About the Biology of Beavers
Beavers are vegetarians and like to eat trees as well as the
cambium that lies beneath the bark. Beavers will also feed on a
variety of aquatic plants including water lilies, pondweed and
roots. Beavers prefer the wood of the quaking Aspen, Cottonwood,
Willow and Alder trees. During winter months they stockpile branches
on the bottom of the pond and have a cachet they can rely on until
spring. They eat the bark from the inside of their dams allowing
them to avoid hibernation during the winter months.
Beavers do not actually eat the wood. Beavers eat the leaves,
small twigs and the bark. Beavers to not eat the inside of trees
because they have trouble digesting cellulose and wood. The leaves
twigs and bark contain most of the nutrients in a tree and there is
much less cellulose. Beavers have to eat the same food twice. A
beaverís stomach has special bacteria to help them digest their
food. The first time a beaver eats its food a soft pellet is
produced, which the beaver eats again. The beaver has a special
gland that works with the stomach to pre-digest food. The food that
is digested the first time is made into a moist green pellet that
passes through the cloaca. The cloaca is the beavers vent for
excretion, reproduction and scent discharges. A second digestion
extracts more nutrients before the final pellet is produced and
eaten for the second time. Fermentation by special intestinal
microorganisms allows beavers to digest 30% of the cellulose they
ingest. The process is called coprophagy and allows the beaver to
get all the nutrients they need from their food.
Beavers are monogamous and both parents help raise the
children, called kits. Beavers are born in litters of 1-9 with the
average litter being 4 kits. On average 25% of the kits survive to
adulthood. Beavers rarely overpopulate the area they live in because
they only mate once a year. Mating season runs from January to March
in cold regions and November to December in the South. Most beavers
are born in early spring. The gestation period of beavers is three
months. Once the kit is born the baby beaver stays with its parents
for two years. Older offspring will also live in beaver families to
help their parents until they are about two. Older siblings will
help feeding, grooming and protecting younger offspring. Older
siblings may take over parenting duties if one of the original
parents die or are separated from them. This practice is called
alloparental care. The extended family helps increase chances of
survival for younger offspring. Beavers can recognize their families
by differences in anal gland secretions. When beavers reach
adulthood they usually do not settle far from their parents.
The genitalia of beavers look identical for both sexes on the
outside but beavers can detect oil that is produced near the anus to
identify males or females. For females the color of the oil is
yellow and males have a brown coppery oil. The female typically
become sexually active by two years old. When beavers are born they
weigh about a pound each. Female beavers only have four nipples so
they can only nurse four kits at a time. Young beavers developed at
a quick rate and they are able to walk on the day they are born.
They are able to swim soon after. Beaverís milk has 11.2% protein
and 19% fat. By the time the kits are a month old they will no
longer need the nutrition of their mother's milk and they are able
to eat solid foods. When they are weaned they weigh about 4 pounds.
Kits interact with each other and their parents at great distances
by making a high-pitched whining sound. After a beaver has spent the
second winter with their family it is time to go off and find a mate
of their own.
Sensing the Environment
The most unique beaver behavior by far is their ability to
recreate their entire world the way they want it. Beavers are well
known for building dams on rivers and streams. No other animals,
except for humans, create their entire environmental landscape on
their own. Beavers not only build homes in the pond that is created
from their dams but they also build canals to float building
materials that are difficult for them to haul overland. Beavers are
master builders placing vertical poles first then filling in the
gaps between the poles with the crisscross of meticulously placed
branches. Beavers improve the water systems and act as a keystone
species in an active system by creating wetlands used by many other
species. Beaver dams also protect them against predators and provide
access to food during the winter. Beavers work at night carrying
mud, stones and timber between their teeth. Beavers may create a
series of dams along one River. Beavers cover their lodges each
autumn with fresh mud that freezes when frost arrives and becomes as
hard as stone. The beaver lodges have underwater entrances that are
nearly impossible for other animals to get inside.
Beavers have an excellent sense of smell, hearing and touch.
They use these three senses to interact with the world. Beavers have
terrible vision because their eyes are very small and their vision
is not accurate. Beavers cannot see well above water. Beavers have
an extra eyelid called a nictating membrane that protects their eyes
and enables them to see underwater. The beaver sense of smell is
vital for finding food and identifying members of its own family.
Their nose and ears have a special inner flap that seals out water
automatically when it touches water. They have a special passage
from their nose to their throats that connect to the upper lungs.
This allows the beaver to chew and carry wood at the same time. This
also allows the beaver to hold its breath for up to 15 minutes
underwater. Beavers have very small ears but they're hearing is
excellent. They have oversized auditory canals that allow them to
pick up not only sounds but vibrations underwater. Beavers are well
known for their alarm signals. When beavers are startled or
frightened they will use their large tales to slap the water to warn
others over great distances above and below the water of danger. All
the surrounding beavers will get to water as quickly as they can
because they are so slow on land but very fast in the water. Once
submerged in the water beavers will dive as deeply as they can and
wait for predators to pass.
are the largest rodent species in North America and Europe and
generally weigh between 40 to 60 pounds. They are semi-aquatic
animals that spend most of their time in the water. They are
equipped with unique features that allow for easy navigation in
water including webbed hind feet to assist in swimming and dense fur
used as insulation in the cold water. Beavers have a broad flat tail
that helps them swim and communicate with other beavers. Beavers
look uncoordinated and waddle on land but are very graceful in the
water. Their paddle shaped tails act like rudders in the water
allowing them to move fluidly. Beavers can swim up to five miles an
hour. Their fur is naturally oily and waterproof. Their skin acts
like a barrier to chemicals and physical attacks. Under the beaver
skin they have two layers, the dermis and epidermis. The epidermis
has many of its own layers that protect and sustain the body. The
dermis has tissue, nerves and blood vessels. Under the dermis is
where fatty tissue is stored. The beaver has strong claws that can
dig. Beaverís hind feet are perfect for swimming. North American
beavers can walk on two feet but primarily walk on four. Beavers
have rootless teeth that keep growing throughout its life. The
incisors of the beavers must continue to gnaw or they will grow too
long and can cause serious health problems for the beaver.
Beavers have many physical adaptations for their woody diet.
Beavers have large jaw muscles to power the bevel edged continuously
growing incisors. The incisors are hard orange enamel on the front
and softer dentin on the back. Female beavers are often larger than
males. Beavers continue to grow throughout their lives and they live
about 24 years in the wild. European and North American beavers are
similar to each other but there are some differences in the species.
European beavers tend to be larger with smaller heads and narrow
muzzles. European beavers also have shorter shinbones making it more
difficult for them to walk on two legs. North American species often
walk on two legs and run on four legs. North American beavers have
broader tails. In both species the rear legs are more powerful than
the front legs.
Beavers have the standard mammalian respiratory system, just
like humans. Beavers also have large airtight pouches in their
cheek's that allow them to take in large quantities of air and store
it why they swim underwater. They use their lungs to take in oxygen
that is diffused into their bloodstream. The carbon dioxide in the
blood stream is then transferred to the lungs to be expelled during
exhalation. The beaverís lungs and liver allow for more oxygen
storage than in any other mammal. As the beaver goes underwater the
airtight valves in its nose and ears are blocked automatically. As
soon as the beaver is out of water it is able to keep respiring.
Beavers are able to slow their heartbeat while they swim in order to
conserve energy and air. Beavers can say submerged under water for
15 minutes and swim a half a mile without taking another breath and
resurfacing. The beaverís respiratory system works in a similar
fashion as many other mammals. Air enters through the nose and moves
through the pharynx and the larynx. The air then travels through the
trachea and comes to the two bronchi. This separates the air into
the lungs and gas exchange can take place. Blood pumping through the
heart delivers the oxygen and takes the carbon dioxide, which is
then expelled from the body.
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