A grizzly bear’s diet consists of all kinds of plants and animals, including berries, grasses, fish, rodents and even the occasional moose or bison. They also eat a range of insects.
In Montana’s Glacier National Park, one particular critter plays a vital role in the grizzlies’ annual bid to fatten up before their long winter slumber: army cutworm moths.
After hatching on the Great Plains in spring, these 2.5cm-long alpine insects leave the heat of the valley floor for the cooler upper slopes of the park, where they congregate in huge numbers. Packed with calories, they are a feast the bears can’t resist.
As autumn sets in, any surviving moths make their way back to the valley to mate and lay eggs – and so the cycle begins once more.
But the important grizzly foraging sites high up in the mountains only occur in 0.3% of Glacier’s land. In a new study published in the journal Biological Conservation, researchers from Washington State University have mapped these sites. The team say their work offers park managers a tool for protecting these vital habitats as recreation in the mountains grows.
Moth-eating grizzlies
“The moths are full of fat, and they're also quite nourishing in protein, so they satisfy two macronutrients of a grizzly bear's diet like few natural food resources do,” says lead author Erik Peterson from Washington State University. “Their abdomens are swollen with liquid fat.”
Grizzly bears are relatively abundant in Glacier. But with roughly three million people visiting the park every year, encounters between the two animals are inevitable. This can distract the bears from foraging the high-calorie moths.
“There are moth sites in Glacier where there are waves of climbing groups scrambling by each day,” says Peterson. “On a given day, there may be upwards of 20 bears foraging for moths at major sites.
"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.”
Locating the grizzly foraging sites wasn’t easy. The team spent three years (2019–2021) conducting ground surveys of potential moth habitat, before using helicopters to record bear activity.
“The effort involved in collecting this data cannot be overemphasised,” says co-author Daniel Thornton. “Climbing up to talus slopes to survey for moths across Glacier with a team of technicians, going up in helicopters to search for bears ... all happening in a flagship national park.”
The team says their findings can help park managers develop access management strategies to limit recreational impacts on moth-eating grizzlies.
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Find out more about the study, published in the journal Biological Conservation.
Top image credit: John Dukes/Getty Images
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