A beaver eats a poplar leaf: Poplar or aspen leaves and bark are a favorite food of beavers. (Photos by Mary Holland)

Beavers are strict vegetarians – no fish or fowl for our largest rodent. They do, however, make significant seasonal changes in their diet.

In the spring and summer, herbaceous plants, primarily aquatic, comprise roughly 90 percent of what they eat. Their preference would be to eat these plants year round, but in New England, that is not a viable option, so come fall beavers switch to a largely woody diet, including the bark and cambium (growing layer of inner bark) of birches, alders, dogwoods, beeches, ashes and cherry, with poplars and willows at the top of the list. (In the United States, the ranges of trembling aspen (Populus tremuloides) and the beaver are almost identical.)

The woody plants that beavers consume during the winter consist mainly of cellulose, which is indigestible by most mammals. While beavers do not possess enzymes to digest the cellulose, they do possess a cecum, a pouch between the large and small intestine in which colonies of micro-organisms reside. These micro-organisms digest approximately 30 percent of the cellulose that beavers consume, thereby providing beavers with enough energy to survive the winter. Beavers obtain additional energy by eating their fecal pellets, thus running them through the digestive process a second time.

The main hurdle that beavers face once winter arrives is that they are locked under the ice. The only food accessible to them throughout the winter (unless a thaw allows them to come up through the ice) is that which is underwater. Thus, conservation of energy is of utmost importance. Beavers have adapted in many ways: little movement, lower body temperature, blood vessel heat exchange, and constriction in extremities, to name a few. Even with all these energy-saving adaptations, each fall a family of six beavers has to acquire a little over half a ton of branches for its winter food supply. (The daily food requirement of a beaver is 22 to 30 ounces of poplar bark, a minimum of six beavers occupy most lodges, and ice usually seals ponds for at least three months in Harvard.)

Because woody plants are available and can be transported, they provide beavers with the sustenance they need in the winter. However, this food supply, in order to be accessible all winter long, requires some forethought on the beavers' part. Come fall, they must scramble to fell an adequate number of trees, cut the branches off and transport these branches to a location near their lodge before their pond freezes over. Once ice forms, the beavers swim out of their lodge to retrieve these branches, the bark of which they consume in their lodge, and discard what remains back into the water, to be later used for construction purposes.

Beaver lodge and winter food cache: This extensive food cache extends past the beaver lodge in both directions.

Before they begin to gather their food supply they test the wood by biting into the tree's bark. According to naturalist Leonard Lee Rue, if the bark is not in the right condition, or if there is still too much sap in the tree, beavers may either never harvest the tree or speed up the conditioning of the tree by girdling it, thereby hastening the drying process. The condition of the bark is extremely important because if the branch is stored underwater before it is ready, it will ferment and sour, rendering it unfit for food. It is not unusual for a beaver to girdle a tree and come back a week later to fell it.

Because beavers are so vulnerable to predators on land, and because of the relative ease of transporting the branches a short distance, they usually will harvest trees closest to the water first. Once the branches are cut, the beaver takes one in its teeth, swims out close to the lodge, dives to the bottom of the pond to the future location of the food cache and rams the branch into the mud.

Most hardwood trees, particularly poplar and birch, are very heavy and have little buoyancy, so they are relatively easily anchored in the mud. More branches are added, eventually forming a base upon which additional branches are piled. This cache, or winter food supply, continues to build until it rises above the surface of the water. The top portion of the cache (unreachable from under the ice) often consists of small, leafy branches from trees that are not the beaver's most preferred. The entire pile of branches can be as much as 40 feet in diameter and 10 feet high.

Many people, including Native Americans, used to judge the severity of the coming winter by the size of the local beavers' food cache. To them, a large cache meant a long, cold winter. Truth be told, if anything, the opposite is true. Beavers continue to add to their cache until their pond freezes over. A late-arriving or easy winter allows beavers more time to add to their cache, thereby making it larger.

If you go to an active beaver pond this month, you will see a rather haphazard pile of branches sticking out of the water close to the lodge. This food cache is often mistaken for an unfinished addition to the lodge. In fact, it's as, if not more, essential to the family of beavers occupying the lodge than any renovation of their dwelling. It is the product of their labors for the past couple of months, and the key to their survival for the coming winter.

Mary Holland is the author of "Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England." She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com.