Monday, February 25, 2013

Nature's Lumberjack

The North American Beaver (Castor canadensis) is one of nature's greatest engineers and its greatest lumberjack.  Beavers cut down trees for building materials for dams and lodges and for food.  They do this without the help of tools, using only their teeth.  The incisors on a beaver are perfect cutting tools, kept sharp by constant use.  Their teeth grow throughout their life.  Each incisor is made of two materials that wear at different rates, the front of the tooth is covered with a hard enamel, the back of the tooth is made of softer dentin.  The softer dentin wears away faster than the enamel, creating a wedge-shaped profile on the tooth.

Beaver Skull

Close-up of incisors.  Enamel is orange, dentin is white.  Note the wedge profile.

This wedge profile allows the beaver to cut wood quite easily with its teeth.  It uses its upper incisors as a fulcrum, holding its head in place as it chews.  Its lower incisors bite into the wood, shaving off strips.

Strips removed from a tree trunk

With these teeth the beaver is able to fell and buck (cut into lengths) quite large trees like this Red Maple (Acer rubrum).

                               The tape measure is about 3.5 inches long.  This tree measured over 14 inches in diameter.                      Note the chewed log in the background.
A beaver is able to fell a 3 inch diameter sapling in as few as 15 minutes.  Larger trees will inevitably take  longer and may be felled over the course of several days.  Once the trees are felled, small branches are trimmed and used as building materials or stored in a food cache.

A riverbank food cache
Over time the activity of even a single beaver can have a large impact on the local environment.  Even if the beaver does not build a dam, the act of cutting trees for food can quickly change a local habitat.

River floodplain after beaver logging activities. 
The opening of the canopy after beavers logged this area, greatly increased the number of wildflowers and ferns in this location.  Many wildflowers bloomed more heavily in the years following this cutting, taking advantage of the increased sunlight.

While the modern North American Beaver is a medium sized herbivore, reaching lengths of up to 4 foot (including an 8 to 14 inch tail) and weights of 50 to 60 (occasionally 100) pounds, during the Pleistocene the modern beaver was joined in Michigan by an enormous cousin the Giant Beaver (Castoroides ohioensis).  The Giant Beaver could reach lengths of 6 to 8 feet and a weight of up to 600 pounds - although most probably weighed less than 300 pounds.  This behemoth probably did not cut down large trees, but rather ate aquatic plants much like a modern Muskrat.

A replica Giant Beaver skull (left) and a modern North American Beaver skull (right)
This comparison photo shows the size difference between the Giant Beaver and the North American Beaver.  The squares on the photo measure one inch.  The Giant Beaver skull measures approximately 12 inches in length, the modern beaver is closer to four inches.

The modern beaver was once hunted and trapped almost to extinction across much of North America for its fur.  A fashion for beaver felt hats drove this pursuit from the 1600s through the 1830s.  The beaver may have been driven to extinction if fashion had not changed; in the 1830s the fashion shifted from felt hats to silk hats.  Today, the population of beaver has rebounded across much of its historic range and continues to expand.

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