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The Bizarre Food Grizzlies Use to Fatten Up for Winter

Posted by Richard Conniff on November 16, 2013


(Photo: Karen Bleier/AFP/Getty Images)

My latest for TakePart:

Right about now, the grizzlies of Yellowstone National Park are starting to think about hibernation. This week or next, as food becomes scarce, they’ll head up into the mountains and hunker down in dens under rocks and trees. They’ll cut their metabolic rate in half, drop their heart rate down to about 18 beats a minute, and take a breath only once every 45 seconds. And they’ll stay like that until springtime.

Many of the park’s bears will be getting by for the winter largely on the energy built up from gorging on moths. That’s right: The largest and most ferocious predator in North America eats fluttery little army cutworm moths (Euxoa auxiliaris).

A grizzly researcher first discovered this strange behavior in 1952. But nobody paid much attention until the mid-1980s, when a radio-collared grizzly bear wandered up to the steep rocky slopes, and researchers started to wonder just what it was doing there.

A couple of amateur naturalists were at the time spending half of every year in the field watching grizzly bears, and they did the first real study of moth-eating behavior. I met Steve and Marilyn French back then and spent some time wandering with them in grizzly habitat, as they studied how grizzlies adapted to every possible food source in their environment, including the moths.

Those kinds of details mattered then because grizzlies had been listed as a threatened species. The U.S. Fish and Wildlife Service delisted them in 2007, under pressure from politicians in Wyoming. But a federal judge reinstated protection two years later, and grizzly diet was a critical factor. He noted that the bears depended on nuts from whitebark pine trees, a species that had sharply declined because of tree-killing beetles. Environmental groups are now suing to block the continued FWS effort to delist the species, which could happen as soon as next year. Fewer than 1,000 individual grizzlies survive in the lower 48 states, down from 50,000 in the 19th century.

In the course of their research, Steve and Marilyn French documented how some bears depend on the spawning run of cutthroat trout. That’s another food source that has now almost disappeared, because of illegal introductions of lake trout, which not only eat cutthroats but do their spawning on the lake bottom—meaning no run for the grizzly bears.

The Frenches also documented the quirky nuances of grizzly bear foraging. In springtime, when the weather is wet, for instance, earthworms bunch up for unknown reasons under the park’s tufted hair grass. “And then the bears come and flip over the grass, and thp-thp-thp, it’s like spaghetti,” Steve told me.

Later, in the heat of summer, the couple followed at a distance as the bears climbed up into the mountains. There they watched them paw up rocks and flip them one by one downslope. The logic of this behavior mystified them at first. When they realized that the bears were feeding so enthusiastically on–of all possible foods–moths, they didn’t quite believe it.

To farmers across the Great Plains, army cutworm moths are an agricultural pest. The larvae emerge from the ground in farm fields from Wyoming to Kansas and proceed to cut down young plants for food (hence the name “cutworm”). Sugar beets, small grains, and alfalfa are prominent among their victims. Farmers retaliate with pesticides. If the larvae manage to survive, they mature into dusty little moths with a two-inch wingspan. The moths migrate in June, and the ones from Wyoming and Montana end up in the steep rocky slopes high in the Absaroka Mountains, on the eastern end of Yellowstone.

It ought to be a sweet life for the moths: Until mid-September they get to forage by night on nectar from the profuse blossoms of alpine flowers. Then they hunker down by day in dense congregations beneath the rocks. In the process they boost their fat stores from 13 to 83 percent of their body weight, to get ready for winter.

All that fat is just too tempting for grizzlies, because they also need to bulk up then, at a rate of about 20,000 calories a day. Researchers following in the Frenches’ footsteps have since learned that Yellowstone grizzlies can eat 40,000 moths a day.

In this fashion, according to one biologist, a grizzly “can consume half its yearly calories in 30 days.” Then the bears hole up for the long winter to sleep off the feast.

With luck, federal protection will hold, and they won’t wake up next spring to a different and far deadlier world.

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