There are two noticeable differences in the appearance of an active beaver lodge in the fall as opposed to the summer. One is the presence of a growing pile of freshly-cut branches adjacent to the lodge. These branches provide resident beavers with the nutrition they will need during the winter months when herbaceous plants are neither available nor accessible.
The second change noticeable in a fall lodge is the presence of massive amounts of mud. Branches are often placed on top of this layer of mud, so you have to observe the lodge before that happens in order to see the extent of the mud layer. It provides protection from harsh winter winds which would significantly lower the temperature inside the lodge. Together, a blanket of snow and a layer of mud serve as excellent insulation for beavers living in the lodge.
In the Northeast, where the temperature often dips into the single digits or lower in the winter, the interior of an active beaver lodge maintains a relatively stable 33° F. – 35° F., roughly the temperature of the water, thanks to the ingenuity of these rodent architects.
It is common knowledge that beavers are herbivorous but the extent of their herbivory is not always appreciated. Examining their skull would tell you that their massive four front incisors (as well as the muscles attached to their jaws) are designed to do some serious chewing. And serious chewing does take place, especially in the fall. Poplars, birches, alders, willows, maples and many other deciduous trees as well as a few conifers are felled in order to reach and consume the inner bark, or cambium layer. (The de-barked logs and branches are subsequently used to repair dams and lodges). Not only do Beavers need to meet their daily nutritional needs but they must cut enough trees to last them through the winter.
However as spring approaches and they can access land, their diet changes from the woody branches they’ve been eating all winter (from their winter food pile under the water) to a diet that consists mainly of herbaceous plants. Ninety percent of their time is spent eating non-woody plants, often skunk cabbage, water lily rhizomes and grasses in the early spring. As summer progresses, they seek out aquatic plants, ferns, sedges and a variety of flowering plants. Usually it’s not until late summer/early fall that their incisors are once again given a good workout.
The pictured Beaver had the good fortune of having a large patch of tasty Jewelweed (Impatiens capensis) growing on and adjacent to its lodge.
Most beavers in northern New England have been trapped inside their lodge for several months, with their only food supply being the branches they cut last fall and piled up on the floor of the pond or river near their lodge. Ice has sealed them in, not only to a life of darkness and dampness, but to a diet of cambium.
Needless to say, they are quick to take advantage of melting ice that allows them to exit their lodge and make their way to shore to sample fresh food. One of the first delicacies they dine on, if it’s available, is skunk cabbage. The rhizomes, leaves and flowers of both yellow and white pond lilies are also favorites. No longer restricted to woody plants, beavers head for grasses, sedges, ferns, fungi, berries, mushrooms, duckweed and even algae as the water warms. Their palate must jump for joy with the melting of ice in early March.
Regardless of the outside temperature, the interior of occupied beaver lodges has a fairly stable temperature of about 32°F. This is due to several factors, one of which is insulation. Beavers spend much of the fall collecting and stuffing mud into the cracks between the branches that provide a framework for their lodge (leaving a mud-free air vent at the top of the lodge). This mud as well as any snowfall that occurs during the winter help keep out the cold and retain the warmth that the resident beavers’ bodies radiate.
A comparison of the (cooler) interior temperature of bank dens and that of open-water lodges confirms that the temperature of the substrate underlying a lodge also contributes to the air temperature of the chamber.
Lastly, heat produced by beavers raises the temperature of a lodge above that derived from the lodge substrate.
(Photo: beaver lodge on a morning cold enough to show “beaver breath” escaping through the air vent that runs up through the center of the lodge to its peak. Thanks to Kay Shumway for photo op.)
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Beavers are busy reinforcing their lodges and repairing dams as the days shorten and temperatures fall. Once these tasks are taken care of, they begin gathering and storing all of the food that they will need this winter, for once ice forms they will no longer have access to land.
Their winter food cache is placed as close to the entrance of their lodge as possible, as the inhabitants of the lodge will be swimming out to this pile frequently to obtain food. After hauling branches and saplings to the lodge, the beaver dives down and jabs the butt end of the branch into the mud at the bottom of the pond. Additional branches are woven into this base layer until eventually most caches weigh two or more tons. One can often see the pile’s top branches and leaves rising above the surface of the water.
Beavers are known for their ever-growing incisors which allow them to cut trees down, eat the cambium (a nutritious layer just beneath the bark) and cut what’s left into pieces they are able to haul and use as building material for dams and lodges. More often than not, it’s straight forward work.
Occasionally not every step is taken – you can find standing trees that had the bottom three or four feet (as high as the Beaver could reach) of cambium removed without the trees being felled. You can find de-barked logs that have been left where they fell and not carried or floated to the dam or lodge as construction material. It’s also not unusual to find standing trees where several times a Beaver has attempted but failed to cut all the way through.
Recently John Twomey brought to my attention a tree felled by Beavers unlike any other I’ve ever seen: one or more Beavers had cut down a Paper Birch and eaten the cambium layer, leaving the tree clean of bark. At some point they cut into the tree every 18 inches or so, not quite severing the pieces, but leaving them connected by a core of wood that ran the length of the tree. If any readers have seen anything similar to this, or if you have an idea as to why Beavers would have cut the tree in this fashion, Naturally Curious would love to hear from you. (Photo by Prentice Grassi of his sons investigating said tree earlier this spring.)
Even in the coldest of winters, there is often a thaw around this time of year that frequently allows beavers to escape the cold (+/- 34°F) dark lodge where they reside during most of the winter. Our most recent thaw was such that in many beaver ponds, ice didn’t even have to be broken in order for resident beavers to forage for food on land.
When beavers are confined to complete darkness (under the ice) their 24-hour circadian cycle extends to a 28 hour day. During this time beavers sleep for longer periods at a time and thus need less food. As spring approaches and the days lengthen, the slightest exposure to daylight will reset the beaver’s biological clock back to the circadian cycle. (Leonard Lee Rue, Beavers) (Photo by Alice Trageser)
In summer Beavers spend considerable time on land searching for, cutting and bringing vegetation back to their young in the lodge. Their woody plant preference (they eat large amounts of herbaceous plants during the warmer months, but also some trees) tends towards the inner bark (cambium) of willow, aspen, maple, birch, cottonwood, beech, poplar, and alder trees. This beaver, however, has retrieved a Red Oak sapling for its offspring.
If you are fortunate enough to have a beaver pond near you, you should give the lodge more than a cursory glance this time of year. It is common to find Canada Geese nesting on beaver lodges, for obvious reasons – safety from most land predators. While Common ravens have been known to raid Canada Goose nests for eggs and goslings, the overall rate of survival of the goslings of lodge-nesting geese is very high.
A Canadian study showed that ponds with beaver lodges (and therefore Beaver activity which warms the water and thaws the ice) thaw at least 11 days sooner than ponds without Beavers, allowing early access to water for Canada Geese returning for the spring nesting season. Battles between pairs of geese vying for these coveted nesting sites are not uncommon.
Canada Geese have much to thank Beavers for. Not only can geese get an early nesting start on beaver lodges, they have a relatively safe spot to incubate their eggs and raise their young.
Muskrats, or “rats,” as they’re sometimes derogatorily called, are semi-aquatic, mostly plant-eating rodents that live in ponds, streams, lakes and marshes. During the winter they seek shelter in lodges that they build out of grasses, reeds, cattails and sticks. Muskrat lodges are much smaller than Beaver lodges, which are constructed out of mud and sizable branches, sticks, stones and mud.
In the spring Muskrats often build nests by burrowing into a stream or pond bank, which they enter under water. Muskrats are also known to set up residence in active Beaver lodges. After dining on aquatic vegetation, the pictured Muskrat made a beeline for the beaver-occupied lodge nearby, and dove under as it approached it. Beavers and Muskrats tolerate each other’s presence in the same pond (and lodge) even though they both consume much of the same vegetation. Unlike Beavers, Muskrats supplement their diet of plants with frogs, crayfish, clams, snails, and fish. It may be that when cohabiting a lodge, they may help one another keep an eye out for predators. (Photo: Muskrat eating pond vegetation)
Over the past century beaver trapping has declined and beavers have returned to many of their former habitats. Wolves also have come back in a few areas (not the Northeast yet) — but most places where beavers now live remain free of wolves. As a result, the beaver population has continued to increase, limited only by a few predators, primarily humans and Eastern Coyotes.
Coyotes are major beaver predators and have established themselves throughout the Northeast partly because of the abundance of prey and partly because of the absence of wolves, who keep coyotes out of their habitat. During most of the year, coyotes can take advantage of beavers that leave their pond to feed on land. When they are in their lodges, however, beavers are fairly safe from coyote predation, especially if their lodge is surrounded by water. Come winter, when ponds freeze and beavers remain in their lodges, coyotes can easily approach an inhabited lodge by walking over the ice. Thanks to the lodge’s two to three-foot-thick walls of frozen mud and sticks, the beavers within are safe. (Photo: signs showing a coyote’s attempt to access a beaver lodge)
This photograph conveys the essence of a beaver’s summer – eating and grooming… more eating, more grooming. During the summer months, beavers feed on non-woody vegetation (grasses, ferns, aquatic plants, etc.) 90% or more of the time. (During March/April and October/November, their diet switches to 60%-90% tree bark, and during the winter, bark from trees stored under water composes 100% of their diet.)
When beavers are not eating, much of their time during the warmer months is spent grooming, both themselves as well as each other. Combing debris out of their coat (with the help of a split nail on both hind feet) and applying oily material from their anal glands to waterproof their fur consume much of their waking hours, both at night as well as at both ends of the day. (Castoreum, produced in castor sacs, differs from anal gland secretion, and is used primarily to mark territory.) (Thanks to Roger and Eleanor Shepard, and Sara and Warren Demont for photo op.)
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Although most of the Northeast is snow-free, parts of northern New England still have a while to go before bare ground is visible. Most beaver ponds, however, are open and the beavers have been taking advantage of their ability to access fresh food after a long winter of water-logged woody plants. While this particular beaver is dining on bark, most beavers head for any green plants, buds, grasses, etc. that are poking their heads above the ground. A particular beaver delicacy this time of year are the leaves and blossoms of skunk cabbage.
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We’re right on the verge of when beavers will no longer be able to smell fresh air, see the sun and obtain fresh bark. Until the temperature drops to around 16 degrees F. they continue to break through the thin ice covering their pond. Once the temperature remains in the teens or lower for several days, they no longer try to break through the ice and are sealed under it until spring, unless there’s a mid-winter thaw.
Once beavers are confined by the ice, their activities outside the lodge are minimal. Beavers leave their lodge in winter primarily for three reasons: 1) to swim out to their winter food supply pile and retrieve a branch which they bring back into the lodge to eat, 2) to defecate in the water, and 3) to mate in January or February. Other than these excursions, they spend most of their days in the dark, enduring life in a lodge that has a temperature of about 34 degrees F. (Thanks to Kay and Peter Shumway for photo op.)
The lazy, hazy days of summer are dwindling, and beavers’ internal clocks are telling them it’s time to batten down the hatches and prepare for several months of life below the ice. This entails adding a significant amount of mud to the outside of their lodge. The mud freezes and creates an impenetrable barrier between them and predators such as coyotes that, thanks to ponds being frozen, will have access to beaver lodges. This mud is anchored by the addition of debarked branches and logs that have provided the beavers with meals of cambium during the summer.
A beaver can transport its own weight in material (roughly 45-60 pounds). Retrieving debarked pieces of wood in many cases involves carrying them over both land and water, using only jaws and sometimes a shoulder for support. A beaver’s short, muscular neck and its powerful lower jaw muscles make this possible. Try lifting one of the larger logs on a lodge or beaver dam sometime. Then imagine carrying it any distance in your mouth with no assistance from your hands. (This feat rivals that of a moose carrying two 25 – 30 pound antlers around for several months.) While there are recorded cases of beavers felling trees 150 feet tall and 5 feet in diameter, logs of this size are not used as building material for dams and lodges, but rather the bark and upper branches provide them with food. (Thanks to Roger and Eleanor Shepard and Sara and Warren Demont for photo op.)
Under ice-covered ponds and lakes in dark, cold water, sometime between December and March, beavers mate. Latitude and climate affects the length of the breeding season, which is shorter and later in colder, more northern locations and longer and earlier in warmer, more southerly regions. February is the peak mating season for New England beavers.
Beavers are classified as monogamous, as once a bond has formed, they remain as a pair throughout their life until one of the pair dies, at which point a new mate is found. However, this does not mean they don’t stray. In a study of beaver colony genetics, researchers discovered that more than half of the litters had been sired by two or more males. So much for monogamy.
What exactly is it like inside an active beaver lodge in winter? It’s dark, damp and around 32°F. The living chamber inside usually has a ceiling no more than two feet high with a diameter of 4 to 6 feet, depending on the number of individuals in the family. (A typical beaver family is composed of an adult male and female, 2 to 3 yearlings, and 2- 4 kits that were born in the spring.) Fresh air enters and carbon dioxide leaves through a central vent (where mud is not applied) and through small holes that remain under logs on the side of the lodge. When there isn’t much snow and the outside air falls well below zero, the temperature inside may drop to a degree or two below freezing, but if the sun is out, it warms right back up again during the day.
The dampness is due to the beavers’ repeated need to enter the water both to retrieve sticks from their nearby food supply pile and to defecate. Upon returning to the lodge, the humidity inside increases due to the water draining from the beavers’ fur. No small wonder that if a January thaw permits, beavers will exit their pond for some fresh air, food and a little bit of sunshine.
Once locked under the ice, beavers have only the food that they have had the foresight to store in their pond prior to it freezing to sustain themselves for the next four to five months. Sometime in September or October beavers start cutting down trees and limbing them. (The more northern the latitude, the earlier they begin this process.) Beavers have been found foraging over a third of a mile from their pond in the fall. At this time of year they tend to go further afield in order to find their preferred trees and shrubs – poplar, willow, alder and sugar maples. The branches are carried to the pond and hauled through the water to the lodge. When they approach the lodge the beavers dive down and push the butt end of the branches into the mud at the bottom of the pond and proceed to weave additional layers of branches into them.
Most caches are built as close to the entrance of the lodge as possible. A cache, or winter food supply pile, that feeds a colony of beavers consists of 1,500 to 2,500 pounds of edible bark, twigs and leaves. (On average, a beaver consumes 1 ½ pounds of food per day in the summer, and 2.2 pounds in the winter.) Because beavers don’t eat the wood, they must gather several tons of saplings and branches in order to have enough to survive.
If you look closely at yesterday’s close-up view of the food cache, you will see larger limbs on top of the pile. These larger logs are used to weight down the pile –they often consist of species that beavers aren’t particularly partial to, if they eat them at all. (Note proximity of food cache to the lodge, which is to the left in photo.)
Little did I know when I wrote yesterday’s post about the silver lining of our low water levels that I would so quickly encounter another predator benefiting from the current drought. I have spent a considerable amount of time this summer watching three generations of beavers do their best to survive as their pond proceeded to diminish to the point of exposing one of their lodge entrances and confining them to an increasingly small body of water. The underwater entrances to a beaver lodge are vital to their protection, and predators are well aware of this.
Yesterday the importance of water as a protective barrier was made very clear to me when a coyote appeared on the opposite shore of the beaver pond from where I sat. It stood for several seconds exactly where the beavers leave the pond on their way to nearby woods to cut poplars and birches which they haul back to their pond to eat. A well-worn trail marks the spot. You could imagine the coyote, upon surveying the shallowness of the pond, telling itself to be patient, as better days were just around the corner.
Moments after the coyote left, the mother beaver got out of the pond precisely where the coyote had been standing and took a few steps before sniffing the ground and then the air (see insert). Being nocturnal, beavers have an acute sense of smell which they use for detecting danger, food and for communication with each other. It took mere seconds for the beaver to detect the scent of the coyote, at which point she turned and sought refuge in the dwindling amount of water remaining around her lodge. May the heavens open up soon.
Last January or February beavers mated (in the water) and in May or June gave birth to 1 – 9 (average 2 – 4) precocial kits. Young beavers are born fully furred, their eyes are open and their incisors are visible. Within four days they are able to swim, but it’s usually a month or more before they are seen outside the lodge. Part of the reason for this is their buoyancy, which prevents them from diving down through the tunnel to get out of the lodge. By the age of two months they are able to submerge themselves and they have begun grooming the water-repelling secretions of their anal glands onto their fur, so they are fully water-repellent. They are ready for their semi-aquatic life. Sometime between two and three months is when they are often first observed by humans — usually in late August or September you can see three generations of beavers in an active pond.
When they are three days old, kits begin to eat vegetation brought to them inside the lodge by their parents and year-old siblings. By the time they are two months old, they are weaned. The three-month-old kit pictured (measuring about a foot from head to base of tail) is able to debark a twig as deftly as its parents.
One associates Beavers with a fairly strict diet of bark and twigs. While their winter diet consists primarily of woody plants, they consume a variety of herbaceous and aquatic plants (as well as woody) during the spring, summer and fall months. Shrubs and trees make up roughly half the spring and autumn requirements, but as little as 10% of the summer diet when herbaceous plants such as sedges and aquatic plants become available.
Recent observation of a local active Beaver pond revealed that Interrupted Fern (Osmunda claytoniana), Jewelweed/Touch-Me-Not (Impatiens capensis) and grasses are high on the list of preferred foods of one Beaver family during the summer, although woody plants such as poplars (Populus spp.) and Paper Birch (Betula papyrifera) have also been consumed in fairly large quantities. All too soon Beavers in the Northeast will be limited to the bark of branches they’ve stored under the ice. Until this time, they take advantage of the accessibility of more easily digested herbaceous plants. (Thanks to the Shepards and Demonts for photo op.)
A recent light snow provided an opportunity to confirm that northern New England beavers have gained access to land and are seeing the sun for the first time in several months. They, along with any beavers living north of the 39th parallel, may well reap some benefit from the change in our climate. Further south, there is no real winter and beavers do not have to cope with a limited amount of stored food for there is usually no ice on ponds. Milder winters and early springs mean more time for Northeastern beavers to access herbaceous food and fresh bark, and less time locked under the ice.
Unbeknownst to many, a large portion of a beaver’s spring, summer, and fall diet consists of herbaceous food – grasses, sedges, ferns, fungi, berries, mushrooms, duckweed and even algae. When beavers first leave their ponds in the spring, one of the first foods they head for is skunk cabbage, as it is one of the earliest flowering plants to emerge (often when snow is still on the ground). Beavers also relish the new foliage of aspen, willow and alders. When they are accessible, the rhizomes, leaves and flowers of both yellow and white pond lilies are favorite foods. Come late fall, when lush greenery has disappeared, beavers up their intake of bark (cambium) and store a pile of branches on the bottom of the pond close to their lodge, where they have underwater access to it all winter.
Yesterday’s design was made by a beaver as it removed bark from a tree. The light-colored, curved little “bumps” that run horizontally across the middle of the tree were made by the two incisors in the beaver’s upper jaw. When eating the sought-after cambium layer of a tree, beavers grip the tree with their two upper incisors as they scrape towards their upper jaw with their two bottom incisors, sometimes creating this pattern. (Individual marks where the upper incisors gripped the bark and the four incisors didn’t quite meet can be seen in the insert.)
The ever-growing incisors of rodents are harder on the front surface (outer layer is hard enamel, colored orange from iron in a beaver’s diet) than the back (softer dentine), so the back of each incisor wears away faster than the front, creating a sharp, chisel-like edge to these four specialized teeth. So functional are beavers incisors as cutting instruments, Native Americans used to insert a beaver incisor in a wooden handle and use it to cut bones and to shape their horn-tipped spears.
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Reproductive activity begins when a beaver reaches the age of three years. Beavers mate in January and February, with the peak activity in mid-February. Typically mating takes place in the water (under the ice), but can occur inside their lodge. Kits, usually three or four, will be born in May or June. Beavers are monogamous and pair for life. (Note: ponds still frozen – photo not recent)