on windswept ridges or plateaus where the snow is thinner.

Using their broad hooves like shovels, they dig through snow to nibble any buried moss or lichens they can sniff out. It is a sparse menu, but they make it last. Starvation is unfortunately the most common cause of death for Svalbard reindeer, especially in late winter when those fat reserves run low.

Yet they have some surprising food sources when times get really tough. During recent warmer winters, Svalbard has had odd rain episodes that freeze into ice sheets on the ground, locking away the plants under glassy ice. In those bad years, some desperate reindeer wander down to the seashore for an unconventional snack: kelp and seaweed.

Researchers have seen more reindeer feeding on seaweed along the coast in icy winters. It is not their preferred diet (and too much salt can give them digestive issues), but this remarkable behavior shows their resourcefulness in coping with Svalbard's harsh winters.

In the darkest months, with no sun and very little to eat, Svalbard reindeer endure an extreme Arctic regimen. By spring, they are often at their weakest. But as the snow melts and the tundra greens up, the cycle of feasting begins anew, and the reindeer bounce back by devouring the fresh vegetation under the 24-hour summer sun.