Biology Reference
In-Depth Information
The manner in which the dispersed two-year-olds find mates is an interesting one, worthy of a bit of
exploration. The beaver's genus name,
Castor,
comes to us through Latin from
Kastor,
the Greek name
for the beaver. Eventually,
castor
even became a term for a beaver hat during the heyday of that article
of apparel.
Of greater relevance in this instance is that beavers of both sexes secrete an oily, pungent substance
that's also known as castor. This rather malodorous liquid is produced by a pair of glands in the inguinal
region, close to the anus. Beavers are highly territorial and have a well-developed sense of smell, so as
soon as a young beaver finds a suitable site for a dam, it marks the boundaries of its territory with what
are called
castor mounds.
Castor mounds are piles of mud and debris carried by the beaver with its front paws. These heaps are
normally placed close to the water, and the beaver deposits castor on them. Such mounds can grow into
sizable structures, especially where the territories of two or more colonies intersect. In that situation, a
beaver from one colony will try to cover the scent of another colony by heaping more material on the
mound and then depositing its own castor. As this “castor duel” continues, the mounds can sometimes
grow to waist height on a human!
Because of its excellent sense of smell, a two-year-old beaver scouring the countryside for a home
will soon locate and investigate castor mounds. If the mounds' originator proves to be an unmated
beaver of the opposite sex, a new colony has its start.
Beavers are obviously highly aquatic mammals, and they have a number of wonderful evolutionary
adaptations that make it easy for them to function in and under the water. When it comes to moving
about in the water, beavers are extremely well equipped. In addition to the nearly all-purpose tail, the
beaver has large, fully webbed hind feet to provide efficient propulsion. These can thrust the big animal
through the water with ease, even when the front feet are occupied with carrying mud or rocks.
Beavers can also stay underwater for an extraordinarily long time—at least fifteen minutes at a
stretch—and they accomplish this feat with lungs of normal size. How is this possible? First, their heart-
beat drops to half its usual rate when the beavers are underwater, thereby reducing their metabolic rate
and their oxygen consumption. Second, they can also utilize most of the oxygen in their lungs—an as-
tonishing 75 percent versus 15 percent in humans!
Nor does working underwater pose problems for the beaver. The instant a beaver submerges, certain
reflex actions take place. A transparent “third eyelid,” called a nictitating membrane, covers each eye
to allow good underwater vision. Simultaneously, special membranes close off the ears and nostrils.
Meanwhile, a similar membrane blocks the throat and permits the beaver to gnaw wood or gather aquat-
ic plants beneath the surface without swallowing water.
To top off the long list of adaptations, the beaver also has a double fur coat. The outer coat consists
of long, glossy guard hairs, while the inner coat is so dense that water doesn't even reach the beaver's
skin. With this sort of protection, the beaver has no difficulty swimming and diving in the most frigid
water.
