Tuesday, November 1, 2011

How Beavers Otter-proof Their Lodges

A beaver lodge is one of the classic shapes in nature. From a distance its conical outline is unmistakable. On closer look, that this conical shape is made with a myriad of logs cut by the beavers adds to our high estimation of the building skills of the beaver. In her book,
Lily Pond, Hope Ryden cites evidence that the beaver's lodge building skills are inherited, that "stereotypical motor sequences used in the construction of dams and lodges are expressed spontaneously by each individual" beaver as it matures. No wonder beaver lodges look so much alike.

That said, I can think of exceptions, some eccentric touches to the iconic design, especially the tendency for lodges to have some rather sharp logs stabbing out from the top of that classic cone shape. In most cases it is hardly noticeable. In my web page on beaver lodges, I wanted to use my most typical photo of a lodge, and chose this one:

I forgave that ragged look on top perhaps because it reminded me of my uncombed hair when I was a kid. Both the lodge and my haircuts were unremarkable manifestations of life's messy ends that need not mar its underlying perfection. I probably should have chosen one of my many photos showing a tamer beaver lodge, like this one.

Then I saw the lodge the photo of which graces the top of this blog. Here is another view of that lodge taken after a snowfall that emphasizes its eccentric top:

I had a long familiarity with this lodge before I saw that log sticking up out of it like a flag pole. Sometimes beavers will build a lodge around a tree, but not in this case. Here is how the lodge looked two months before it became the beaver version of the Iwo Jima Memorial:

That photo was take February 6, 2005. I saw the log sticking out of the lodge in April. My immediate reaction was that a human did it. During hunting season the previous fall, someone dug a hole in the dam. The beavers repaired it promptly and through out the winter and early spring there was no other vandalism that I noticed. There were no other signs of a human, other than me, being around. There were signs, just before the pole went up that otters returned to the pond.

I have been fortunate in being able to observe ponds that both beavers and otters share. Over the years I've taken many videos of otters on top of beaver lodges. Here are 6 otters on another lodge in the same pond taken a few years earlier.

I know that it is widely believe that otters eat beavers but they don't. Please don't use my photos and observations to argue that beavers are afraid of otters. They aren't. And I am not sure that beavers worry that much about otters lounging and scatting on top of their lodges. Not that the beavers think otters are especially cute when they are on top of a lodge, which might be your reaction to the video from which I lifted that photo of the 6 otters.

Some beavers seem to go to great lengths to keep otters from getting into the top of their lodge. Beavers need to vent air from the lodge and usually that vent is at the top where the logs used to build the lodge meet or crisscross. But I've noticed that often the loose construction at the apex of the lodge can result in a small chamber that's not of much use to a beaver. This is especially the case in large lodges that can lose that neat conical shape and tend toward the profile of a Bacterian camel. Here is a photo of a family of otters crawling over an ungainly lodge in another pond.

But these otters weren't just visiting this lodge. They were denning there, at least for that day. I saw them disappear into a hole at the top of the lodge, and then one otter pup came up to poop and went right back into its penthouse apartment.

Beavers were in that lodge at the same time, and maybe even a muskrat or two. A year before that October 2001 video, I saw a beaver try to keep a family of otters away from the lodge. At first it seemed like the otters were content just to fish in the pond and after they did and swam close to the lodge, a beaver seemed to intimidate them enough that they scooted into the grasses on shore. But then just as it got dark enough to no longer see, the otters came out from the grasses, and to their screeching delight gained the top of the lodge while the beaver vainly slapped its tail.

I have never tried to sleep in close proximity to otters. (Most beavers sleep in their lodges during the day.) I have gotten e-mails from people who have had otters denning under their cottage and they reported troubling noises and smells. From observing otters on top of lodges, I've seen that otters are restless even when they sleep. Beavers may not be afraid of otters, but they might object to their high-strung behavior and the fishy smells attached to and often ejected from a piscivorous animal. I offer in evidence a photo of a typical otter scats, the smell of which can be redolent for several days.

One trouble with this thesis of mine is that beavers and otters share all the ponds I watch and not all the lodges in them show any otter-proofing on top. In most ponds beavers have other lodges so if otters get too annoying they can move. However, this year I got pretty good evidence that the beavers added logs to the top of their lodge with the intention of keeping otters off of it. The brief back story goes like this: beavers abandoned a large pond in 2005 and after the water in it mostly drained away, otters that had raised their pups in the fall there for several years stopped visiting. Then in 2010 beavers moved back into the upper end of the pond and built a new dam that flooded what had been an area popular with otters. In 2011 otters returned to the pond.

Just as with the flag-pole lodge, I had an Ah Ha! moment with this lodge. When otters returned to the lodge, I dutifully took photos of their mounting piles of poop on a sloping rock on shore where, by the way, the otters had an excellent view of the lodge. Here is a photo of the poop:

Before the poop appeared here is how the lodge looked on October 19:

Here is how the lodge looked on October 31, 5 days after I noticed that otters had returned:

I rest my case. I also have evidence that the beavers were well aware of the otters presence. Of course, once I noticed that the otters were regularly leaving their scats by the pond, I checked the pond more often hoping to see otters. One sunny morning as I walked down to check where they latrine on the rock, I heard a splash and then an otter snort from under a bush. I waited for the otter to appear but it didn't. Then I saw the trail of something swimming under water also leaving the bush. It was a beaver. Evidently it had been keeping an eye on the otter. The video below is not that good.

Eventually I will do a blog on the propensity for one beaver in a beaver family to stay out in the pond during the day when otters are around. At this pond I saw that beaver on patrol several times, but never an otter even though their scats kept piling up at their latrine. One morning the beaver climbed up on the lodge as if to inspect the otter-proofing, though that might be reading a bit too much into the 2 minute video below.

With winter upon us, the beavers no longer have to worry about otters on top of their lodge, or fashioning a penthouse den up there. The outsides of beaver lodges freeze hard in the St. Lawrence River valley. As winter wears on, otters will probably visit the pond. There is a resident mink now using the burrow of a very old bank beaver lodge that otters used to den in several years ago. Otters get into beavers during the winter, usually through an underwater entrance but often make the whole pond like a den by digging a hole through the dam so much of the water drains out of the pond leaving an airy under ice world congenial to otters and usually convenient for the beavers too. I think the otters fashion dens among crisscrossed logs on the bottom of the pond, but that is hard to observe.

The otter-proofing of this season will probably eventually fall into disrepair. The flagpole like log that one beaver pushed up on the top of its lodge slowly, very slowly fell down.

Some otters made themselves quite at home there years later after the beavers moved into another one of the 5 lodges in pond.

Here is a link to the monograph I wrote about beaver and otter encounters published on-line by the Otter Specialist Group of the International Union of Conservation Naturalists Encounters Between Otters (Lontra canadensis) and American Beavers (Castor canadensis) The paper is based on videos I took over the years of beavers confronting otters.

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