Beavers: Nature's Hydrologist - Part 1
Excerpted from: Water - A Natural History by Alice Outwater, 1996.
Beavers do more to shape their landscape than any other mammal except for human beings. Their ancestors
were building dams 10 million years ago. Until European colonization of the New World, the North American beaver
was one of the most successful mammals on the continent. They lived everywhere from the arctic tundra to the
deserts of northern Mexico.
Along thousands of streams lived colony after colony, dam after dam of beavers in close succession. There were
as many as 300 dams per square mile — each with its own ring of wetlands. It is estimated that as many
as 200 million beavers once lived in the continental United States! Today the current North American beaver
population is estimated to be between 10 and 15 million individuals.
Beavers made dams that created meadows out of forest, their wetlands slowly capturing silt. The result of the
beaver's engineering was a remarkably uniform buildup of organic material in the valleys, a checkerboard of
meadows through woodlands, and a great deal of edge — that fruitful zone of where natural
Beavers are a 'keystone species'. Where beavers build dams, wetlands spread out behind them
providing home and food for dozens of species from migrating duck to moose, from fish to frogs to great blue
|North American Beaver
||10 - 20 years
||28 - 70 pounds
||3 - 4 feet
||Wolves, Coyotes, Bears,
The beaver is North America's largest rodent and is built for life in the water. Adults can be up to four feet long
and weigh over 60 pounds. The beaver has webbed hind feet and a large, flat, nearly hairless tail. It uses its tail
to help maintain its balance when it is gnawing on trees. It will also slap its tail against the water to signal danger
or to warn away predators. The beaver has short front legs with heavy claws. Their rear legs are longer, and
they use their rear webbed feet help propel themselves through the water. When the beaver is under water,
its nose and ears close up and a special membrane covers its eyes
It has dark brown fur on its back and sides and lighter brown fur on its chest and belly. The beaver waterproofs
its thick fur by coating it with castoreum, an oily secretion from its scent glands. The beaver has
a thick layer of fat under its skin that helps keep it warm underwater. Beavers have long sharp upper and lower
incisor teeth that they use to cut into trees and woody vegetation. These teeth grow throughout the beaver's life.
Beaver are vegetarian generalists with sophisticated foraging preferences. Beavers consume a mix of
herbaceous and woody plants, which varies considerably in both composition and species diversity by
region and season. They prefer aspen and poplar, but also eat cattails, water lilies, and other aquatic
vegetation, especially in the early spring. Contrary to widespread belief, they do not eat fish.
When herbaceous plants are actively growing, they make up much of the beaver's diet. In the winter,
beavers switch to woody plants and the food they have stored over the winter. Beavers do not necessarily
use the same trees as construction material and as food. Inedible material is more likely to be used as
the cap of a beaver family's food cache, the upper part which is frozen in the ice, while the cache itself
is composed of edible, high quality branches, which remain unfrozen and accessible
Beavers live in family groups or colonies. A colony is made up of a breeding male and female beaver
and their offspring. Beavers are very territorial and protect their lodges from other beavers. They mark
their territory by building piles of mud and marking it with scent.
Beavers are active mainly at night. They are excellent swimmers and may remain submerged up to 15
minutes. More vulnerable on land, they tend to remain in the water as much as possible. They use their
flat, scaly tail both to signal danger by slapping the surface of the water and as a location for fat storage.
They construct their homes, or lodges, out of sticks, twigs, rocks, and mud in lakes, streams, and tidal
river deltas. These lodges may be surrounded by water, or touching land, including burrows dug into
river banks. Beavers are well known for building dams across streams and constructing their lodges in
the artificial ponds which form.
When a beaver kit reaches 3 years old they leave home to find a companion with whom they mate for life.
During this quest for a new territory, they are at their most vulnerable to predation. When the new couple
finds a suitable location, they mark the area with scent mounds and dig out a den in a river bank.
Beavers do not hibernate in winter, so they need to have food available, but most living plants are dormant,
trees are not moving sap through their bark, and frankly snow cover makes it difficult for beavers to move
around. So, the beaver has an interesting solution, although it may not seem that novel to us — it stores food
for the winter.
Communication is highly developed in beaver, including scent marking, vocalization, and tail slapping.
Beaver deposit castoreum on piles of debris and mud called scent mounds, which are usually placed
on or near lodges, dams, and trails less than a meter from water. Over 100 of such mounds can be
constructed within one territory. Beaver colonies with close neighbors constructed more "scent
mounds" than did isolated colonies, and the number of scent mounds at each active lodge is correlated
with the distance to the nearest occupied lodge.
Although 7 vocal sounds have been described for beaver, these are the most common: a whine,
hiss, and growl. Vocalizations and tail slapping may be used to beg for food, signal to family
members to warn of predators, or to drive away or elicit a response from predators.
Beavers mate for life, but if one mate dies, the other one finds another mate. Beavers mate when they are
about three years old. Mating season runs from January and March in cold regions and in late November
or December in the south. Gestation lasts about three months, and females have one litter of kits a year
between April and June.
Before birth, the female makes a soft bed in the lodge. The babies' eyes are open when they are born, and
they can swim within 24 hours of birth, and they will be exploring outside the lodge with their parents within
a few days.
Young beavers are weaned in about two weeks. Both the male and the female take care of the young beavers.
They stay with their parents for two years. Beavers can live to be 20 years old.
Beavers are burrowers. They have powerful, curved claws and a number of features that aid underwater
construction. Beavers have valves to close off their nose and ears, thin membranes over their eyes that
serve as goggles and skin flaps behind their front teeth that allow them to tow tree branches in the teeth
without swallowing half the pond water.
With these adaptions, beaver are able to dig their burrow into the stream bank with the entrance well below
the surface of the water. Slanting the tunnel upward to the high--water line, they clear out a room 3 feet
wide and line it well with shredded wood and grass.
After beavers choose a dam site where the stream is not too deep and the bottom muck is firm, then then
fell sapling first and then larger trees. Working by night — sometimes on separate trees and sometimes
together on a single trunk — they sit with their paws around the tree, their tails either folded beneath
them like a seat or extending behind them like a prop. They tilt their heads from side to side and make
deep bites in the tree, driving their long, yellow teeth into the wood to wedge, pry or pull out a chip. They
chisel the tree trunk until it falls.
After cutting the tree trunk into manageable lengths, the push and pull the logs into position on the dam,
pointing the but ends upstream, and hold them fast with piled mud and stones. As the dam grows higher,
the water slows and the beavers weave in more branches and put on more mortar until a substantial
barrier is completed.
Dams must be continuously maintained and beavers to this every night, replacing shifted sticks and poles
and patting on more mud. They build dams throughout their territory, some for water control; some, it
seems, just for fun! A family of beavers can build a 35-foot-long dam in a week.
Beavers — Nature's Hydrologist, Part 2
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