In the high, stony slopes of Glacier National Park, grizzly bears climb to places few humans ever see. They're searching for what researcher Erik Peterson calls "bear butter" — army cutworm moths, tiny insects packed with fat that help grizzlies bulk up for winter.
Now, scientists have discovered exactly where these moths live, and the answer is surprising: the moths gather in just 0.3% of the entire park. That tiny slice of land — steep, rocky terrain called talus slopes — is incredibly important for both the moths and the bears that depend on them.
"Of all the foods that a grizzly bear eats, the story of this moth might be the most amazing," said Peterson, who led the research as a graduate student at Washington State University.
The moths begin life on the Great Plains as caterpillars that can damage farmers' crops. Then they transform into adult moths and fly hundreds of miles to the cool, shaded rock fields high in the Rocky Mountains, escaping summer heat. Grizzlies follow, hoovering up thousands of moths each day during the summer months.
"Their abdomens are swollen with liquid fat," Peterson explained. "The moths are full of fat, and they're also quite nourishing in protein, so they satisfy two macronutrients of a grizzly bear diet like few natural food resources do."
From 2019 to 2021, Peterson and his team spent three years mapping exactly where the moths cluster. They hiked to remote talus slopes to survey moth habitat and flew in helicopters to watch bear behavior from above. It was difficult, dangerous work.
"The effort involved in collecting this data cannot be overemphasized," said Daniel Thornton, a co-author and associate professor at WSU. "Climbing up to talus slopes to survey for moths across the glacier with a team of technicians — it was a huge effort."
The research, published in the journal Biological Conservation, gives park managers a new tool to protect these special places. Glacier National Park attracts about 3 million visitors each year, and while most stay near roads and visitor centers, backcountry recreation like mountain climbing is growing.
Peterson has seen climbers and bears sharing the same slopes. "There may be upward of 20 bears foraging for moths at major sites on a given day," he noted. "If a grizzly bear is having to watch people pass by, whether it's alarmed or habituated to people, it's there to eat calories, not to watch us."
By knowing exactly which tiny patches of land matter most, the park can make smarter choices about where to guide visitors away — giving grizzlies space to fuel up for their long winter sleep.