An uptick in the number of grizzly bear deaths this season in Yellowstone National Park has biologists looking for answers. Of the 16 bears that died, 10 passed away from natural causes, reports the Interagency Grizzly Bear Study Team.
“We’ve seen more natural mortality this year,” team leader Mark Haroldson told the Bozeman Daily Chronicle. “It is something that we haven’t seen before.”
Typically, 75 percent of bear deaths that occur in the first half of the summer are human-related. This year, only 37 percent of deaths have been human-related.
"The fact that there were two females with cubs that were killed inflates the numbers a little bit," Frank van Manen, study team leader, told the Jackson Hole News & Guide. "We're seeing an aging of the population as well. It wouldn't surprise me if we start to see more of these bears dying from old age."
Van Manen’s team aims to determine whether the grizzly bear should remain a “threatened” species on the endangered species list. The bears were added to the list in 1975 when there were only 150 bears in Yellowstone. In 2010 biologists found that number had risen to 600 bears.
However, the Yellowstone grizzly population remains vulnerable. According to van Manen, the female population can only absorb up to 9 percent mortality, while the male population can absorb up to 15 percent mortality. Biologists estimate two bear deaths for every one that is detected, making the estimate for the summer’s overall mortality 8 percent.
Experts guess that among the bears dead from human-related causes is “Brownie,” one of the two yearling cubs of bear “399,” a mother grizzly made famous in Teton National Park for her decision to raise her cubs near the park’s main roads.