Biology Reference
In-Depth Information
aren't that fast. Such a feat would tax most people with a sharp ax and saw! The fact is that a beaver
requires at least two or three nights of hard work, and sometimes more, to take down a single large tree.
Beavers are indisputably dam builders
par excellence,
justly famous for this ability. They live ex-
clusively in colonies, and construction of a dam is the first step in founding a colony for a newly mated
pair of young beavers. There's a good reason for this. A high percentage of our North American beavers
live where winter is severe and the ice becomes very thick. Beavers store their winter food supply un-
derwater, and they require water deep enough that neither the underwater entrances to their home nor
their food supply will be encased by ice.
This is purely instinctive behavior that has nothing to do with reasoning. Beavers evolved under
harsh conditions that necessitated water depth sufficient for winter survival. Now they'll mindlessly
build dams wherever they possibly can, even if there's no apparent reason for it. Further, they'll con-
tinue to raise the height of a dam year after year, oblivious to the fact that the depth of their pond was
more than sufficient from the beginning. Even in large lakes and ponds, beavers will attempt to dam the
outlet, despite water depth many times what they require for survival.
It's incorrect to assume that beavers always build dams, however. Beavers often live in wide, deep
streams and rivers that are too difficult a challenge even for their formidable dam-building skills. Under
such conditions, beavers tunnel up into the riverbank to create a home. Beavers living in this fashion
are often referred to as “bank beavers,” and old-timers frequently claimed that they were a different
species. They aren't, but they do seem to have learned that they can't dam that particular stream, and
have somehow managed to override their age-old compulsion to build a dam.
Dam construction itself is a fascinating process. The beavers begin by laying down brush, with the
butts pointed downstream. Mud and rocks are brought to the site to anchor and coat the brush, and
various materials, from pieces of driftwood to old bottles, are gradually incorporated into the dam as it
grows higher and higher. Mud is constantly packed in among the sticks, brush, and other material, and
the upstream surface of the dam is coated and recoated with mud.
The whole affair is really a beaver version of concrete. Just as we use cement, weak by itself, to
bind gravel and iron rods into extremely strong concrete, beavers use mud as their cement, with brush,
sticks, and other materials as the reinforcing medium. The end result is a structure that's strong and
nearly impervious to water, except for the overflow through the brush and sticks at the very top of the
dam.
A beaver dam is such a fascinating edifice that it's worth inspecting at close range, both for its in-
genious construction and for the variety of materials often incorporated into it. Given the near ubiquity
of beavers today, most people shouldn't find it difficult to locate a beaver dam that they can scrutinize
at leisure and thereby gain a much greater appreciation of this clever rodent's abilities.
In addition to the physical construction of the dam itself, beavers have some related talents. For one
thing, they seem to have an uncanny eye for selecting dam sites that offer a high ratio of pond acreage
to size of dam. From the beaver's point of view, the perfect dam site combines a short span between
