This is the time of year when two-year-old beavers leave their lodges and strike out on their own, primarily because the woods surrounding a pond usually can’t support more than one family of beavers. Beavers are exceptionally territorial; once they’ve established a lodge, they do not take kindly to interlopers. In order to make this perfectly clear to house-hunting young beavers, in the spring resident beavers build what are called scent mounds — piles (up to three feet in height, but usually much smaller) of mud, leaves and pond-bottom debris — around the perimeter of their territory. They then smear castoreum, a substance that comes from their castor sacs, over the mound. Chemicals in the castoreum convey to roaming young beavers that this particular pond is spoken for.
A study of coyote prey (through stomach contents) in the Adirondack Mountains of New York revealed that beavers were second only to white-tailed deer. This photograph shows that, possibly for the last time this winter, a coyote recently took advantage of a still-frozen-but-fast-thawing pond by walking across it in an attempt to reach an active beaver lodge. Once there the coyote attempted to dig into it in order to reach the inhabitants. A hard, two-to three-foot-thick wall of frozen mud, logs and sticks kept the beavers well protected, as it was designed to.
It is not coincidental that you often find otters residing in beaver ponds. There appears to be a commensal (one animal benefits while the other is unaffected) relationship between these two animals. The beaver is unaffected – it is a herbivore, so its food supply is not threatened by the presence of otters. (While an occasional beaver is eaten by an otter, it is a rare occurrence.) The otter, on the other hand, benefits from abandoned as well as active den sites (both beaver bank dens and lodges) as well as an ample supply of fish due to the impoundment of streams by beavers. While I was aware that otters often take over abandoned beaver lodges, I only recently learned that the lodge does not have to be uninhabited for otters to move in. This was confirmed when I discovered a large amount of otter scat (mostly fish scales and crayfish shells) on top of a beaver lodge, right next to the hind foot print of a beaver. Freshly placed sticks on the lodge (it is in open water) indicated that it was occupied by beavers, while an otter’s stream of air bubbles could be seen as it exited the lodge and popped its head up above the surface of the water.
If beavers have bank dens on rivers that remain open all winter, they are not subjected to the confines of a dark, damp lodge for several months. They have access to fresh food year round, and aren’t limited to the pile of aging branches under the ice that they stored last fall near their lodge. On the other hand, beavers that do live in ponds that freeze over often can find an opening in the ice if there’s a big enough January or February thaw. In either case, signs of their activity on land can be found.
Ice is starting to form on ponds, and days are numbered when beavers can be out grooming themselves on land or eating freshly-cut branches. As long as the ice is thin enough to crack by swimming up under it and bumping their heads against it, they will do so, for soon it will be thick enough to lock them into their pond. Life under the ice is challenging. At 32 degrees F. a beaver’s resistance to heat loss in water is about 1/8th of that in air at the same temperature. This is due to the fact that its fur is compressed in the water, allowing the insulating air between the hairs to escape — a beaver’s pelt accounts for about 24% of its total insulation in water and body fat is responsible for the rest. Heat is also retained through a beaver’s tail and hind legs, which serve as heat exchangers. In the summer, a beaver can lose 25% of its body heat through its tail, but it only loses 2% in winter. Even so, it’s no wonder beavers risk getting a headache in order to see the sun for the last time until spring.
If you find a mushroom hanging in an unlikely spot, such as from tree branches or tucked into the bark of a tree where it didn’t grow, it’s likely that you have happened upon the work of a red squirrel (Tamiasciurus hudsonicus). Red squirrels are known for their habit of snipping mushrooms and hanging them from branches and rough bark in order to dry them before collecting them and caching them for dining on later in the winter. Unlike beavers, which share their stored cache of winter food with family members, red squirrels keep their cached fungi all to themselves.
Shorter days and longer nights trigger a flurry of activity for beavers. There is a lodge to be built, rebuilt, enlarged or repaired and a dam to be built, repaired or reinforced. As, or more, important than these tasks is cutting, gathering and transporting a supply of food for winter. Once the pond is frozen, the only food available to beavers is that which they have stockpiled under the ice. Thus, beavers spend many an autumn night adding to a growing pile of submerged branches close to the lodge. More thought is put into the harvesting of a winter food supply than one might imagine. Before cutting down a tree a beaver often tests its readiness by biting into the bark. If it is not in just the right condition — for instance, if there is still too much sap in the tree — they may speed up the drying of the bark by girdling it, and returning in several days to cut it down. If limbs and branches are stored underwater before the bark is ready, they will ferment and sour, making them unfit for food.
Beavers are constantly grooming and oiling their fur to waterproof it. Typically when grooming, a beaver sits upright with its tail curled under its body and extended in front of it between its two hind legs. This allows the cloaca (an opening which contains ducts for everything from evacuation to reproduction, plus oil and castoreum glands) to be exposed. The beaver uses its front feet to retrieve creamy-yellow waterproofing oil from its inverted oil glands and then rubs it carefully over all of its body. Without constant waterproofing the beaver’s fur would soon become soaking wet and the beaver would not be able to tolerate the cold water.
Beavers are meticulous housekeepers, in that they almost always defecate in the water, not in their lodge, and rarely on land. The best place to find their scat, should you be so inclined, is where they have been working for an extended period of time — for example, in the water adjacent to their dam. Their scat consists of kumquat-size pellets, which, as you might expect, are full of tiny bits of woody fiber. The pellets are essentially little balls of sawdust, and disintegrate easily if disturbed. Their light color makes them visible even under water. Congratulations to all who guessed correctly — I’ll make the next mystery post even more challenging!
Although some ponds have had open water in spots all winter, many have remained frozen over until the recent warm weather started to melt the ice. The first open water often appears close to the lodge and along the dam of a beaver pond. It doesn’t take long for resident beavers to detect an opening, for it’s a ticket to fresh food! The first plant that beavers head for, if it’s growing in the area, is skunk cabbage. Being the first wildflower to push up through the snow, it’s usually available when ponds first open up. Aspen, willow and alder leaves, grasses, the rhizomes, leaves and flowers of water lilies, sedges, ferns, fungi, berries, mushrooms, duckweed and algae are eaten in the spring and summer by these large rodents we think of as strictly bark eaters. Photograph by Kay Shumway.
2-29-12 Beaver Sign of Spring
Anyone who buys and consumes the pale, relatively tasteless, store-bought tomatoes in the winter, and then, finally, can eat their own garden tomatoes right off the vine, will identify with the winter and spring diets of beavers. While they are locked under the ice, the beavers’ entire winter supply of food is a pile of branches they store at the bottom of the pond near their lodge. Once the ice on the pond begins to melt, beavers take immediate advantage of any escape holes, enlarging them if need be, in order to make their way to fresh, nutritious food. While their preferred spring food, herbaceous plants, are not yet up, the fresh cambium of living trees is most likely a welcome change from their water-logged winter food. It is always fun to come upon signs of their activity when there’s still snow on the ground – it’s one of my favorite signs of spring.
Beavers, like many mammals, communicate with chemical signals. In scent marking, beavers actually build a mound of mud on which to place their scent. First they dive down to the bottom of their pond, dig up an armful of mud with their front feet and swim to shore with the mud held against their chest. Walking on to the shore on its two front legs, the beaver deposits this mud next to the water. Beaver scent mounds can be quite small, or as high as three feet or more, depending on how many loads of mud they contain. The beaver straddles this pile of mud and applies castoreum from its castor glands, or anal gland secretions, or both. The purpose of building a mound is to elevate the odor (helps with scent dispersal), to intensify the odor by putting it on a moist substrate, and to protect it from flooding when the pond level fluctuates. Beavers build most of their scent mounds in the spring, when young beavers are dispersing and claiming new ponds, but I have found several fresh ones this fall, including the one in the photograph. (Click on photo to enlarge.)
There’s a period of time in the fall, and again in the spring, when pond ice is thin enough to break under the pressure of a beaver’s head punching it from underneath, but is not quite hard enough to support the beaver’s weight. After an audible crack, a beaver’s head emerges from the recently-made hole in the ice, and immediately the beaver lifts its front feet up onto the edge of the ice in front of it and then lunges forward, breaking a path with its body through the ice to where the beaver wishes to go. Over and over the beaver lunges, pausing periodically to catch its breath. Why the beaver doesn’t swim under water from point A to point B (beavers can swim up to half a mile underwater, and remain submerged for up to 15 minutes), which would mean having to break through the ice only once, when it gets to where it’s headed, instead of laboriously breaking trail through the ice is unknown to me, but the process is great fun to observe.
In March and April, beavers consume a lot of tree bark, but come summer, approximately 90 % of a beaver’s diet consists of grasses, aquatic plants, and other herbaceous vegetation. Of the woody plants that they do eat during the warmer months, aspen/poplar (in photograph) and willow are favorites. It appears that beavers use their sense of smell in order to find their tree of choice. The greater the distance from the pond, the more selective beavers are in terms of species chosen – they go as far as a tenth of a mile away and up steep slopes for aspen!