Beaver anatomy and physical make-up
The average length is 35 to 52 inches measured from the head to the tip of the tail. Adult beavers weigh 40 to 70 pounds (18 to 32 kg) . Records of 96 pounds (44 kg) for a Missouri beaver and the largest castor canadensis was 110 pounds (50 kg) from Wisconsin. After late summer beavers start putting on weight. This makes it easier to deal with the freezing winter temperatures relying o extra body fat for insulation. Kits weigh 9 to 15 pounds (4 to 7 kg), by the age one they are 21 pounds (10 kg) and by age 2 they are 30 pounds (15 kg). From the time they are born the beaver uniquely never stops gnawing.
The beaver tail has many uses. On land they use it when tree felling to support themselves, to balance when carrying mud walking on hind legs. In the water beavers can turn their tails in four directions to act like a rudder while swimming. The surface is leathery with a fish like look, coarse hairs cover the tail where it joins the body. Tails measure 6 to 12 inches (15 to 30 cm) long and 4 to 5 inches (10 to 13 cm) wide and about 3/4 inch (15 mm) thick.
The fur has two different lengths. Guard hairs measure 2.5 inches long and the dense mat of underfur is about 3/4 inches long. The two layers of fur trap air in between while swimming under water and helps protect the beaver from the cold water.
Beavers have a stout body size with legs that have separate lower leg bone, unlike rats and mice that have fused leg bones. The feet have evolved to carry out different jobs. The front feet are short and look like small web-less hands with sharp pointed claws that are excellent for moving branches and digging into mud banks or tearing up vegetation for feeding or construction.
The webbed hind feet are much larger and like the front feet have five digits. One digit has two claws, this is used to clip out or remove burrs, chips or twigs from their coats. These rear legs are what propel the beaver through the water.
Digesting food the beaver has to eat the same meal twice. Breaking down high amounts of cellulose from bark and wood in the diet the beaver has a special gland that works with the stomach and a large appendix to pre-digest food. While the food passes through the first time its made into a moist green pellet that is discharged through the cloaca. It is then eaten for the second time and expelled as excrement into the water as brown dung. The process is called coprophagy allowing the beaver to get maximum nutritional benefit from their food.
The cloaca is the beavers only vent for excretion, reproduction, and scent discharges. This cavity is where intestinal and genitourinary tracks empty. Both scent glands empty into the cloaca. Each pair has different uses. The smaller pair are oil glands the beaver uses to waterproof their fur. Castoreum is discharged from the larger pair and is a thick orange yellow like liquid used by the beaver for scent marking its territory.
Brain, Ears, Lips, Eye Vision and Nose
Beavers intelligence is above all rodents other than the Norwegian rat. The hearing sense is highly developed and can easily classify dangerous to friendly sounds. The ears of the beaver are small and when swimming valvular flaps close tightly, stopping water entering the auditory canal. Beaver lips are large and close behind the teeth. They are covered with fur. This allows beavers to carry branches while swimming and cut material under water. Beaver eyes are not really good. They can see but they rely more on sound and smell. When underwater the beaver has a thin film that covers the eyes and acts like goggles. This helps their sight. The eyes have 3 eyelids that help protect the eyes from branches and twigs. With an excellent sense of smell the beaver can know most of the time what is happening around it. The nose can close up for underwater swims with the use of built in valves.
The beavers mouth is made up of 20 teeth and 16 molars, mainly used for grinding bark or other food for digesting. The 4 front incisors never stop growing and have a length of 2 1/2 inches (4.5 cm) long. They are hardest on the front and softer on the inner surfaces. This keeps the teeth sharp for gnawing of tree trunks and branches.