A "highly predatory" brown bear woke up from hibernation and killed 38 reindeer calves in a single month, then 18 young moose the next month, according to a new study.
The unnamed 13-year-old female was one of 15 brown bears researchers tracked in northern Sweden to understand how the bears use their landscape. They found that the bears change habitats to target reindeer and moose calves in spring. The exact spaces bears occupied varied depending on how many calves they hunted, with some bears like the unnamed female killing more than others.
Why are some bears more predatory? "It must be a combination of different factors," study co-author Antonio Uzal Fernandez, a senior lecturer in wildlife conservation at Nottingham Trent University in the U.K., told Live Science in an email, "such as innate behaviour related to personality (for instance, some people are more aggressive than others)."
The researchers don't indicate these "highly predatory" bears pose anymore of a threat to humans.
Related: 6 surprising facts about reindeer
Brown bears (Ursus arctos) are the most widely distributed bears on Earth; they can be found in 45 countries across North America, Europe and Asia, according to the International Union for Conservation of Nature. The new paper is part of a long-term project to study the lives of brown bears in Norway and Sweden.
In the new study, researchers tracked bears with GPS collars between 2010 and 2012 and recorded the bears' kills. As the reindeer calving period begins, brown bears in Sweden move to the high-elevation, rugged terrain preferred by reindeer with young, and then on to habitats used by calving moose, such as near deciduous forests, according to a statement released by Nottingham Trent University. This allows them to hunt vulnerable calves.
The team mapped the habitats different bears used and compared individual kill rates. Bears were called "high predatory" if they killed more than the average kill rate across all of the bears — 0.4 kill per day, and "low predatory" if they killed at a lower rate than that, according to the study. The team found some differences between where high- and low-predatory bears went. For example, high-predatory bears favored forested areas occupied by more reindeer than the more open areas low -predatory bears selected.
Eight of the 15 bears were deemed to be highly predatory and regularly killed more than 20 reindeer calves and 5 moose calves in a calving period, according to the statement. Bears are not as effective at hunting larger adult prey and so they focus on hunting calves until July, after which the calving period ends and they rely on berries for the rest of the year until re-entering hibernation, according to the study.
"Our study shows the differences between individual bears' predatory behaviour and how this helps to explain individual variation in their habitat selection," Fernandez said. "Differences among individuals are also important from a management perspective; for instance, mere predator removal, without targeting specific individuals, may not necessarily reduce conflict." That's because some bears are more aggressive and bolder than others.
Reindeer are semidomesticated in Sweden and herded by the Indigenous Sámi people. Every year, people kill bears in response to reindeer being attacked. The new findings may help researchers develop forecasts for potential bear-reindeer hotspots to help reduce this conflict, according to the study. The hotspots could inform livestock owners about where bears are most likely to attack during calving season and help them take preventative actions to reduce losses, such as increased vigilance in those areas, the researchers said.
The findings were published Dec. 17 in the journal Diversity.
Originally published on Live Science.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Patrick Pester is a freelance writer and previously a staff writer at Live Science. His background is in wildlife conservation and he has worked with endangered species around the world. Patrick holds a master's degree in international journalism from Cardiff University in the U.K.