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All About the Biology of Beavers

Digestion

     Beavers are vegetarians and like to eat trees as well as the cambium that lies beneath the bark. Beavers will also feed on a variety of aquatic plants including water lilies, pondweed and roots. Beavers prefer the wood of the quaking Aspen, Cottonwood, Willow and Alder trees. During winter months they stockpile branches on the bottom of the pond and have a cachet they can rely on until spring. They eat the bark from the inside of their dams allowing them to avoid hibernation during the winter months.

    Beavers do not actually eat the wood. Beavers eat the leaves, small twigs and the bark. Beavers to not eat the inside of trees because they have trouble digesting cellulose and wood. The leaves twigs and bark contain most of the nutrients in a tree and there is much less cellulose. Beavers have to eat the same food twice. A beaver’s stomach has special bacteria to help them digest their food. The first time a beaver eats its food a soft pellet is produced, which the beaver eats again. The beaver has a special gland that works with the stomach to pre-digest food. The food that is digested the first time is made into a moist green pellet that passes through the cloaca. The cloaca is the beavers vent for excretion, reproduction and scent discharges. A second digestion extracts more nutrients before the final pellet is produced and eaten for the second time. Fermentation by special intestinal microorganisms allows beavers to digest 30% of the cellulose they ingest. The process is called coprophagy and allows the beaver to get all the nutrients they need from their food.

Reproduction

      Beavers are monogamous and both parents help raise the children, called kits. Beavers are born in litters of 1-9 with the average litter being 4 kits. On average 25% of the kits survive to adulthood. Beavers rarely overpopulate the area they live in because they only mate once a year. Mating season runs from January to March in cold regions and November to December in the South. Most beavers are born in early spring. The gestation period of beavers is three months. Once the kit is born the baby beaver stays with its parents for two years. Older offspring will also live in beaver families to help their parents until they are about two. Older siblings will help feeding, grooming and protecting younger offspring. Older siblings may take over parenting duties if one of the original parents die or are separated from them. This practice is called alloparental care. The extended family helps increase chances of survival for younger offspring. Beavers can recognize their families by differences in anal gland secretions. When beavers reach adulthood they usually do not settle far from their parents.

     The genitalia of beavers look identical for both sexes on the outside but beavers can detect oil that is produced near the anus to identify males or females. For females the color of the oil is yellow and males have a brown coppery oil. The female typically become sexually active by two years old. When beavers are born they weigh about a pound each. Female beavers only have four nipples so they can only nurse four kits at a time. Young beavers developed at a quick rate and they are able to walk on the day they are born. They are able to swim soon after. Beaver’s milk has 11.2% protein and 19% fat. By the time the kits are a month old they will no longer need the nutrition of their mother's milk and they are able to eat solid foods. When they are weaned they weigh about 4 pounds. Kits interact with each other and their parents at great distances by making a high-pitched whining sound. After a beaver has spent the second winter with their family it is time to go off and find a mate of their own.

Sensing the Environment

      The most unique beaver behavior by far is their ability to recreate their entire world the way they want it. Beavers are well known for building dams on rivers and streams. No other animals, except for humans, create their entire environmental landscape on their own. Beavers not only build homes in the pond that is created from their dams but they also build canals to float building materials that are difficult for them to haul overland. Beavers are master builders placing vertical poles first then filling in the gaps between the poles with the crisscross of meticulously placed branches. Beavers improve the water systems and act as a keystone species in an active system by creating wetlands used by many other species. Beaver dams also protect them against predators and provide access to food during the winter. Beavers work at night carrying mud, stones and timber between their teeth. Beavers may create a series of dams along one River. Beavers cover their lodges each autumn with fresh mud that freezes when frost arrives and becomes as hard as stone. The beaver lodges have underwater entrances that are nearly impossible for other animals to get inside.

      Beavers have an excellent sense of smell, hearing and touch. They use these three senses to interact with the world. Beavers have terrible vision because their eyes are very small and their vision is not accurate. Beavers cannot see well above water. Beavers have an extra eyelid called a nictating membrane that protects their eyes and enables them to see underwater. The beaver sense of smell is vital for finding food and identifying members of its own family. Their nose and ears have a special inner flap that seals out water automatically when it touches water. They have a special passage from their nose to their throats that connect to the upper lungs. This allows the beaver to chew and carry wood at the same time. This also allows the beaver to hold its breath for up to 15 minutes underwater. Beavers have very small ears but they're hearing is excellent. They have oversized auditory canals that allow them to pick up not only sounds but vibrations underwater. Beavers are well known for their alarm signals. When beavers are startled or frightened they will use their large tales to slap the water to warn others over great distances above and below the water of danger. All the surrounding beavers will get to water as quickly as they can because they are so slow on land but very fast in the water. Once submerged in the water beavers will dive as deeply as they can and wait for predators to pass.

Support and Movement

     Beavers are the largest rodent species in North America and Europe and generally weigh between 40 to 60 pounds. They are semi-aquatic animals that spend most of their time in the water. They are equipped with unique features that allow for easy navigation in water including webbed hind feet to assist in swimming and dense fur used as insulation in the cold water. Beavers have a broad flat tail that helps them swim and communicate with other beavers. Beavers look uncoordinated and waddle on land but are very graceful in the water. Their paddle shaped tails act like rudders in the water allowing them to move fluidly. Beavers can swim up to five miles an hour. Their fur is naturally oily and waterproof. Their skin acts like a barrier to chemicals and physical attacks. Under the beaver skin they have two layers, the dermis and epidermis. The epidermis has many of its own layers that protect and sustain the body. The dermis has tissue, nerves and blood vessels. Under the dermis is where fatty tissue is stored. The beaver has strong claws that can dig. Beaver’s hind feet are perfect for swimming. North American beavers can walk on two feet but primarily walk on four. Beavers have rootless teeth that keep growing throughout its life. The incisors of the beavers must continue to gnaw or they will grow too long and can cause serious health problems for the beaver.

      Beavers have many physical adaptations for their woody diet. Beavers have large jaw muscles to power the bevel edged continuously growing incisors. The incisors are hard orange enamel on the front and softer dentin on the back. Female beavers are often larger than males. Beavers continue to grow throughout their lives and they live about 24 years in the wild. European and North American beavers are similar to each other but there are some differences in the species. European beavers tend to be larger with smaller heads and narrow muzzles. European beavers also have shorter shinbones making it more difficult for them to walk on two legs. North American species often walk on two legs and run on four legs. North American beavers have broader tails. In both species the rear legs are more powerful than the front legs.

Gas Exchange

      Beavers have the standard mammalian respiratory system, just like humans. Beavers also have large airtight pouches in their cheek's that allow them to take in large quantities of air and store it why they swim underwater. They use their lungs to take in oxygen that is diffused into their bloodstream. The carbon dioxide in the blood stream is then transferred to the lungs to be expelled during exhalation. The beaver’s lungs and liver allow for more oxygen storage than in any other mammal. As the beaver goes underwater the airtight valves in its nose and ears are blocked automatically. As soon as the beaver is out of water it is able to keep respiring. Beavers are able to slow their heartbeat while they swim in order to conserve energy and air. Beavers can say submerged under water for 15 minutes and swim a half a mile without taking another breath and resurfacing. The beaver’s respiratory system works in a similar fashion as many other mammals. Air enters through the nose and moves through the pharynx and the larynx. The air then travels through the trachea and comes to the two bronchi. This separates the air into the lungs and gas exchange can take place. Blood pumping through the heart delivers the oxygen and takes the carbon dioxide, which is then expelled from the body.

Works Cited

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