Beaver


 * By Brendan McKee **

**1. Classification/Diagnostic Characteristics **
 * Kingdom: Animalia**
 * Phylum: Chordata**
 * Class: Mammalia**
 * Order: Rodentia**
 * Family: Castoridae**
 * Genus: Castor**
 * Species: Canadensis**


 * The beaver, known as //Castor Canadensis,// is classified as a chordate. It usually has a lifespan of up to 24 years, with 5-10 years living in the wild. As an adult, the beaver can be around 30-51 inches long from head to tail and can weigh 40-60 pounds. The beaver contains a notochord (dorsal supporting rod), a dorsal hollow nerve cord, and a tail extending beyond the anus. The notochord is composed with fluid-filled vacuoles, which provides rigidity and flexibility. The beaver is a type of rodent, the greatest populated group of living mammals in the world. It is usually defined by its teeth that are adapted to be able to gnaw through substances like wood. As an aquatic animal, it uses a transparent membrane when swimming underwater to cover its eyes and flaps to cover its nostrils and ear holes. The beaver has hand-like front paws for human-like dexterity, and large webbed hind feet for swimming. It also has excellent senses of hearings and relies heavily on this due to their lack of developed eyesight. The large teeth are used as incisors to cut down trees, and though they can be damaged during usage, they continue to grow throughout the lifespan of the beaver.**



**2. Relationship to Humans** **The beaver has a common ancestor with humans, with the break off in the phylogenetic tree occurring in the differences as eutherian mammals. Humans are categorized as primates, while the beaver is categorized as a gnawing mammal. Both man and beaver have many internal systems in common and reproduce in the same sexual manner. Many humans hunt beavers for their pelts and destroy their dams.** **In the past, the beaver was widely hunted for its pelts, testicles, and a substance called castoreum found in the beaver's castor sacs. The pelts were used for clothing and were used by the Native Americans in the 17th century to barter with European traders. The testicles when dried had many "medical" uses, as a pain reliever, or ironically as a contraceptive. Castoreum had uses as an analgesic, anti-inflammatory, and antipyretic. It still sees use today in perfumes.**

**3. Habitat and Niche** **The beaver is found in the Northern he misphere, and many different species are dispersed throughout North America and Eurasia. It has specific habitats usually located near water sources as it requires water to survive. The beaver constructs its habitat by making dams and lodges in rivers. The dams are made primarily out of trees, which a beaver cuts down using its razor-sharp teeth, and mud, which is gathered from the bottom of the river. Dams disrupt the flow of water in the river, creating a pond. Dams are useful to the beaver for food storage during the cold winters. It is considered a keystone species, a species that greatly affects the ecosystem that it inhabits to the extent that many are dependent on the changes that the species creates in the environment and ecological relationships. This consideration is because the pond that beaver dams create initiates a new habitat for many other species that would not normally live there. The beaver also creates lodges made out of the same materials as dams to provide a safe shelter during the winter. ** **Beavers often live in colonies, made up of a male female and offspring. They mark their territory by spraying a special scent from their scent glands. Beaver dams also play a huge role in succession. Abandoned dams allow for the growth of new plants, creating a marsh, and eventually a woodland. The wetland environments that are formed by the creation of a dam help provide habitats for new species, purify water, and also slow water erosion. However, negative impacts include the slowing of water flow and more floods in area prone to water buildup.**



**4. Predator Avoidance** **Beavers, similar to other mammals with a strong sense of smell, are able to sense predators based off of their smell and avoid confrontation with the predator. Young beavers are especially susceptible to predation. The predators of beavers are bobcats, coyotes, wolverines, wolves, fisher cats, and mountain lions. Also, many humans catch beavers for their pelts and destroy their dams. Sometimes the dams or "lodges" that they build are what helps them avoid predators. These habitats make them difficult to catch. Also, beavers use their heavy, flat tails to warn predators in the area by flapping them violently on the surface of the water. This also warns other beavers in the vicinity that danger is nearby. Lastly, the few predators that beavers have are not nocturnal as the beaver usually is, which allows it to get a lot of the work done on its dam and lodge without disruption from predators.**

**5. Nutrient Acquisition** **Beavers change their diet according to the season. With the ability to break through wood with their teeth, beavers are commonly found eating bark off of trees along with other wood-like materials in order to obtain nutrients. During the winter months, this makes up half of their diet. During the summer months, woody plants make up only 10 percent their diet. Aspen trees are usually a beavers preference, although they also enjoy black cherry, beech, maple, alder and birch trees. Beavers also eat the "cambium" of trees, which is the soft and smooth layer just below the bark.** **Because of it's diet, the beaver ends up consuming a lot of cellulose. Because mammals cannot digest cellulose, the beaver has adapted a special digestive system. This diet completely lacks meat, as beavers only eat twigs, foliage, cambium, tree bark, buds, roots, ferns, shrubs, grass, and water plants. Beavers will sometimes gnaw a tree in order to sharpen its teeth, as the back of it's teeth has less strength than the front, which allows the teeth to be shaped in a more sharp orientation. Knocking down trees is a common method used by the beaver to get to the more tender branches.**

 **6. Reproduction and Life Cycle** **Male beavers have a vomeronasal organ, which is a small tubular structure deep within the nasal epithelium. Every time the beaver sniffs, nasal fluid is released over the chemoreceptors on the nasal wall. This information travels to the olfactory bulb located in the brain, which is able to activate sexual and other instinctive behaviors. This VNO is what allows mating to occur, as it is essential for male beavers to detect sexually receptive females. The sexual intercourse that occurs between beavers is the same as most other mammals, with sperm from the male fertilizing the egg from the female, causing impregnation.** **Beavers stay with the same mate for life. They mate in winter (January to March) and females give birth in a lodge sometime between April and June. A single litter each year usually contains four kits which are baby beavers (but may have as many as nine). Young kits spend most of their time in the lodge where they are relatively safe from predators. Although they are weaned by three months of age, young beavers will stay with their parents through two winters before dispersing the following spring. A single family unit of beavers is typically made up of two adults, that year's kits, and young from the previous year. Such a group is called a colony and usually contains six to eight individuals in areas where harvest pressure is low. Most beavers become sexually mature in their third year at which time they leave, or are driven out by the parents, to seek mates and territories of their own. Adult beavers have few predators, and they may live up to twenty years or more in the wild.**

**7. Growth and Development** **Beavers grow as most multicellular organisms do, with number of cells or individual cell size increasing over a large period of time during its lifespan. The average adult beaver grows up to 4 feet and weighs 45-60 pounds, along with having a tail up to 16 inches long and 5-7 inches wide.**

**8. Integument** **Beavers have sweat glands. The sweat glands allow the body to secrete sweat onto the skin, which eventually evaporates and allows the body to be cooled. Beavers have fur coats that are coated with castoreum, an oily secretion from scent glands, which help waterproof the thick fur. Also, they have adipose tissue underneath their skin which provide more insulation and protection from the cold environment. They have webbed hind legs which propel them through the water and short front legs with claws for protection from predators and to provide grip when gnawing on trees. Additionally, beavers have flat hairless tails which give balance when they swim and signal any danger in the surroundings.**

**9. Movement** **Beavers have tissues within their bodies, one of which is muscular tissue. This tissue contracts, which allows movement to occur for a beaver. On land, the beaver moves in an awkward, waddle-like manor, though it may swim at speeds up to 8 kilometers per hour. The beaver's aerodynamics are due to large webbed feet, waterproof and oily fur, and a tail which moves in a rudder-like fashion. To see clearly underwater, the beaver uses its pair of transparent eyes.**

**10. Sensing the Environment** **Beavers, though able to see color, have small eyes that force nearsightedness. The nictitating membrane located on the eyes of the beaver activates and protects the eyes when beavers submerge in water, which allows them to see more clearly. To compensate for bad eyesight, beavers have very good olfactory systems which allow them to smell at long distances. This sense is used by beavers to find food, identify other beavers, and detect obstructions or predators. The nose and ears also have flaps that seal water from entering the body during submergence. Hormones will be secreted by the body in order to allow the body to be regulated to its set point. The nose also has a passage to the throat that connects to the lungs, which allows beavers to be submerged for long periods of time. Also, small ears are made up for by the large auditory canals within the ear, which allows the beaver to be able to sense danger while underwater.**

**11. Gas Exchange** **Beavers are similar to all mammals, having a typical respiratory system involving lungs taking in air with high concentrations of oxygen and then diffusing it into the blood stream when a beaver inhales. The carbon dioxide from the blood stream is displaced to the lungs and breathed out when a beaver exhales. The flaps that cover the ears and nose while underwater also serve to prevent the body from expelling air until brought back to the surface.**

**12. Waste Removal** **Beavers, like all mammals, have kidneys which filter the blood and remove waste by diverting it to the urinary bladder.** **Like other mammals, beavers convert the nitrogenous portion of their wastes from ammonia to urea before removal, a process which takes place in the liver and kidneys. Mammals do this despite a net energy cost to their metabolism due to the great toxicity of ammonia, which would require great dilution to excrete and therefore frequent drinking and urination.** **Beavers are able to excrete solid wastes through the anus.**

**13. Environmental Physiology (Temperature, Water, and Salt Regulation)** **Due to its endocrine system, beavers are able to maintain homeostatic needs (temperature, water regulation, etc.). The endocrine system works as a feedback mechanism, with signals being sent to activate hormones due to irregularities within the body. Whether positive or negative, the feedback is intended to regulate the body back to its set point. Additionally, the tail of the beaver stores hair and is virtually hairless. Therefore, when necessary, the tail can release heat and regulate the body temperature.**

**14. Internal Circulation** **Beavers have the same closed circulatory system as all vertebrates, with the pulmonary circuit taking blood from the heart to gas exchange organs and back to the heart, and systemic circuits taking blood from the heart through the body and back to the heart. The blood removes wastes and picks up oxygen at the lungs, which is then provided throughout the rest of the body to keep its functionality at top shape.**

**15. Chemical Control (i.e. Endocrine System)** **Beavers have the same endocrine system as any other mammal, as they have many different glands that work to keep the body regulated. Also, beavers have a gender-dependent body part (testes for males, ovaries for females) to keep the system functioning.**

**Review Questions:** **1. Why do beavers have sweat glands?** **2. Why must beavers change their diets depending on the season?** **3. Why do beavers build dams and lodges?** **4. What role do the teeth of a beaver have in development of habitat and nutrient acquisition?** **5. How do beavers compensate for terrible eyesight?**

"HowStuffWorks "Fooled by Nature: Beaver Dams"" HowStuffWorks. Discovery Communications, n.d. Web. 16 Nov. 2013.  [] http://en.wikipedia.org/wiki/Kidney http://animals.pawnation.com/beaver-eat-1661.html http://animals.pawnation.com/beavers-prey-predators-3787.html [] http://www.bio.umass.edu/biology/conn.river/beaver [] http://www.theanimalspot.com/americanbeaver.htm http://wdfw.wa.gov/living/beavers.html
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