 |
| Beavers
(Castor canadensis)
are the largest living rodents in North
America. (Photo
by Ginger Holser.) |
|
|
Beavers
(Castor canadensis) are the largest living rodents
in North America, with adults averaging 40 pounds in
weight and measuring more than 3 feet in length, including
the tail. These semi-aquatic mammals have webbed hind
feet, large incisor teeth, and a broad, flat tail.
Once
among the most widely distributed mammals in North America,
beavers were eliminated from much of their range in
the late 1800s because of unregulated trapping. With
a decline in the demand for beaver pelts, and with proper
management, they became reestablished in much of their
former range and are now common in many areas.
Beavers
are found where their preferred foods are in good supply—along
rivers, and in small streams, lakes, marshes, and even
roadside ditches containing adequate year-round water
flow. In areas where deep, calm water is not available,
beavers that have enough building material available
will create ponds by building dams across creeks or
other watercourses and impounding water.
Beavers
dams create habitat for many other animals and plants
of Washington. In winter, deer and elk frequent beaver
ponds to forage on shrubby plants that grow where beavers
cut down trees for food or use to make their dams and
lodges. Weasels, raccoons, and herons hunt frogs and
other prey along the marshy edges of beaver ponds. Migratory
waterbirds use beaver ponds as nesting areas and resting
stops during migration. Ducks and geese often nest on
top of beaver lodges since they offer warmth and protection,
especially when lodges are formed in the middle of a
pond. The trees that die as a result of rising water
levels attract insects, which in turn feed woodpeckers,
whose holes later provide homes for other wildlife.
 |
Figure
1. The beaver’s incisors (front teeth)
are harder on the front surface than on the back,
and so the back wears faster. This creates a sharp
edge that enables a beaver to easily cut through
wood.
(From
Christensen and Larrison, Mammals of the Pacific
Northwest: A Pictorial Introduction.) |
The
beaver’s incisors (front teeth) are harder on
the front surface than on the back, and so the back
wears faster. This creates a sharp edge that enables
a beaver to easily cut through wood.
Like
many rodents, beavers construct nesting dens for shelter
and for protection against predators. These may be burrows
in a riverbank or the more familiar lodges built in
the water or on the shore (shown here). However, the
basic interior design varies little and consists of
one or more underwater entrances, a feeding area, a
dry nest den, and a source of fresh air.
Facts
about Washington’s Beavers
Food
and Feeding Habitats
-
Beavers eat the leaves, inner bark, and twigs of
aspen (a favorite food), alder, birch, cottonwood,
willow, and other deciduous trees. Beavers also
eat shrubs, ferns, aquatic plants, grasses, and
crops, including corn and beans.
- Coniferous
trees, such as fir and pine, are eaten occasionally;
more often, beavers will girdle and kill these trees
to encourage the growth of preferred food plants,
or use them as dam building material.
- Beavers
have large, sharp, upper and lower incisors, which
are used to cut trees and peel bark while eating.
The incisors grow their entire lives, but are worn
down by grinding them together, tree cutting, and
feeding. (Fig.1)
- Fermentation
by special intestinal microorganisms allows beavers
to digest 30 percent of the cellulose they ingest.
- When
the surface of the water is frozen, beavers eat
bark and stems from a food “cache” (a
safe storage place) they have anchored to the bottom
of the waterway for winter use. They also swim out
under the ice and retrieve the thick roots and stems
of aquatic plants, such as pond lilies and cattails.
- Food
caches are not found consistently where winters
are comparatively mild, such as in the lowlands
of western Washington.
|
Figure
2. Like many rodents, beavers construct
nesting dens for shelter and for protection
against predators. These may be burrows in a
riverbank or the more familiar lodges built
in the water or on the shore (shown here). However,
the basic interior design varies little and
consists of one or more underwater entrances,
a feeding area, a dry nest den, and a source
of fresh air.
(Drawing by Jenifer Rees.) |
Beaver
Dams
-
Beavers flood areas for protection from predators,
for access to their food supply, and to provide underwater
entrances to their den. Flooded areas also wet the
soil and promote the growth of favored foods (Fig.
3).
- Beavers
living on water bodies that maintain a constant level
(e.g., lakes, large rivers) do not build dams.
- Dams
are constructed and maintained with whatever materials
are available—wood, stones, mud, and plant parts.
They vary in size from a small accumulation of woody
material to structures 10 feet high and over 165 feet
wide.
- The
feel and sound of flowing water stimulate beavers
to build dams; however, they routinely let a leak
in a dam flow freely, especially during times of high
waters.
- Beavers
keep their dams in good repair and will constantly
enlarge the dams as the water level increases in their
pond. A family of beavers may build and maintain one
or several dams in their territory.
- In
cold areas, dam maintenance is critical. Dams must
be able to hold enough water so the pond won’t
freeze to the bottom, which would eliminate access
to the winter food supply.
Lodges
and Bank Dens
- Depending
on the type of water body they occupy, beavers construct
freestanding lodges or bank dens.
- Lodges
and bank dens are used for safety, and a place to
rest, stay warm, give birth, and raise young.
- Freestanding
lodges are built in areas where the bank or water
levels aren’t sufficient for a safe bank den.
- Lodges
consist of a mound of branches and logs, plastered
with mud. One or more underwater openings lead to
tunnels that meet at the center of the mound, where
a single chamber is created.
- Bank
dens are dug into the banks of streams and large ponds,
and beavers may or may not build a lodge over them
(Fig.2). Bank dens may also be located under stumps,
logs, or docks.
- One
family can have several lodges or bank dens, but will
typically use only one den during winter.
Reproduction
and Family Structure
-
A mated pair of beaver will live together for many
years, sometimes for life.
- Beavers
breed between January and March, and litters of one
to eight kits (average four) are produced between
April and June. The number of kits is related to the
amount of food available (more food, more kits), and
the female’s age.
- The
female nurses the kits until they are weaned at 10
to 12 weeks of age.
- Most
kits remain with the adults until they are almost
two years old. (Some leave at 11 months and a few
females may stay until they are three years of age.)
The kits then go off on their own in search of mates
and suitable spots to begin colonies, which may be
several miles away.
- Beavers
live in colonies that may contain 2 to 12 individuals.
The colony is usually made up of the adult breeding
pair, the kits of the year, and kits of the previous
year or years.
- Populations
are limited by habitat availability, and the density
will not exceed one colony per ½ mile under
the best of conditions.
Mortality
and Longevity
-
Because of their size, behavior, and habitat, beaver
have few enemies.
- When
foraging on shore or migrating overland, beavers are
killed by bears, coyotes, bobcats, cougars, and dogs.
- Other
identified causes of death are severe winter weather,
winter starvation, disease, water fluctuations and
floods, and falling trees.
- Humans
remain the major predator of beavers. Historically,
beavers have been one of the most commonly trapped
furbearers. In Washington, from 1991 to 2000, an annual
average of 5,289 beavers were trapped. However, the
average for the past three years has dropped to just
over 1,000, due to the passing of Initiative
713.
- Beavers
live 5 to 10 years in the wild.
Viewing
Beavers
Beavers
are nocturnal, but are occasionally active during
the day. They do not hibernate, but are less active
during winter, spending most of their time in the
lodge or den.
Probably
no animal leaves more obvious signs of its presence
than the beaver. Freshly cut trees and shrubs, and
prominent dams and lodges are sure indicators of their
activity.
Look
for signs of beavers during the day; look for the
animals themselves before sunset or sunrise. Approach
a beaver site slowly and downwind. (Beavers have poor
eyesight but excellent hearing and sense of smell.)
Look for a V-shaped series of ripples on the surface
of calm water. A closer view with binoculars may reveal
the nostrils, eyes, and ears of a beaver swimming.
If
you startle a beaver and it goes underwater, wait
quietly in a secluded spot and chances are that it
will reemerge within one or two minutes. However,
beavers are able to remain underwater for at least
15 minutes by slowing their heart rate.
When
seen in the water, beavers are often mistaken for
muskrats. Try to get a look at the tail: Beavers have
a broad, flat tail that doesn’t show behind
them when swimming, whereas muskrats have a thin tail
that is either held out of the water or sways back
and forth on the water’s surface as the animal
swims.
Beavers
stand their ground and should not be closely approached
when cornered on land. They face the aggressor, rear
up on their hind legs, and hiss or growl loudly before
lunging forward to deliver extremely damaging bites.
Harvest
Sites
Beavers cut down trees, shrubs, and other available
vegetation for food and building materials. Large
stumps are pointed, 1 to 2½ feet high, and
sometimes the tree trunk is still attached. (Fig.
4)
Tooth
marks look like twin grooves, each groove measuring
1/8 inch or more. There will be a pile of wood chips
on the ground around the base of recently felled trees.
Limbs that are too large to be hauled off are typically
stripped of bark over the course of several days.
The cut on small wood usually involves a 45-degree
cut typical of rodents, but at a larger scale. Branches
and twigs under ¾ inch in diameter are generally
eaten entirely.
Most
harvesting is done within 165 feet of the water’s
edge. In areas with few predators, but a lean food
supply, toppled trees and other signs of feeding may
be found twice that distance from the den site. Beavers
transport woody material even farther through upstream
and downstream sites.
By
late fall (earlier in cold winter areas of Washington),
all family members concentrate on repairing and building
up dams and the family lodge in preparation for winter.
Harvesting is at its most intense level at this time
of year.
 |
Figure
5. The paths beavers make where they enter
and leave the water are called slides.
(Photo
by Russell Link.) |
Slides
Slides are the paths beavers make where they enter
and leave the water. They are 15 to 20 inches wide,
at right angles to the shoreline, and have a slicked
down or muddy appearance (Fig. 5).
Channels
Beavers construct channels or canal systems leading
to their ponds, using them to float food—such
as small, trimmed trees—from cutting sites.
Canals are also safe travel ways for swimming instead
of walking. With receding water levels during summer,
beaver activity shifts toward building and maintaining
channels to access new food supplies. Channels often
look man-made, have soft, muddy bottoms, and are filled
with 15 to 25 inches of water.
Food
Storage Sites
Beavers that live in cold climates store branches
of food trees and shrubs for winter use by shoving
them into the mud at the bottom of ponds or streams
near the entrance to their bank den or lodge (Fig.
2).
Droppings
Beaver droppings are seldom found on land; those
that are will commonly be found in the early morning
at the water’s edge. Individual beaver droppings
are usually cylindrical, up to 2½ inches long
(sometimes shorter), and look as if they were formed
of compressed sawdust. The diameter is an indication
of the animal’s size, with 1 inch being average
for adults. The color of fresh deposits is dark brown,
with lighter-colored bits of undigested wood, all
turning pale with age.
Sounds
In order to warn each other of danger, beavers
slap their tails against the water, creating a loud
splash. Sounds also include whining (noises made by
kits), a breathy greeting noise, and loud blowing
when upset.
| A
Beaver’s Tail |
The
tail of a large beaver may be 15 inches long
and 6 inches wide. It is covered with leathery
scales and sparse, coarse hairs.
The
beaver’s tail has important uses both
in the water and on land. In the water, the
animal uses its flexible tail as a four-way
rudder. When diving after being frightened,
a beaver loudly slaps the water with its tail;
the sound warns all beavers in the vicinity
that danger is near, and perhaps serves to frighten
potential predators.
On
land, the tail acts as a prop when a beaver
is sitting or standing upright. It also serves
as a counterbalance and support when a beaver
is walking on its hind legs while carrying building
materials with its teeth, front legs, and paws.
Contrary to common belief, beavers do not use
their tails to plaster mud on their dams.
The
tail stores fat, and because it is nearly hairless,
releases body heat, helping the beaver to regulate
its body temperature. |
Preventing
Conflicts
Despite
an appreciation for beavers and our best intentions
to live with them, beavers can become a problem if
their eating habits, and dam or den building activity,
flood or damage property.
Before
beginning any beaver control action, assess the beaver
problem fairly and objectively. Are beaver really
causing damage or creating hardship requiring control
action? The very presence of beavers is often seen
as a problem when, in fact, the beavers are causing
no harm. You should also determine the type of damage
or problem the animals are causing, and then match
the most appropriate and cost-effective controls to
the situation.
Once
you have decided to control beaver damage, you have
three control options: prevention, beaver translocation,
or lethal control.
To
prevent conflicts or remedy existing problems:
Choose
and place plants carefully.
Plant areas with Sitka spruce, elderberry, cascara,
osoberry (Indian plum), ninebark, and twinberry, because
they are not the beavers’ preferred food plants.
Densely plant aspen, cottonwood, willow, spirea (hardhack),
and red-twig dogwood because, once their roots are
well established the upper parts of the plants often
resprout after being eaten. Planting preferred plants
away from known beaver trails will limit losses.
Note:
Beavers do use plants as construction materials that
they might not eat.
Install
barriers.
The trunks of individual large trees can be loosely
wrapped with 3 foot high, galvanized welded wire fencing,
hardware cloth, or multiple layers of chicken wire
(Fig. 6). Barriers can be painted to make them less
noticeable. Welded wire fencing coated with green
vinyl that helps the fencing blend in is also available.
Lengths
of corrugated plastic drainpipe can be attached around
the trunks of narrow-diameter trees.
Note:
Dark-colored pipe can burn trunks in full sun; wider
diameter pipe or pipe with holes in it may prevent
overheating problems.
Painting
tree trunks with a sand and paint mix (2/3 cup masonry
grade sand per quart of latex paint) has proven somewhat
effective at protecting trees from beaver damage.
The animals presumably don’t like the gritty
texture.
Note:
Preventing access to food sources may force beavers
to eat other nearby plants, including roses and other
ornamentals.
Surround
groups of trees and shrubs with 3-foot high barriers
made of galvanized, welded wire fencing or other
sturdy material (Fig. 7). (A beaver’s weight
will pull down chicken wire and similar lightweight
material.) Stake the barriers to prevent beavers
from pushing them to the side or entering from underneath.
An electric fence with two hot wires suspended 8
and 12 inches off the ground is also effective at
protecting groups of plants.
 |
| Figure
7. Groups of plants can be protected from beaver
damage by surrounding them with wire fencing. |
Protect
large areas that border beaver habitat by installing
4-foot high field fencing. Keep the bottom of the
fence flush to the ground, or include an 18-inch
wide skirt on the beaver side of the fence, to prevent
beavers from entering underneath.
Groups
of plants can be protected from beaver damage by
surrounding them with wire fencing.
Apply
repellents.
Commercial taste and odor repellents have provided
mixed results, perhaps because they need to be reapplied
often, particularly in moist weather. Taste and
odor repellents are most effective when applied
at the first sign of damage, when other food is
available, and during the dry season. Two repellents
that have had some success are Big Game Repellent®
and Plant-skydd®. (See “Repellents”
in Deer for additional information.)
Control
the height of water behind a beaver dam to prevent
flooding.
It may be possible to make a small change in
the depth of a beaver pond by installing a flow
device at the intended depth, extending upstream
and downstream of the dam. This keeps the rise in
the water level at a minimum by using one or more
plastic pipes to continually drain the pond area
(see “Flexible Leveler” below. Click
to enlarge). For leveling systems to work properly,
you will have to have at least 3 feet of water in
the pond area for the beaver to stay.
The
flow device can be constructed from plastic pipe
measuring 4 to 12 inches in diameter, depending
on the volume of water in the stream. The end extending
upstream from the dam must be baffled to prevent
beavers from damming or blocking the pipe.
 |
|
Prevent
beavers from plugging culverts.
To a beaver, a culvert probably looks like a hole
in an otherwise fine dam. When they plug the hole,
a flooded road can result. However, V-shaped, semicircular,
or trapezoidal fences of woven wire mesh can prevent
culverts from being plugged (see “Beaver Deceiver”
below. Click to enlarge.). Large flow-control devices
that include a solid framework can be covered and
used as a deck or wildlife viewing spot.
Note:
Installation of flow control devices is complicated
and generally requires a permit (see “Legal
Status”).
Dam
removal.
It
is pointless to destroy a beaver dam because beavers
(frequently many at one time) often begin rebuilding
them immediately after they are removed. Furthermore,
it may be illegal to remove a beaver dam without a
permit (see “Legal Status”).
Live
Trapping and Moving Beavers
Live
trapping and moving beavers elsewhere (translocating)
is often cost-prohibitive. Also, research has shown
that beavers seldom survive relocation, and those
that do often move great distances from the release
site. However, in urban areas where lethal trapping
may be illegal or unsafe, live trapping may be the
only alternative.
Hancock
or Bailey suitcase-type traps are the most commonly
used live trap. (Due to the weight and dangers associated
with suitcase traps, it is recommended that only people
experienced with these traps use them.) Some success
has also come from using a 4 foot long cage trap set
right at the water’s edge next to the beaver
slide. Bait for live traps include freshly cut tree
sprouts or branches of preferred foods, apples, and
commercial scents and lures.
If
you are moving beavers to a different site, particularly
a site where other beavers are not already present,
there must be cooperation between adjacent landowners
and local wildlife officials. A cooperative evaluation
of existing habitat quality and potential adverse
beaver activity is also very important. (Expect beaver
to cut and use a large number of trees for dam construction
during the first year or two.)
To
help ensure the survival of beavers, the gradient
of the watercourse at the release site should be less
than 3 percent, and the site should have adequate
food supply. Move beavers during their principal dam-building
period, August to October. This will allow them time
to gather a food cache, but limit their time to explore
before having to settle in for the coming of winter.
It may be helpful to provide beavers with a pickup-truck
load of aspen or other trees to use as building material
at or near the release site. This may encourage the
beavers to stay nearby.
See
the handout Trapping Wildlife
for additional information on trapping and moving
beavers.
Lethal
Control
Lethal
control may become necessary when all efforts to dissuade
problem beavers fail. Removing beavers is rarely a
lasting solution since survivors have larger litters,
and others will resettle good habitats.
Lethal
trapping has traditionally been the primary form of
controlling beaver damage. If you feel you need to
have a beaver trapped, private individuals who work
directly with property owners on a fee basis to resolve
problem beaver situations can be hired. Call your
local Fish
and Wildlife office for contact information. See
"Legal Status"
and the handout Hiring a Wildlife
Damage Control Company for additional information.
Note: State wildlife offices do not
provide animal removal services.
Shooting
beaver requires skilled marksmanship, but can be an
effective control technique where it is safe. For
safety considerations, shooting is generally limited
to rural situations and is considered too hazardous
in more populated areas, even if legal.
Public
Health Concerns
Beavers
can be infected with the bacterial disease tularemia.
Tularemia is fatal to animals and is transmitted to
them by ticks, biting flies, and via contaminated
water. Animals with this disease may be sluggish,
unable to run when disturbed, or appear tame.
Tularemia
may be transmitted to humans if they drink contaminated
water, eat undercooked, infected meat, or allow an
open cut to contact an infected animal. The most common
source of tularemia for humans is to be cut or nicked
by a knife when skinning or gutting an infected animal.
Humans can also get this disease via a tick bite,
a biting fly, ingestion of contaminated water, or
by inhaling dust from soil contaminated with the bacteria.
A
human who contracts tularemia commonly has a high
temperature, headache, body ache, nausea, and sweats.
A mild case may be confused with the flu and ignored.
Humans can be easily treated with antibiotics.
Beavers
are among the few animals that regularly defecate
in water, and their droppings (like those of humans
and other mammals) may cause a flu-like infection
when contaminated water is ingested. The technical
name for this illness is “giardiasis.”
It is more commonly referred to as “giardia”—derived
from giardia, the single-cell protozoan that causes
the disease. Another popular term, “beaver fever,”
may be a misnomer. It has never been demonstrated
that the type of giardia beavers carry causes giardiasis
in humans. Giardia has been found in many animal species,
including pets, wildlife, and livestock.
Legal
Status
Because
beavers’ legal status, trapping restrictions,
and other information change, contact your local wildlife
office for updates.
The
beaver is classified as a furbearer (WAC
232-12-007). A trapping license and open season
are required to trap or shoot a beaver.
The
owner, the owner’s immediate family, an employee,
or a tenant of property may shoot or trap a beaver
on that property if a threat to crops exists (RCW
77.36.030). In such cases, no special trapping
permit is necessary for the use of live traps. However,
a special trapping permit is required for the use
of all traps other than live traps (RCW
77.15.192, 77.15.194;
WAC
232-12-142). There are no exceptions for emergencies
and no provisions for verbal approval. All special
trapping permit applications must be in writing on
a form available from the Department of Fish and Wildlife
(WDFW).
It
is unlawful to release a beaver anywhere within the
state, other than on the property where it was legally
trapped, without a permit to do so (RCW
77.15.250; WAC
232-12-271).
To
remove or modify a beaver dam you must have a Hydraulic
Project Approval (HPA)—a permit issued by WDFW
for work that will use, obstruct, change, or divert
the bed or flow of state waters (RCW
77.55). A permit application can be obtained from
your WDFW
Regional Office or from the Hydraulic
Project Approval (HPA) web page.
In
emergency situations (when an immediate threat to
property or life exists), verbal approval from WDFW
can be obtained for work necessary to solve the problem.
A 24-hour hotline (360) 902-2537 is available for
emergency calls during nonworking hours. During normal
hours, contact your nearest WDFW
Regional Office.
Additional
Information
Books
Hygnstrom,
Scott E., et al. Prevention and Control of Wildlife
Damage. Lincoln, NE: University of Nebraska-Lincoln,
Institute of Agriculture and Natural Resources, 1994.
(Available from: University of Nebraska Cooperative
Extension, 202 Natural Resources Hall, Lincoln, NE
68583-0819; phone: 402-472-2188; also see Internet
Sites below.)
Link,
Russell. Landscaping for Wildlife in the Pacific
Northwest. Seattle: University of Washington Press
and the Washington Department of Fish and Wildlife,
1999.
Maser,
Chris. Mammals of the Pacific Northwest: From the
Coast to the High Cascades. Corvalis: Oregon State
University Press, 1998.
Muller-Schwarze,
D., and Lixing Sun. The Beaver: Natural History
of a Wetland Engineer. Ithaca, NY: Cornell University
Press; and London: Comstock Publishing, 2003.
Verts,
B. J., and Leslie N. Carraway. Land Mammals of
Oregon. Los Angeles: University of California
Press, 1998.
Internet
Resources (General)
Burke
Museum’s Mammals of Washington
Prevention
and Control of Wildlife Damage
U.S.
Forest Service Wildlife Species Life Form Information
Woodland
Fish and Wildlife Project
Internet Resources (Flood Control
Devices)
Beaver
Management
Beaver
Stop
Clemson
Beaver Pond Leveler
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