Some grizzly bears survive by eating up to 40,000 moths a day.
Each summer, moths of the army cutworm fly into tall mountain rocky slides, where they burrow away from the intense mountain sunlight into dark crevices. Hundreds of thousands of them. These moths come from farmland many miles away to these high, remote mountain slopes in Yellowstone.
At these places of slide rock and sunshine, both grizzly and black bears gather each year, climbing high above timberline to feed on the moths. The bears will dig through the slide rock and eat the moths that they uncover. It is estimated that some 40,000 moths per day can end up in the stomach of a hungry bear.
While fat in the diet is not the best thing for humans, it is important to bears. A single moth has a high enough fat content that it accounts for as much as a half a calorie. That means that 20,000 calories of just moths per day can be consumed by a rock-turning grizzly bear.