Tuesday, December 1, 2009

Beaver Trail

On 11/23/09, Lars was at the northern end of Delaney in Stow. A nice trail was seen in the forest litter, making it easy to imagine this beaver's girth. I expect that he would have made a skinnier trail in the spring.

It was a second try for the beaver, as you can see by the aged central scar on the tree. Oak is a very hard tree to cut, but he does have a brain the size of a, well, beaver....


  1. As I thought about it, I wondered if the beaver began felling trees from the same side of approach each time. On this tree, the beaver approached from the left of the tree. Is there handedness in beavers? Or is it only dependent on sunlight/open space/slope of land (the lean of the tree), or the orientation of water/dam/lodge? Here's one reference I found:


    Hmm, Lars

  2. There's a great book about beavers, relatively recently published, on the reading list at our website. Also a nice chapter on beavers in Tom Wessel's wonderful book, Reading the Forested Landscape.

    I don't recall all the details, but I do know that oak is actually fairly high up on the beaver's list of preferred woody foods. Aspens and willows are favorites, conifers are last resort. The other deciduous trees fall somewhere in between, with oak pretty high, and red maple and witch hazel quite low.

    The hardness of the wood doesn't seem to be an issue for them. They base their selection on scent of the plant, its size, and closeness to the water.

    Oaks have tannins, a class of phenolics, which are those toxic compounds that plants have evolved for defense against herbivores. However, tannins don't bother the beaver, which has responded by evolving a way to inactiviate them.

    One strategy they have for dealing with some phenolics (I don't recall if this is what they do with tannins) that is particularly fascinating is by sequestering them into their castor sacs...and then using them for scent marking as a component of castoreum!!

    Your question about handedness in beavers is interesting. I've never thought about that. But even if they do display handedness, would that cause them to have a preferred direction of approach to their work? If so, it seems like we might observe some peculair asymmetries to their dams and lodges. Especially since they are presumably not intelligent enough to step back and observe their work and make adjustments to compensate....But who knows...

    As you know animals reuse the same trails in snow just as we do, and for the same reason: because it's easier than trudging through unbroken snow.

    But they also sometimes reuse trails even when there is no apparent advantage, as in your photo above, and even if they have to go out of their way to do so.

    I've heard some very interesting debates about why they do that, including speculation that it is evidence of higher thinking, such as a ritualistic, religious behavior.

    Call me concrete, but I tend to think these things have far simpler explanations. Maybe it is a ritual, but only in the sense that brushing your teeth before combing your hair every morning is a ritual. That ritual is good for you because it ensures you won't forget to do either task. The trail reuse ritual is good for the animal because sometimes (as in snowy or muddy conditions) it saves the animal energy. Why waste effort thinking about something when doing it automatically is a good thing much of the time, and rarely a bad thing.

    Anyone else have ideas about this?