Unusual Beaver Activity
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.)
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Porcupine Artistry
The patterns that a porcupine’s incisors leave when a porcupine has been removing bark down to the cambium can be a work of art. The way in which a porcupine makes these patterns is as intriguing as the patterns themselves. “The porcupine removes the bark in small triangular patches, each patch composed of five or six scrapes converging on an apex, like sticks in a teepee. The apex represents the position of the upper incisors, held fixed against the bark. The lower incisors scrape, moving over a fresh path as the lower jaw swivels in a narrow arc.” (Uldus Rose, The North American Porcupine) Fortunately, porcupine incisors, like those of all rodents, grow continually. Even though each incisor loses 100% of its length to wear in a year’s chewing, its length always remains the same. Juvenile porcupines leave a much less “organized” set of incisor marks (overlapping, randomly placed) than adults.
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Meadow Vole Nests
Meadow voles (Microtus pennsylvanicus), tiny mouse-like rodents, spend a majority of their time on the surface of the ground, particularly in moist fields filled with grasses and sedges. They have elaborate runways through the vegetation – well-worn trails about the width of a garden hose which they keep mowed down with their incisors. Latrines of small brownish-green pellets can be found intermittently along the trails.
Most meadow vole nests are constructed out of dried grasses, also on the surface of the ground, although they are sometimes built at the end of shallow burrows. When above ground, the nests are often located in the center of a grass tussock, where they are less apt to be flooded. When there is snow on the ground they are a bit easier to find, as the heat of the voles inside the nest melts away the snow, forming a chimney that sometimes reveals the nest below.
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Rodent Incisors
Rodents have two pairs of incisors that oppose each other; two in the front of the upper jaw and two in the front of the lower jaw. These four teeth never stop growing, and thus have to be constantly filed down. (Squirrel incisors grow about one-half inch a month.) While nipping herbaceous plants and cutting into bark help do this, it isn’t enough to significantly wear down the teeth. When mice, beavers, woodchucks and other rodents nip or gnaw food, they move their jaws in such a way that these two pairs of teeth grind against each other. Most rodents have hard orange enamel on the outer side of their incisors (woodchucks are our only rodent with white enamel), and softer dentine in the back. When the incisors grind against each other, the dentine wears away faster than the enamel, creating a sharp, chisel-shaped edge to the incisors. This, more than anything else, keeps the growth of rodent incisors under control, just as a nail file keeps our nails from getting too long.
If a rodent breaks an incisor, there is nothing to wear down the opposing incisor. The tooth opposite the missing incisor will continue to grow unchecked in a circle until it causes the death of the rodent, either by piercing its skull or by preventing the animal from being able to eat. In the pictured woodchuck skull, the lower left incisor has broken off, allowing the upper left incisor to grow through the woodchuck’s palate and into its brain.
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Beaver Incisors
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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Rodents Recycling
Bones, antlers, skulls, turtle shells – all are recycled relatively quickly by rodents seeking a source of minerals, particularly calcium and phosphorus. All rodents possess four incisors, two in the front of the upper jaw and two opposite these, on the bottom jaw. These incisors, unlike other teeth, never stop growing. By gnawing on hard objects such as bones, rodents keep their incisors paired down. If an incisor is broken or lost, the opposing incisor will continue growing in a circle, having nothing to grind against, causing the rodent to die of starvation or from having its brain pierced (through the roof of the rodent’s mouth) by the ever-growing incisor. In this photograph, a gray squirrel is obtaining minerals and sharpening its incisors on a moose skull that a human wedged into the crotch of a tree.
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Red Squirrels & Sugar Maples
We’re approaching what is often a very stressful time of year for many animals, including red squirrels. In the fall they feed on all kinds of conifer seeds, mushrooms, insects, nuts and the many fruits and berries that are available. They also have caches of cones, which they turn to once there is a scarcity of food elsewhere.
Once these caches are used up, usually by late winter or early spring, red squirrels turn to sugar maples for nutrients. Their timing is perfect, for this is when sap is starting to be drawn up from the roots of trees. Red squirrels are known to harvest this sap by making single bites into the tree with their incisors. These bites go deep enough to tap into the tree’s xylem tissue, which is where the sap is flowing. The puncture causes the sap to flow out of the tree, but the squirrel delays its gratification. It leaves and returns later to lick up the sugary residue that remains on the branch after most of the water has evaporated from the sap.
Not only do red squirrels help themselves to sugar maple sap, but they have developed a taste for the buds, and later in the spring, the flowers, of both red and sugar maples. Red squirrels are not the only culprits – gray squirrels and flying squirrels also make short work of buds and flowers from these trees.
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Porcupine Feeding Technique
In the winter, the bulk of a porcupine’s diet is the inner bark, or cambium, of trees. The porcupine removes the outer bark (unless the tree is young or has thin outer bark and then it eats the outer bark as well) in order to reach the cambium layer, which lies directly beneath the outer bark. At this point the exposed surface is very smooth, more finely finished than the work of a beaver. Then the porcupine removes the cambium in small, triangular patches, each patch composed of five or six scrapes converging at one point, like sticks in a tepee. The point where the scrapes meet is where the upper incisors are placed and held fixed against the tree. The lower incisors scrape, making a fresh path with each scrape, as the lower jaw swivels in a narrow arc.
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