Thursday, December 10, 2009


Nature Watch
By Susan Benson,
CNHM Director of Education

Some weeks seem like my best natural history observations are from a car. On my way back from teaching in South Shore School District, I kept observing signs of beavers in many of the wetlands along the way. Beavers are endowed with a series of very helpful skills that express their intelligence. Beavers are flexible, learn from observation, have imagination, an ability to plan, and can be innovative in their design. Generally, mammals rarely build structures, but beavers do far more than that. They build elaborate dams and canals to control water flow, create water levels that ensure the safety of their family, and make essential transportation pathways to find food and move building materials. In their “free time,” they build artificial islands and homes we call lodges. They are probably one of nature’s best architects!

Not only are they great builders and shapers, but at a rather large sixty or more pounds, beavers make their living while eating bark. Yum. Technically, beavers eat the ring of live tissue just under the bark. Located in this area is the phloem, containing water rich in sugars. Secondly is the xylem, the plumbing that carries mineral-rich water up from the roots. Between these two “transportation tubes” is the cambium, which is responsible for making new rings of xylem and phloem as it is needed. When they are finished eating, the leftover pile of indigestible bark chips is used as nesting material, and the leftover trunks and branches becomes their “lumber” for building.

Part of what makes this species so amazing is that no situation in their world is ever alike – they must solve each hydraulic problem before them, expressing tremendous creativity to make things work. First, they build dams as a defense from predators. To build their dams, they will often put logs in the bottom of the stream across the direction of flow, pile on more logs, use stones to hold it in place, and seal this foundation with extra sticks and packed mud. Other times they will fell one tree across the stream and use its mass to hold the water back. They will drive sharpened stakes into the stream bed. Scientists have observed an amazing array of techniques that beavers use to build their dams. The longest known dam is 2,200 feet! What a lot of work!

No matter how the dams are built, they are crucial when cold weather comes. A large pond that freezes over its entire surface can make breathing for an underwater mammal a big challenge. If the ice is thin, beavers can break open breathing holes, or can return to their burrows or lodges for air. What appears to be the best method of breathing, however, is that many beavers will cut a small channel through their dam. This allows the water to drain out just below the ice, where the gap between the water and ice provides a layer of air across the entire pond.

Next, transportation of beaver food creates a continual challenge, as we know that trees are heavy! When a beaver pond first runs out of trees close to the water’s edge, building a dam floods the area, providing more food. In time, though, they reach the point at which no more food is available. Beavers then begin excavating canals into the forest, cutting down trees along the edges, eating the tasty inner bark, and floating the branches back to the pond for use in future dams or lodges.

Another important feature beavers build is a lodge. Beavers seem to always understand what is needed, and come up with a strategy. For example, sometimes they will dig a burrow to live within, and other times will build a lodge. A family of beavers can include up to eight individuals. Their front door is in their floor, and this entrance must be at least a foot under water. The “bedroom” chamber must be above the water, and must be above the highest water level that could possibly occur throughout the year. Keeping safety in mind, they will even turn their lodges into island retreats by excavating a moat around them. Even on the coldest days, their lodge is built so well that it remains at a temperature above 40 degrees Fahrenheit.

Learning about beavers certainly puts my own life into perspective. When I complain about rising costs or the economy, or about the weather, or even the amount of energy I put into my job this week, it seems like I have nothing to complain about – my life is pretty cushy. Beavers work hard, and I sound rather lazy in comparison. On top of all the hard work they do, they coexist peaceably with many uninvited guests to their home, including muskrats, flying insects, mice, or water voles. If only I could do the same!

For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility and exhibits, the Curiosity Center and Brain Teasers 2, in Cable at 13470 County Highway M. Also find us on the web at to learn more about exhibits and programs.

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