In an earlier post I pointed out the complex structure of beaver dams and the many decisions that the beavers have to make in building them. there is another complex aspect to beaver dams and a scientific mystery as well. Beavers build many dams, but why?
Most people know that the beaver builds a dam, which creates a pond, and in that pond they then build a sturdy lodge with an underwater entrance. This lodge becomes the beavers home, usually occupied by a mating pair, where they raise a family.
What is less well known is that the beaver pair also build other dams nearby, sometimes a lot of other dams. Why the do this is not well understood, but I have a conjecture about the reason. More on that in a bit.
Two examples will help. In one case we saw the beavers had built five dams in a row on a steep mountain stream. These dams were massive, each almost six feet high. In another case a pair of beavers started building dams on two small side streams that were close together. They built 36 dams in all, mostly relatively small one's.
In both cases the beavers also built their customary lodge in one pond, but why go to the extraordinary extra effort of building all those other dams? Not only does this take effort but it is dangerous.
The beaver has to leave the safety of the pond to get the wood to build a dam and during that time it is exposed to predation. The more dams it builds the further it has to go, and the more time it spends exposed, greatly increasing the danger. Why do this I wondered?
When I looked into it I found that this behavior was well know but the only explanation offered was that the beaver could not stop building dams. In other words all this dangerous activity was an irrational compulsion.
To me this explanation was unacceptable because evolution could easily modify this behavior. There had to be a good reason for building all these dams. The basic principle is animals do what they do for good reasons.
My conjecture is that these extra dams clear land in what would otherwise often be dense forest. Moreover the first trees to grow after the cleared land dries are often aspen and alder, which are the beaver's standard food sources. Thus if I am correct then the beavers are tree farming, not for themselves but for future generations. I would be interested other possible explanations.
The basic point is that animal behavior may be complex and subtle, and hard to explain, but it is unlikely to be irrational on a large scale.