Beaver Basics
by Amy Himes
Ancient Giants
One of the largest members of the rodent family, beavers evolved from the so-called giant beavers—Castoroides of North America and Trogontherium of Europe—that roamed the Earth more than one million years ago. Dated remains suggest that giant beavers lived as recently as 10,000 years ago. These beavers may not have depended upon water as much as their successors, as no evidence suggests they built dams. Rather, they lived in large burrows in the ground. The species resembled the modern beaver, only much, much larger. With a massive tail more than two feet long and incisors up to six inches long, some fossilized specimens were as large as a black bear.
A beaver on the Zoo's Valley Trail.
Stephanie Garnett/FONZ
Dams and Families
Beavers live an average of ten to 12 years in the wild. They generally mate for life and live in family units that consist of an adult male and female and three to four kits. Once they are old enough to leave the nest, kits spend most of their time playing in the water around the lodge, but are buoyant and cannot dive. Young beavers usually stay with their parents—helping out with the family business of maintaining the lodge and dam—until they reach sexual maturity at about two years of age. Then, chased away from their natal territory by their parents, they go stake their own claim, often downstream.
Nature's Builders
Beavers cut trees and drag them to the water to form dams and lodges. Using any rocks or branches lodged underwater or on the bank to build upon, beavers begin adding branches hauled to the water. Lodges are mounds built in the middle of the water, usually not far from the dam. Each lodge has one nesting chamber located above the water's surface and several entrances through underwater channels. Wonderful diggers, beavers may also burrow underground tunnels from the banks up to favorite feeding grounds and excavate channels to other parts of the stream or river. Beavers eagerly maintain their structures by patching them with branches and mud scooped from the floor of the waterway or pool. At times, flooding or storms may destroy lodges and dams, but many withstand the weather.
Engineering the Ecosystem
Beaver dams prevent the normal flow of water, flooding the banks of waterways and creating pond environments. These pools attract many insects, birds, and other creatures that thrive in still water. Certain animals—such as fish that must migrate long distances up or down a river—may not benefit from the beaver's work, and may perish. Flooding caused by the dam may also kill engulfed trees, plants, and insects. But beavers also encourage new life, as they create a marshy area. New insects are attracted to dead wood, and the dampened soil encourages new trees to sprout and grow. As beavers cut trees, branches then grow from the remaining stumps, providing beavers with fresh food.
ZooGoer 31(2) 2002. Copyright 2002 Friends of the National Zoo. All rights reserved.
