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Posts Tagged ‘North American beavers’

I’ve had several people ask me about this article that appeared on a Finnish news website about introduced North American beavers losing their damming instinct. The story goes as follows:

Canadian beavers were introduced to Finland in the 1930s after the indigenous European beaver population had been hunted to extinction in the 19th century.

There are now an estimated 10-thousand beavers in the country, up to half of them in the South Savo region.

Population pressures, competition for living space and food, have brought more into large waterways such as Lake Saimaa. With a surface area of 4,400 square kilometres, Lake Saimaa provides plenty of room to spread out.

Beavers usually build damns in rivers and streams, to create ponds where they build their lodges. Those who have moved into Lake Saimaa, have had to adapt to a lifestyle of fluctuating water levels, rather than damming and regulating the waters themselves. In the process, they seem to have lost the desire to do so.

These beavers now create burrows or dens for themselves along the shores of the lake.

“They have had to adapt so that if the water level of Lake Saimaa falls, they change dens and diets. They change locations, and then if the waters rise too high for their dens, then they abandon them and build new dens higher up the shore,” explains Risto Hirvonen, a member of a local hunting club.

None of this should be of any surprise to anyone who knows about North American beavers.

We tend to think that beavers always build dams and then build lodges with underwater entrances.

The problem with this popular conception is that is not an absolute.

Most beavers in West Virginia don’t build dams at all. Instead, they dig dens into the sides of the banks of rivers. These dens always have an underwater entrance, which can sometimes be exposed if the water level in the river drops.

We call these beavers “bank beavers.”

beaver bank den

A beaver bank den in Kansas. Source for the image.

Does this mean that these beavers have lost their damming instinct?

I am very skeptical.

My guess is that these animals in Finland are now accustomed to the constant water levels in Lake Saimaa, just as they become accustomed to constantly shifting water levels in relatively slow moving rivers in the United States.

There is a way to test this hypothesis scientifically.

Catch some beavers from known populations that engage in bank denning, Put them in an enclosure with shallow rapidly running water and give them some material to make a dam.  Measure how long it takes for them to build a dam, if they built it at all.

Then do the same with beavers from known dam-building populations and run exactly the same test.

Then you’ll actually know if they’re actually losing the instinct.

My guess is that North American beavers, which are among the most successful of all large rodents, which have been introduced not just to Europe but Tierra del Fuego as well, are pretty good at adjusting their behaviors to fit their surroundings.

That hypothesis seems to fit with what we already know about them.

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