Grizzly Bears Can Eat 40,000 Moths in a Day

Although this grizzly bear is in the Central Park Zoo, the animals have been seen high in Rocky Mountain peaks, and many wondered why they would voyage there. Subsequent research reveals they travel to these areas to eat moths, and can eat up to 40,000 of them per day. Brendan McDermid / REUTERS

Grizzly bears will eat just about anything. They make a living by consuming whatever is edible in their immediate surroundings, happily dining on huckleberries, dead animals, small mammals like rodents, nuts, fish, or, occasionally, human food.

As seen in the film Wild Yellowstone, which debuts this Sunday (Dec. 6) at 9 p.m. ET on Nat Geo Wild, grizzlies also voyage way up into the seemingly barren, rock-strewn highlands of mountains. For decades, people have witnessed grizzlies in these locations, puzzled as to why the bears would make their way up there. But why would they go to a virtually life- and food-less area, above nearly all vegetation and animal inhabitants?

Researcher Don White Jr. set out to get to the bottom of this enigma when he began studying grizzly bears in Glacier National Park in the 1990s. He soon found out—and Wild Yellowstone reveals—the reason the bears climbed to such frosty, rocky heights: to dine on moths. Lots and lots of moths, says White, who is now a wildlife ecologist at the University of Arkansas-Monticello. His calculations suggested that grizzlies can eat up to 40,000 of these army cutworm moths—a species common throughout the Western U.S.—in a single day, sometimes spending all day turning over rocks to feast on these "lipid Chiclets," as he calls them.

White made that estimation by watching grizzlies with a scope for up to 14 hours per day, while writing his dissertation on the animals at Montana State University-Bozeman. He also counted how many dead moths he could find in each bear "scat," or pile of poo, and multiplied that by the number of times these bears defecated.

Often, grizzly bears tolerate each other's presence while feeding, getting closer together than they normally would. But sometimes, the gathering of bears in a relatively small area can lead to fights. In Wild Yellowstone, a female grizzly takes exception to the presence of a younger male, and violently drives him off.

In a month's time, one bear can consume at least 300,000 calories from moths, accounting for more than one-third of its nutritional needs, he says. The moths' bodies are mostly fat, making them energy-dense and perfect for packing on the pounds before hibernation.

Bears on Vulture Peak, GNP
Grizzly bears gathering in a talus pile to eat moths, on Vulture Peak in Glacier National Park. Don White, Jr.

It's typical for bears to take advantage of whatever food source is plentiful at the time. Moth populations fluctuate; some years, they'll make up much less of the animal's food intake, but other years, "bears will hammer [moths] when they're abundant," he says.

Black bears also get in on the action. Researcher Hilary Robison, who has spent years at Yellowstone National Park studying these animals, witnessed a black bear "digging up moths and they were flying up and landing on his forearms and he was licking them off his forearms," she writes at "And then he wandered on and kind of took a nap for most of the day."

Army cutworm moths, also known as miller moths, are a major agricultural pest, causing millions of dollars in damage to wheat and other crops during some years. The insects fly west from the Great Plains as the summer wears on, and up into the Continental Divide to escape the heat and to find flowers to feed upon. During the day, the moths take shelter in high elevation rock piles, also known as talus piles. Within the talus are large pockets of air where the moths hide, escaping the sun and the heat of the day.

The army cutworm moth, or miller moth, may not look like much, but its body is mostly fat, and it provides grizzly and black bears with a lot of energy. Whitney Cranshaw, Colorado State University,

Unless, of course, a grizzly finds them. Although it hasn't been shown how the bears know to find them, it is almost certainly by using their incredible noses. White says these bears have an extremely acute sense of smell, perhaps an order of magnitude more sensitive than even the best bloodhounds, and can sniff out the insects from miles away.

White is currently working with Robison to create a model showing the locations in the Rockies where bears likely feed on these moths, so that these areas might be protected from development, hikers and human interference. This moth-eating isn't confined to Yellowstone or Glacier National Park, but has been observed as far north as Banff National Park in Alberta, and all the way south to Santa Fe, New Mexico, White says.

Just because they're moth-eaters doesn't mean these grizzlies are gentle beasts. White has had some closer encounters during the 1,000-plus hours he's spent watching bears. Twice he accidentally surprised grizzlies, which "huffed" at him and bounced forward and back, warning signs that they are about to charge. He got out of those situations by "talking softly, not looking at their eyes...which is viewed as a threat in the bear world...and slowly backing up."