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People often disagree about the beaver. Some think this master dam builder is the smartest thing in fur pants, and they use such words as intelligent, energetic, and helpful when speaking of it. Others claim the paddle-tailed engineer is dumb, stubborn, and destructive. A look at the beaver and its lifestyle may show why opinions are so different.
No other creature besides humans controls its environment as completely as the beaver – changing the surroundings to meet its needs. And since water rules the beaver's life, most of its efforts are aimed at making sure it has a good supply.
The chubby beaver is slow and clumsy on land, waddling along at the mercy of its predators. However, in water it becomes a sleek, stream-lined swimmer that can travel at five or six miles an hour. No wonder the beaver chooses to surround itself with water and builds a dam to make sure there's enough. Water provides protection as well as an easy way to travel. As water backs up behind the dam, it brings the beaver's food supply within reach of the water. Raising the dam deepens the water and spreads it to new food sources as old ones are used.
Going back in time a million years or so, we discover the beaver is the descendant of an 800-pound prehistoric rodent. Adult beavers, which average between 30 and 60 pounds, may seem rather puny by comparison, but they still rank as the second largest rodents in the world, right behind the South American capybara (kap-ah-BARE-ah). A record beaver found in 1921 weighed 110 pounds, and heavyweights that tip the scales between 80 and 100 pounds still are caught occasionally.
The beaver has been described by some as a "furry hunk of homeliness." This description may be close to the truth, but from its big orange teeth to its paddle-like tail, the beaver is designed for its aquatic life and lumberjack activities.
Its fur coat, which looks messy when wet and ungroomed, has an outer layer of long guard hairs that cover an inner layer of wooly underfur. The underfur is so thick and well oiled that water usually cannot reach the skin beneath it. Those who have felt the beaver's fur underwater describe it as smooth and slippery. This slickness keeps water drag to a minimum as the animal swims. The dark chestnut brown of its winter fur may fade to a golden tan by spring. Fur colors range from almost black to reddish brown to blond.
Frequent grooming is necessary to keep the fur tidy. Each hind foot has a split or double toenail that serves as a comb. Drawing the hairs through these double nails removes excess water and straightens any tangles. Despite its large size, the beaver can twist and bend to reach every part of its wet fur with these toenail combs. The toenails also serve as toothpicks to remove wood splinters left between its teeth.
Once the fur is combed, it must be oiled. The beaver does not always sit on its tail during the combing process, but it cannot oil itself until it does. Sitting back with its tail folded under its body exposes the opening at the tail base in which the beaver's sex organs and its large oil glands are located. Using its front feet, the beaver reaches down, scoops up the strong-smelling oil released from the glands, and rubs it on the fur. The toenail combs then spread the oil evenly through the fur. This grooming process takes place right after the beaver leaves the water and requires about ten or fifteen minutes to complete.
The yellowish oil, called castoreum, is also used by the beaver to mark its territory. Using its front feet, the beaver rakes together a mud patty, which often contains wood chips and other debris, and places it in a highly visible spot. It then soaks the patty with castoreum. Researchers have found that this oil contains at least forty-five chemical ingredients, and humans have used the oil in making perfumes. The scented patty reveals the sex of the beaver that made it and many other messages only another beaver can understand. It serves as a chemical communication system.
Probably the most distinctive part of the beaver is its tail, which looks like a large scaly paddle. The adult's tail is about a foot long, six or seven inches wide, and less than an inch thick. The base is covered with the same type of fur as the body, but the flat, scaly part of the tail has only a few short, bristly hairs.
The tail serves as a prop on dry land, bracing the beaver as it cuts down a tree and helping the beaver sit up so it can use its front feet for various chores. At any sign of danger the beaver slaps its flat tail on the water's surface with a loud smack, creating a big splash before it dives to safety. Some observers believe that slapping the tail in water and on land not only is a warning signal for fellow beavers, but also may be a sign of anger or an attempt to frighten away an approaching predator.
A downward thrust of the tail as the beaver submerges helps push its body under. Once underwater, the tail acts as a diving plane, determining the angle of descent. It also serves as a rudder, keeping the beaver on a straight course, especially when the animal is towing a tree or large branch. Seldom, if ever, is the tail used for actual swimming.
Large, webbed hind feet deliver the powerful strokes that push the beaver through the water. The webbing folds together ducklike as the foot is brought forward and spreads wide for the backward thrust. If the beaver is in no particular hurry, its back feet stroke at the same time, sending it forward in a smooth glide. As the forward movement slows, the feet stroke together again. But when the beaver needs to turn on the speed, it switches to alternate foot strokes. Since each foot is six or seven inches long and spreads to a width of about six inches, each individual kick produces quite a bit of forward thrust.
To further streamline its shape in the water, the beaver hugs its front legs to its chest. The front feet are clenched into fists and serve as bumpers. These flexible front feet can be used almost like hands. Even though the beaver doesn't have thumbs, it has five fingers that are able to pick up and grip objects with ease.
While the beaver is underwater, its heartbeat slows so less oxygen is needed. An extra large liver and big lungs make it possible for the beaver to store air and oxygen-rich blood for long dives. Three minutes is the beaver's usual time underwater, but it can stay down for as long as fifteen minutes. It's not unusual for the beaver to travel as far as half a mile underwater before surfacing, especially if frightened.
The beaver has several waterproofing devices that go into operation as it submerges. Valves in the ears and nostrils snap shut and membranes slide over the eyes like goggles to protect them and increase their underwater vision. Folds of skin meet tightly behind the large front teeth to seal off the mouth. These folds of skin also prevent splinters from entering the mouth as the beaver chomps away on land or in the water.
Speaking of chomping, another unmistakable feature is the beaver's large, orange buckteeth. As with other rodents, these front teeth grow continually and are kept worn down by constant gnawing. The front surface is very hard enamel that wears slowly. The backs are of a softer material that wears faster. This unequal wear gives the teeth a chisel-like edge, and the angle the edges form makes it possible for the beaver to sharpen them against each other. The sixteen molars used to grind the beaver's woody food seldom show any sign of wear.
Small twigs are fed into the mouth with the front feet, nipped into small pieces by the front teeth, chewed by the molars, and swallowed. When eating bark off larger sticks, the beaver picks up a piece about a foot long. One "hand" closes around it and the other grips it between the little finger and the other four. The stick is turned quickly and evenly by the fingers and moved slowly sideways. The sharp front teeth strip off the bark as you might eat corn off a cob. When its mouth fills with bark, the beaver pauses, chews, swallows, sharpens its teeth, and then continues eating. Upon nearing the end, it holds the stick against the "palm" of its "hand" so the teeth can eat right up to the edge. The cleaned stick is then tossed aside and another is picked up. The discarded sticks have an almost threaded appearance because of the grooves the teeth cut while removing the bark from the turning stick. Later these cleaned sticks will be used to reinforce the beaver's dam.
A beaver dam is a remarkable structure, but the builder's reputation for engineering intelligence may be slightly exaggerated. Researchers who have spent time studying these dams say that persistence and hard work may be more responsible for the finished product than any so-called engineering skills. In many cases a dam that would hold the same amount of water could have been built with only a fraction of the work if it had been placed either upstream or down-stream from the chosen site. But whether the location seems ideal by human standards or not, once the site has been selected, the beaver stubbornly refuses to build elsewhere. Why the beaver chooses one site over another has puzzled people for years.
Beavers observers don't always agree on the way the dam is started. One explanation for the different methods observed may be that beavers are adaptable. Some claim beavers cut down a tree so that it falls across a creek. Logs and branches are then floated downstream to catch in the fallen tree. Others say beavers cut down branches, tow them to the middle of a stream, stick them in the mud on the bottom, and add rocks and more mud to keep them there. More branches and logs are added and, when the structure is tall enough, the beavers begin building from it toward the shore.
According to Swedish researcher Lars Wilsson, who spent several years studying a beaver colony and experimenting with its residents, dam building is an instinctive action triggered by the sound of running water. He discovered that when plenty of water is available, beavers may live in an area for some time without starting a dam. But if the water level drops low enough to ripple between or over some rocks or debris, the sound of the shallow, running water attracts the beavers and triggers a building urge. His captive beaver tried to build a dam in a bathtub full of water when a recording of running water was played beside the tub.
In the wild his beavers pushed or carried mud and rocks from the bottom of the stream to the low spot where the sound was occurring naturally. The low spot was built up with these materials until water no longer flowed over the top. The only problem was the water now flowed around either side of it, continuing to make the sound. This, too, had to be stopped, so the barrier was extended on each end. Grass, leaves, twigs, and other such materials were used, along with more rocks and mud.
Eventually each end of the barrier reached land and the structure could be called a dam. But this wasn't the end of the work. As the water built up behind the low barrier, it got deep enough to flow over the top again. The beavers' solution was to add more material and raise the height of the dam.
Whatever building material is available is used in the construction of a dam. If you or I were adding sticks or small trees, we probably would place them lengthwise across the stream, but this is not the beavers' way. They place each stick or limb parallel to the flow of the water. When possible, the butt end is wedged into the muddy bottom and points up-stream while the top leans toward the dam. Small trees may be included, but mud, grass, leaves, and twigs are used to fill spaces between the larger pieces and stop leaks. Busy "hands" poke, probe, and ram small sticks into place. Debris carried downstream also catches in the dam and helps plug any holes. A dam in good repair leaks very little.
A pair of beavers can build a dam across a small stream in two or three days. Their untidy pile of wood may not look too impressive when the dam is finished, but don't sell them short. When the need arises, they are able to build some fantastic structures.
In his book The World of the Beaver (J. B. Lippincott, 1964), Leonard Lee Rue III describes some large beaver dams he has seen. The longest was about 800 feet and the tallest stood about 8 feet above the water level on the downstream side. Historical beaver dams included one in Wisconsin in 1919 that was 12 feet high and 640 feet long. Another located in Wyoming in 1955 was only 30 feet wide, but it stood 18 feet tall. Montana's Jefferson River contained a beaver dam 2,140 feet long, but the length record may be held by a 4,000-foot dam in New Hampshire. The lake created by this record dam contained forty beaver lodges.
A dam described in Lewis H. Morgan's book The American Beaver and His Works (B. Franklin, 1970; reprint of 1868 edition) was more than 450 feet long and 18 feet thick at the base; it contained probably 250 tons of materials. Such a structure takes years of work and generations of beavers to build and maintain.
In their place in the wilderness, beavers usually are considered constructive animals. Their ponds hold water for dry times, help control floods, and create attractive environments for many different wildlife creatures. However, when the beaver builds too close to human neighbors, it usually is considered a nuisance or a destructive animal. It's hard to consider the beaver's needs when its pond floods your crops, roads, or pastures or when its menu includes your trees and shrubbery. When a beaver's dam conflicts with human interests and attempts are made to remove it, we learn just how stubborn or persistent beavers can be. The animals will spend each night repairing or rebuilding a dam that has been damaged or destroyed during the day, and they will continue to do this indefinitely.
County road engineers often clash with beavers. Instead of building a small bridge over a ditch or creek, the engineers install a culvert pipe and then build the road on top. This allows the water to continue to flow under the road through the pipe. With just a little work, the beaver can plug the pipe and back up water that eventually will flood the new road.
Such culvert pipe battles have continued for years. Each time the road workers remove the plug, the beavers replace it. One county engineer claims the beavers in the area have developed their ability to plug a culvert pipe to a fine art. He is sure they must have found some way to measure the inside diameter of the pipe because they go out each time and cut one-inch sticks about three-fourths the length of the pipe's diameter. The sticks are placed around the inside of the pipe with one end caught in the pipe corrugations. At their other end, the sticks are overlapped and interwoven toward the upstream direction. Pieces of brush and other small debris are added to the upstream side and the whole thing is plastered with mud. When the water backs up, the pressure wedges the entire mass even tighter.
The only practical solution to such a problem is destroying the animals or live-trapping them and moving them to a more suitable location. However, either solution may last for only a season or two. By then a new pair of beavers may move into the area, discover the pipe, and decide it is perfect for their needs.
And where do these wandering beavers come from? They are two-year-olds that have been driven from their home lodges in the spring. Forced out on their own, they must find mates and establish their own territories.
Let's assume that during his travels downstream, a two-year-old male beaver meets a female beaver and they decide to set up housekeeping. Together they choose a suitable territory and, when necessary, start building a dam. Once their water supply is assured, they build a place to live. The home structure may be either a lodge or a burrow.
Lodges, like dams, are built of mud, rocks, debris, and wood. They may be completely or partially surrounded by water and will have two or more underwater access tunnels.
Burrows, most commonly used by southern beavers, are built into the bank of the river, pond, or stream. From underwater entrances the beavers tunnel into the dirt and hollow out a living area above the waterline. When the burrow comes close enough to the surface or breaks through, a pile of interlacing sticks and branches plastered with mud is constructed over the burrow site. This gives added protection from land predators that might try to dig into the burrow from the top. As time passes and more surface material is added, the burrow may begin to look more like a lodge.
The living chamber for the new couple may start out small – one and a half to two feet tall and no more than three feet or so in diameter. However, as the family grows, the living area will be expanded. Large families may construct two separate chambers divided by a wall. Both chambers will have their own en-trance tunnels, called plunge holes. Living chambers three or more feet tall and six to ten feet in diameter are common, and lodges eight to fourteen feet tall and forty feet across at the base have been reported.
Each living chamber, regardless of its size, is divided into two basic parts – one for eating and one for sleeping. The main floor, which is built a few inches above the water level, is used as a feeding and grooming area. Any water brought in by the beaver's fur drains off the elevated floor into the lower area around the plunge holes. In the sleeping section, which is built like a shelf a few inches above the main floor, each beaver has its own soft bed of shredded wood.
In April or May of the following year the beaver pair probably will become parents. As the time for birth approaches, the female takes sole possession of the burrow. The male takes up temporary residence elsewhere. She prepares the bedchamber for the young by shredding fresh bark into the softest of bedding. Her litter may contain one to eight young, but four is the usual number. The young, called kits, are born fully furred, weigh about one and a half pounds, and have their eyes open. They are miniatures of their parents, complete with teeth. During the first two weeks they remain in the burrow, feeding on a rich, yellow milk. By the end of the first month they are ready to go with their mother on swimming and feeding excursions. The male moves back home when they are a couple of weeks old.
The young beavers stay with their parents in a family group, sharing the lodge and helping to keep the family dam in good repair. When the next litter is born the following year, they are evicted with the father and return to the home burrow when he does. However, in two years, when the third litter arrives, they are not allowed to return to the family lodge with the father. They are forced to make their own way as their parents did. Sometimes they are allowed to build their own lodges and remain in the parents' area, but if all the offspring from a beaver pair stayed, the colony soon would strip the area of food. Forcing the two-year-olds to find new territories extends the period of time an area can support its resident beaver population.
Beavers live along many wooded rivers, lakes, and streams in Texas, but their highest populations are found in the northeastern part of the state. Legal trapping efforts help keep their numbers under control.
Admired or despised (depending upon how close a neighbor it happens to be), the beaver is an interesting member of the wildlife community that always makes its presence known.
1990 – Beavers: Introducing Mammals to Young Naturalists. The Louise Lindsey Merrick Texas Environment Series, No. 10, pp. 31-38. Texas A&M University Press, College Station.