When grizzly bears clamber onto the talus slopes high in Glacier National Park, they're searching for an abundant, fatty meal: army cutworm moths. The inch-long (2.5-centimeter-long) moths hatch on the Great Plains and fly en masse to escape the heat in the stony upper reaches of the Rocky Mountains. Grizzlies hoover them up by the thousands—at about a half-calorie each—as they fatten up for winter.