Foraging in a Beaver’s Pantry

Abstract

Every evening in the fall, as the sun sets a minute or two earlier than yesterday, beavers enter the forest surrounding their ponds. This forest is their pantry. The beavers are searching primarily for aspen or willow but sometimes maple, birch, oak, cottonwood, or ash. During the evening and into the night, the beavers will cut, haul overland, and float branches and small stems across the pond back to the food cache a few meters beside their lodge. The beavers weigh down the branches with mud and rocks, so this food cache is mostly underwater. When winter comes, the layer of ice atop it seals it in. Protected from predators by the overlying ice, the beavers swim out from underwater entrances to their lodge and bring back small branches from the nearby food cache. After dragging the branches to platforms above the water within the safety of their lodge, the beavers then nibble the bark off them for their daily meals during the long winter. Because the beavers will not emerge from their lodge or from beneath the ice until the next spring to forage again in the surrounding forest, the food cache is their entire winter’s food supply. The amount and quality of food in the cache under the ice therefore determine whether the beaver colony (usually a family unit of two parents and three to five children) survives the winter. In his classic study of the natural history of the American beaver, Lewis Morgan estimates that the food cache can range up to a full cord of wood (a cord is a stack of wood 4 feet wide by 4 feet deep by 8 feet long).