Beavers, those industrious engineers of the Northern Hemisphere, are known for their dam-building prowess and semi-aquatic lifestyle. But what exactly do these large rodents eat? The answer is more complex than simply "wood," as their diet is diverse, adaptable, and varies with the seasons and available resources.
Beaver Basics
Beavers belong to the rodent genus Castor, with two extant species: the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). They are the second-largest living rodents after capybaras, weighing up to 50 kg (110 lb). Beavers possess stout bodies, large heads, powerful jaws, and continuously growing incisors perfectly adapted for gnawing through wood. Their webbed hind feet and flat, scaly tails aid in swimming and balance.
Herbivorous and Generalist Diet
Beavers are strict vegetarians. They have an herbivorous and generalist diet, meaning they consume a wide variety of plant matter. While they are known for felling trees, wood is not their sole source of nutrition.
Seasonal Variations in Diet
The beaver diet depends significantly on the season.
Spring and Summer
During the warmer months, beavers primarily feed on herbaceous plant material. This includes:
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- Leaves
- Roots
- Herbs
- Ferns
- Grasses
- Sedges
- Water lilies (Nymphaea and Nuphar)
- Water shields
- Rushes
- Cattails
Succulent, actively growing herbaceous plants constitute the bulk of their diet when available. Aquatic plants, especially water lilies, are a crucial component where accessible.
Fall and Winter
As vegetation dies back in the fall and winter, beavers shift their focus to the bark and cambium (the inner bark layer) of woody plants. Favored tree and shrub species include:
- Aspen
- Birch
- Oak
- Dogwood
- Willow
- Alder
- Cottonwood
- Poplar
- Maple
- Black cherry
- Apple trees
While they may consume fir, pine, and other conifers, they generally do not favor them for food, instead using them for dam building.
Specific Plant Preferences
Beavers exhibit preferences for certain plant species. For example, in North Texas, they show a clear preference for Black Willow trees, capable of reducing willow stands to stumps in short order. They also favor Pickerel Weed, a succulent plant found near water bodies. Giant Ragweed is another plant they enjoy, especially in late August.
However, preferences can vary depending on location and availability. A surprising example is the underwhelming reaction of some beavers to quaking aspen when their main woody food source is eastern white pine. This suggests that factors such as familiarity and gut microflora may influence food choices.
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Food Caching for Winter
To survive the winter when fresh vegetation is scarce, beavers create food caches. They pile up tree and shrub branches in the water near their lodge, anchoring them to the bottom. This underwater stash remains accessible even when the pond freezes over, allowing them to feed throughout the winter. They may also cache water lily rhizomes.
Nutritional Adaptations
Beavers have several adaptations to aid in their herbivorous diet. Their intestine is six times longer than their body, and their caecum (a pouch in the digestive system) is double the volume of their stomach. Microorganisms in the caecum help them process about 30 percent of the cellulose they consume.
Beavers and Trees
Beavers do not simply eat any tree they come across. They select certain varieties for food and others for building dams and lodges. They fell trees by gnawing around the base, using their strong jaws and sharp incisors. The sound and feel of flowing water stimulate beavers to build dams. You can spot areas of beaver lumbering where there are sharply pointed tree trunks, with grooves from their teeth marks, as well as piles of wood shaving around the trunks.
Beavers and Agriculture
In agricultural areas, beavers may raid crops such as corn and other grains. They have also been known to eat apples and clover. They may also eat ferns.
Beaver Kits
Baby beavers, called kits, nurse from their mothers' milk. After about six weeks, they begin to eat solid food, with family members helping to provide them with sustenance.
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Habitat and Food Availability
Beavers tend to build their homes alongside rivers and the edges of other bodies of water, within close proximity to trees. If the food supply in an area becomes depleted, they will eventually move.
Impact on the Ecosystem
Beavers are considered a keystone species because of their significant impact on the environment. Their dam-building activities create wetlands that benefit a wide array of other species. Dams increase water depth, allowing beavers to store food where it will not be frozen into ice in the winter. Flooding also lets beavers construct underwater entrances to their lodge, which in turn protects them from predators. The higher water level promotes the growth of favored aquatic food plants. Beaver dams create habitat for many other animals and plants. Moose use the highly nutritious emergent and submergent aquatic plants found in the deeper beaver flowages. In winter, deer and moose may frequent beaver ponds to forage on shrubby plants that grow where beavers cut down trees for food, dams, or lodges. Deer benefit from lush meadows that develop along flowages when beaver dams no longer hold water. Otters, mink, raccoons, and herons hunt frogs and other prey along the marshy edges of beaver ponds. Waterfowl such as black ducks, wood ducks, hooded mergansers, and green-winged teal are closely tied to these flowages to forage, raise young, and rest during migration. Ducks and geese may even nest on top of beaver lodges, which offer warmth (from the beavers that live below) and protection (especially when lodges are located in the middle of a pond). Trees killed by rising water levels provide perch sites for avian predators, habitat for insects, and food for insect-eating birds such as woodpeckers.