Beavers are well-adapted for semi-aquatic life, with thick waterproof fur, a flattened tail that acts as a rudder, and closable nostrils and ears, as well as a transparent eye membrane.
There are only two species of beaver. The American beaver (Castor canadensis) typically weighs 60 lbs. (27 kilograms) and are 23 to 39 inches (60 to 100 centimeters) long. The tail adds another 7.75 to 12 inches (20 to 30.5 cm) to its length, according to National Geographic.
Eurasian beavers (Castor fiber) are around the same size. They usually weigh from 29 to 77 lbs. (13 to 35 kg) and are 29 to 53 in (73 to 135 cm) in length, according to the Animal Diversity Web (ADW) at the University of Michigan. Their tails are narrower, and skulls are smaller than those of the American beaver.
The world’s largest beaver dam stretches 850 meters deep in the thick wilderness of northern Alberta. It was discovered after being spotted on a satellite image in 2007, but scientists believe multiple generations of beavers have been working on the dam since the 1970s.
To gnaw through tree trunks, they need extra-strong teeth. Fortunately, their tooth enamel contains iron, which makes them incredibly strong, sharp, and orange. Because the orange enamel on the front of their teeth wears away more slowly than the white dentin on the back, a beaver’s teeth self-sharpen as he chews on trees.
The beaver’s diet consists almost entirely of woody plant material, mainly bark, though perhaps not surprisingly the wood is indigestible to them (as to most mammals). So, to aid digestion, beavers have microorganisms living in their small intestine that cause the wood to ferment, making the beaver able to absorb its nutrients.
During summer months, their diets often shift from woody plants to aquatic vegetation, including water lilies and the rootstalks of shoreline ferns.
Beavers have large teeth. Their upper incisors are from 20 to 25 mm long, according to ADW. They continue to grow throughout a beaver’s lifetime. Beavers have adapted to a semi-aquatic existence with closable nostrils and ears, and transparent eye membranes.
Both male and female beavers have a pair of scent glands, called castors, at the base of their tails. They use the secretions from these glands, a musk-like substance called castoreum, to mark territory.
All beavers need water to survive. They live in or around freshwater ponds, lakes, rivers, marshes and swamps. American beavers live throughout North America but stay clear of deserts and the far northern areas of Canada. Eurasian beavers once lived all over Europe and Asia. Now, they only live in small numbers throughout southern Scandinavia, Germany, France, Poland, and central Russia due to overhunting.
Beavers are very social and live in groups called colonies. One lodge is often the home for a monogamous couple, their young and the yearlings born the year before.
Beavers mate during the winter, from January to March. The Eurasian beaver has a gestation period of around 60 to 128 days. Then, they give birth to one to six babies that weigh around 8.1 to 22 ounces (230 to 630 grams), according to ADW. Baby beavers are called kits. Eurasian kits are usually weaned after six weeks of life.
American beavers have a gestation period of around 105 to 107 days. They give birth to one to four kits that weigh around 9 to 21 ounces (250 to 600 g). American beavers are usually weaned in around two weeks. At around 2 years of age, the kits leave the lodge and make one of their own. At 3 years, they find a monogamous mate.
Deborah Wilson Nov. 2019
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