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Grizzly Bears Eat 40,000 Moths a Day

Grizzly bears have been seen working mountain slopes for decades. No one knew why, until a researcher discovered that they go there to eat moths. 40,000 moths a day, to be exact.

Bears have keen olfactory senses, everyone knows that. But are they keen enough to smell moths hiding amongst the rocks of mountain slopes? Apparently they are, or so discovered Don White Jr. back in the 1990s when he first began studying grizzly bear behavior in Glacier National Park.

White was interested in why grizzly bears would often head to the rock-strewn slopes of mountains, high above their normal habitat. What he discovered was that the bears rose to those elevations with the intention of eating army cutworm moths, as many as 40,000 a day.

As the summer advanced the moths would come from the Great Plains and up into the Continental Divide to the rocky slopes to escape the summer heat and feed on flowers there. The moths would take refuge, by the millions, in the rock piles or “talus piles” to escape the heat of the sun.

The bears would smell the moths and make the climb, spending hours upon hours each day turning over rocks and licking the fatty moths up like jelly beans.

A bear can consume 300,000 calories worth of moths in a month’s time. That’s about a third of it’s normal calorie intake for that period of time. White estimated the number of moths eaten by bears by counting the moth remains in bear scat and estimating how many times bears relieved themselves during a normal day.

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Forestry Images

There are so many moths that territorial conflicts seem to lessen at such times, with bears allowing one another within closer proximity to themselves. Much like bears forego territorial disputes during salmon runs, so too do they seem to be focused on the moths during the time period when the “lipid Chiclets”, as white calls them, congregate. The moths are mostly fat, providing a high level of caloric intake for relatively little effort.

As the moths feed on the flowers at such high elevations, they increase their body mass fat stores from around 13 percent to 83 percent.

White is currently Associate Professor of Wildlife Ecology at the University of Arkansas-Monticello. He is working with researcher Hilary Robison, who has witnessed the same behavior in Yellowstone bears, on mapping the locations in the Rocky Mountains where the bears spend time feeding on the moths. They hope to be able to identify and protect these areas, as well as consideration for areas where the moths come from if possible.

“If the moths come from specific areas and we lose that habitat, that will affect the bears,” Robison said.

The moth eating behavior of bears, however, has also been noticed from Banff National Park in Alberta in the north to Santa Fe, New Mexico in the south.

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Grizzly Bears Eat 40,000 Moths a Day