Sunny Skies for the Fattest Reindeer

We are deep into the holiday season, and that means a lot of things for a lot of people. To some, it’s a strange reminder of the changing long term weather patterns brought about by climate change. And while the reality of the situation rings out a somber tone, hopefully knowing that some animals have been making the best out of a warming environment will help lift some spirits in time to enjoy the present. And what better way to deliver such nuanced cheer than by telling you about the world’s smallest, fattest reindeer?

In the Norwegian archipelago of Svalbard, between the mainland and the North Pole, lives a species of reindeer endemic to the islands called, appropriately, the Svalbard reindeer (Rangifer tarandus platyrhynchus). What’s special about these fellas is that the Svalbard reindeer are also the smallest, furriest, and fattest reindeer on the planet. And if those weren’t already enough of a resume booster, it’s likely these very traits that have allowed populations of Svalbard reindeer to overcome the effects of climate change that have threatened reindeer and caribou in other parts of the world.

The Arctic Circle is warming faster than anywhere else on the planet–about 4 times faster–and with that increased temperature comes a shift from snow to rain. What ends up happening are these “rain-on-snow” events, where rain will mix with snow on ground and refreeze, turning into a hard, thick layer of ice. This keeps any plants, particularly the mosses reindeer typically enjoy, inaccessible from the reindeer who can’t dig through them with their hooves. These events lead to starvation and are associated with the deaths of around 80,000 reindeer in the Russian and Norwegian winters of 2020-2021.

Svalbard reindeer have some specific traits that allow them to weather these events a bit better than most. First of all, they’re nice and squat with their legs proportionally shorter than other species, allowing them to hold onto heat–and therefore their calories–more efficiently. Their bushier coats also help them for the same reason.

Another quality that works in their favor is how well they store energy as fat. Most reindeer build up to around 20% of their body weight as fat before the winter. This fat gain is a really good indicator of individual fitness and is linked to more successful births and overall survival. In Svalbard reindeer, this jumps up to 40%, making these little guys the fattest on the planet. The idea is that this better allows these reindeer to tough it out during the barren winters.

But that doesn’t explain why we’ve seen Svalbard reindeer populations, specifically those further inland like the Reindelen population, increase almost 4-fold in number over the last thirty or so years. Well it turns out that turning up the thermostat has turned their environment into an all-you-can-eat buffet. Warmer summers, especially in Svalbard where the sun is up 24 hours a day between mid April and late August, has led to increased plant productivity giving the reindeer a better chance to pack on the pounds before autumn.

Svalbard reindeer are also especially good at adjusting their diets to suit their surroundings. Mosses are a go-to, but with the changing temperatures, researchers have noticed an increased abundance of grasses and shrubs in the reindeer’s diets. These are not only more accessible after rain-on-snow events but have also been linked to higher autumn body mass and a more diverse gut microbiome.

It is important to note that not all Svalbard reindeer are enjoying their day in the sun. Inland populations have felt the most of the benefits due to how climate change has interacted with their feeding range. Coastal populations, however, haven’t seen as much plant abundance and still have to deal with rain-on-snow events. Many of them have turned to eating kelp, which is less nutritious and can only support maybe a fifth of the population. After the winter of 2018-2019, over 200 reindeer were found starved in these coastal regions. This sort of population divergence paints a clearer picture of how the effects of climate change vary even over short geographic distances, but it also raises the question of just how long the inland regions will maintain this reindeer paradise.

In the meantime, know that climate change is an incredibly complex process and that a number of species have found themselves in the right place at the right time. For the Svalbard reindeer, that means enjoying full bellies in preparation for winter. So, if you’ll excuse me, I think I’ll be following their lead.