It's not a surprise that bears are some of nature's most deadly predators, but researchers in Sweden caught sight of one brown bear that was downright murderous.
Brown bears (Ursus arctos) hibernate through the winter months and emerge in the spring with voracious appetites. It was this period of activity that conservation experts at Nottingham Trent University, the University of León, Spain, and others sought to study when they identified the especially deadly female.
Using GPS collars, the researchers tracked 15 bears in Norrbotten, Sweden, for two years. The area is also home to herds of reindeer and moose, and the reindeer calving season begins in the spring, just as brown bears emerge from hibernation.
These young reindeer calves are especially vulnerable to hungry bears, with one female bear killing 38 reindeer calves in just one month. Once the moose calving season began the following month, that same bear went on to kill 18 moose calves. All told, the bear killed a hooved calf at a rate of nearly one a day for two months.
Even more remarkable, the studied bears changed their hunting grounds according to these calving seasons. When reindeer calving began, the bears moved from the wetlands and coniferous forests to the more rugged, higher elevations favored by reindeer rearing their young.
Once moose calving began, the bears then moved down to the deciduous forests and old clear-cuts favored by moose during this period. After the moose calving ended, the bears returned to more remote areas away from human activity and mostly subsisted on berries for the remainder of their active period.
"We found that brown bears switched their habitats across pre-calving, reindeer calving, moose calving and post-calving periods,” Dr. Antonio Uzal Fernandez, a wildlife conservation expert at Nottingham Trent University's School of Animal, Rural and Environmental Sciences, said in a statement.
"It is clear that highly predatory bears were mirroring the land cover types of reindeer and moose and to overlap with seasonally available and vulnerable prey," he said. "Such a process shows an active hunting strategy of brown bears in spring, when their diet is more dependent on animal protein than during the rest of the year."
While the unidentified female bear was a particular standout in terms of her body count, she wasn't alone. Eight of the 15 bears tracked were deemed "highly predatory", and typically killed more than 20 newborn reindeer and five newborn moose. The findings were recently published in the journal Diversity.
Analysis: Nature is a messy business
It isn't surprising that brown bears are deadly predators, but tallying their kills puts things in perspective and can help inform wildlife management practices in the future.
While the moose herds in Sweden are entirely wild, the reindeer are partially domesticated and make up a substantial part of the indigenous Sámi people's economic and cultural life.
Up to 30% of reindeer calves are killed by bears every year, leading to economic losses for herders. These deaths also have a significant demographic impact on the reindeer herds, especially with the loss of female calves, which can threaten a herd's long-term viability.
Brown bears are regularly culled to prevent over-predation of reindeer herds, so knowing that some bears are more aggressive killers than others is an important discovery.
"Differences among individuals are...important from a management perspective," study co-author Andres Ordiz, a conservation biologist at the University of León, said, "for instance, mere predator removal, without targeting specific individuals, may not necessarily reduce conflict.”
