A majestic Norwegian reindeer
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Reindeer are shrinking, because of warmer arctic temperatures

Bad news for Santa’s sleigh: Reindeer are getting smaller. As temperatures inch up, winters are getting warmer in Norway, where Svalbard reindeer (Rangifer tarandus platyrhynchus) live. Instead of being able to brush aside the snow that covers the grasses, lichens, and mosses they eat during the 8-month-long chill, the animals’ diet is locked away under a layer of ice when warming temperatures cause rain to fall on existing snow cover, freezing it solid. To see exactly how these conditions affect the reindeer, a team of researchers captured and weighed 135 of the animals on average every year in April from 1994 to 2015. Platyrhynchus body mass decreased by 12% over this period—from 55 kilograms to 48, the team will report tomorrow at the British Ecological Society annual meeting in Liverpool, in an expansion of a Global Change Biology paper published earlier this year. Though the numbers don’t seem extreme, reindeer size has reached a critical point: Female reindeer below 50 kilograms give birth to smaller calves and even terminate their fetuses to save themselves if too little food is available. When these smaller calves begin to reproduce at about 3 years of age, they produce smaller calves themselves. Researchers think this cycle has caused populations to become smaller and lighter. The scientists say there’s a possibility that an even warmer climate could make winters so short, the ice layer could melt altogether and expose plants, allowing reindeer to increase in size by producing larger, heavier young over a handful of generations. But for now, the shrunken Svalbard reindeer are sure to be picked last during reindeer games.