Science Discussion: Why do beavers need to make ponds in the first place?

Why are beavers damming up streams and making ponds? What is so special about having ponds to live in?

Only have five minutes? Here’s the short answer: Beavers need ponds because they need slow moving, deep water to be safe. Beavers are incredibly awkward on land, but graceful and agile in the water. If they have a pond to build their lodge in the middle of, they basically have a giant moat around their house to keep predators away. It also makes the water deep enough to not freeze through in the winter – beavers rely on underwater passages to transport food and swim to safety from the shore. If the water they are in froze through, they’d be much easier targets for predators.


Have a little longer? Here’s the more detailed answer.

Beaver Ponds for Safety
Beavers are pretty round, with short little legs and a very heavy, large, flat tail. They’re certainly not build like a gazelle, and that is clear when you see them running on land. They are awkward, slow, and not particularly graceful.

Beaver on land is awkward. You would be too if you were round and short legged and had to drag around a big flat tail all day.
Beaver on land is awkward. You would be too if you were round and short legged and had to drag around a big flat tail all day.

That all changes when they get into the water though. Much like penguins, beavers transform from a cumbersome blob on land into an agile, acrobatic, elite swimmer in the water. In fact, beavers can swim up to 35 mph in the water and dive rapidly to safety from predators that can sort of swim, like bears. Once underwater, a beaver can stay down without needing to come up for air for 15 minutes.

Beavers are designed to swim and dive. They're virtually invincible if the water is deep enough.
Beavers are designed to swim and dive. They’re virtually invincible if the water is deep enough.

Their big flat tails maybe be a hinderance on land, but they are truly key to their survival. It’s their tails that allow them to swim so fast and dive so easily, acting as a giant paddle propelling them forward. They also use their tails to slap the water when they see a predator. This tail slap warns nearby beavers that a predator is near, and any beaver that hears the slap will dive or return to the lodge. The tail slap also scares the predators, and discourages them.

In order for the tail slap to be most effective, the water needs to be still (slapping still water like a pond makes much more noise than slapping moving water like a stream). In order for the beavers to heed the tail slap warning and dive, the water needs to be fairly deep – at least several feet deep. Beavers know they need fairly deep pools of still or slow moving water to be safe, so instead of searching for existing ponds, they just dam up streams and make a new pond to their specifications.

Beaver Ponds for Winter Survival
As mentioned in yesterday’s beaver fact of the week, lodges keep beavers warm through the winter. Well, beavers need to be able to get in and out of their lodges to access underwater food stores and go on land to collect new food. One of the reasons their lodges are so warm is the only opening to outside air is a small vent in top, almost like a chimney. If beavers had to routinely use above ground entrances to their lodges because the water was too shallow and froze all the way through in the winter, the holes would likely be larger, and the lodge would be colder. They’d also need to be leaving their lodges more often since they’d no longer have access to their underwater food stores.

Great photo/drawing composite of a beaver lodge in the winter. The beavers have water to swim in, access to food stores, and a nice toasty lodge to keep them cozy and safe. This is only possible if the pond is deep enough to not freeze through in the winter! Photo credit to Jon Nelson at www.montananaturalist.org.
Great photo/drawing composite of a beaver lodge in the winter. The beavers have water to swim in, access to food stores, and a nice toasty lodge to keep them cozy and safe. This is only possible if the pond is deep enough to not freeze through in the winter! Photo credit to Jon Nelson at http://www.montananaturalist.org.

As I already mentioned, beavers are easy prey on land. If the water their lodge in froze through, they’d need to go out on land every single day to get food in the winter. If they build a pond to their personal specifications – i.e. one that is deep enough to avoid freeze-through in the winter, then they only need to go out in the open once they’ve eaten through their food stores.

Having water deep enough to not freeze through is critical for the winter survival of beavers. Sure, the beavers could go looking around trying to find an already established pond deep enough to suit their needs. Or, they could just spend the summer and fall damming up a good sized stream and create one wherever they want.

Beavers are the only creature besides humans that move into inhospitable locations and actively alter the environment to suit their needs.
It’s really quite remarkable. Obviously people have moved into places that aren’t naturally hospitable to our survival, and we’ve changed the landscape to make it so we can thrive (looking at you southern California and Arizona…people aren’t designed to live in deserts with no water, but thanks to dams, canals, and pumping groundwater we’ve made it work). Beavers do the same thing. They move into a pond-less area, build a dam, make a pond, and thrive. They really are nature’s engineers.

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