Polar bears are known to be very skilled and intelligent predators. What many people don’t know is that the king of the Arctic is actually often counted as a marine mammal. This is because polar bears are enduring swimmers and actually spend most of their time on the pack ice and in the Arctic Ocean. Until now, it was thought that the animals do not hunt in the water, but only swim. But a Polish research team was able to make a sensational observation on Svalbard: A polar bear chased reindeer into the water and killed the animals there.
The female, which the researchers said had made a healthy, well-fed impression, was observed chasing a reindeer bull into the water and following it there. What happened next astonished the observers: the polar bear killed the reindeer bull with bites and by pushing her prey under water, probably drowning it. “I was shocked,” Izabela Kulaszewicz told Science magazine. “I thought reindeer were good swimmers and the bear would have bigger problems catching the reindeer.” The researchers published the observations and accompanying images in the journal Polar Biology this month.
After making sure the reindeer was dead by pushing it underwater for another 15 minutes, the polar bear brought her prey back to land and consumed about half of the animal in the first go. After that she went to sleep and came back hours later to feast on the carcass again. The research team estimates that about 80 percent of the reindeer was devoured by the bear. The rest was eaten by arctic foxes and gulls, the authors write further.
These observations alone would be spectacular. However, the Polish researchers found the same bear the following day near the station with a new carcass of a reindeer. The team suspects that they just had missed the kill and the bear wanted to satisfy her remaining hunger with a new reindeer. Observations showed that this time the bear did not devour as much of her prey and barely used more than 40 percent of the reindeer. After that, the bear disappeared into the vastness of the Hornsund of Svalbard.
The fact that reindeer are on the menu of polar bears has long been suspected, as a review of the relevant literature shows. Researchers had often documented that polar bears either chased reindeer or even caught calves. But the fact that polar bears kill their prey in the water and do not even stop at adult reindeer bulls has never been observed live before. On land, polar bears would not stand much of a chance against the faster and more agile reindeer, experts agree. Polar bears can sprint very fast over short distances, but they overheat very quickly thanks to their insulation provided by blubber and fur. Also in the water, polar bears are excellent swimmers though. But successfully beating an animal such as a seal or even a reindeer, which weighs around 100 kilograms, in the water seemed unlikely until now. A study carried out on Svalbard had shown that in the summer reindeer make up more than a quarter of the diet, but it was not clear whether this was self-hunted animals or eating deceased reindeer.
Experts are admittedly surprised by the findings of their Polish colleagues. But then again, polar bears are known to be very flexible when it comes to food. Especially in summer, when the sea ice retreats far to the north, polar bears that have remained on land have to look for alternative sources. Then nests are plundered, carcasses of dead animals eaten and even algae consumed. Obviously, however, the agile reindeer are also a possible food source. How big the energy gain is for the bears in the process would still have to be determined. And one thing is certain: despite their wide range of food sources, polar bears are finding it increasingly difficult to find food and meet their energy needs. For they need fat above all, which only seals or whales can supply them with in sufficient quantities. These need ice as a habitat… and that is becoming increasingly scarce in the Arctic.
Dr Michael Wenger, PolarJournal
Link to the study: Stempniewicz, L., Kulaszewicz, I. & Aars, J. Yes, they can: polar bears Ursus maritimus successfully hunt Svalbard reindeer Rangifer tarandus platyrhynchus. Polar Biol 44, 2199-2206 (2021). https://doi.org/10.1007/s00300-021-02954-w