1Hi, it's Doug. You see this?
2This is the Hoover Dam in Nevada, USA.
3A dam is a place where people have walled off part of a river and created a lake behind the wall.
4Hoover Dam is huge, but isn't even the tallest or the longest dam in the world.
5Check out this one in Alberta, Canada.
6It's not very tall, but it's almost twice as long as the Hoover Dam.
7In fact, it's so long that it shows up on satellite images from space.
8And the most amazing thing of all,
9this dam wasn't built with cranes or cement trucks or tractors like most dams,
10this was made almost entirely by beavers.
11Someone named Mateo has a question about beavers and dams.
12Let's give Mateo a call now!
13Hi Doug.
14Hi, Mateo.
15I have a question for you.
16Why do beavers build dams?
17That's a great question.
18This is a beaver dam.
19Why do beavers build these?
20Well, at first, I didn't know the answer to this question either.
21I thought that a beaver dam must be a house where beavers lived,
22but it surprised me to find out that beaver dams aren't where beavers live.
23They're not houses at all.
24So why did they build them then?
25I mean, building a dam seems like a lot of work,
26especially since beavers are so small,
27and it is a lot of work.
28For a beaver to build a dam, can take weeks, months, or even years to completely finish.
29And if you think about why, it makes sense it takes so long.
30First, they need to cut down tall trees,
31then they drag them all the way to the middle of a river.
32Then they pile rocks and sticks and plants on top of it to make it taller.
33So if they're not gonna live in a dam, why do they go through all this trouble?
34There must be a good reason, right?
35Before I say anything more, I'm curious.
36Why do you think beavers go through all the trouble of building a dam?
37Even though dams aren't beaver houses,
38scientists have discovered that they have a lot to do with where beavers build their houses.
39You see, beavers do build houses.
40Their homes are called lodges.
41And when a beaver builds a lodge, it needs to find the perfect place.
42For a beaver, that means a place surrounded by lots of water.
43Like, if they can find an island, in the middle of a pond, that's perfect.
44That's where they'll build their lodge.
45That's because beavers aren't very fast on land,
46but they're great swimmers.
47Look closely. You see it's back feet?
48They're webbed, just like a duck's.
49So a beaver can swim really fast,
50And you notice its flat tail?
51It turns out, that helps a beaver steer, just like the rudder on a boat.
52So building a lodge in the middle of a pond makes sense.
53It makes it a lot easier for beavers to escape from danger and swim home
54if they're being chased by predators, like wolves, wolverines, or bears.
55It's kind of like a moat around a castle.
56The water helps beavers swim home quickly,
57and keeps dangerous animals away, animals that can't swim as well.
58But what if there's no pond around?
59Like, what if the only water nearby is a river.
60A beaver can't just build a lodge in the middle of a rushing river.
61The fast moving current would just carry their home away.
62This is where a dam comes in.
63By building a dam, the beaver blocks the rushing water,
64and turns the river from this to this,
65a totally calm and peaceful pond.
66Once they have a pond, there's no more rushing water or fast moving current that can carry away their lodge,
67and they'll have a safe place to build.
68Now, you may be wondering, what about when beavers have to go outside to build dams or collect food?
69Couldn't they get attacked by a predator then?
70But beavers have that figured out too.
71A beaver dam, floods the land, and plants, and trees around the river.
72So now, instead of having to walk everywhere to get wood or food,
73they can just swim everywhere.
74It's kind of like having a water freeway.
75And this, in my opinion, is the coolest part.
76A beaver builds a secret underwater tunnel, underneath its lodge,
77so that it can swim in and out without anyone or anything seeing.
78In fact, when a beaver sees a wolf or other predator coming close,
79it will start slapping its tail on the water as a warning signal,
80which tells the other beavers to swim to the secret entrance as fast as they can.
81So beavers build dams in order to slow down a rushing river,
82and create a pond where they can live safely.
83But even that's not all,
84they also use these ponds they create for something else.
85It's their refrigerator.
86Beavers love eating bark and leaves off trees.
87So they spend the entire fall storing tree branches underneath the water of the pond near their secret entrance.
88When winter comes and the pond freezes on top,
89they can swim out of their secret tunnels to their stash of sticks whenever they want a snack,
90then take it back into their cozy lodges.
91And they can do this without ever having to go out into the snow.
92So in summary, beavers build dams to stop the flow of rushing rivers,
93so they can create safe ponds to build their lodges on.
94Since these lodges are surrounded by water,
95the beavers who live there are a lot safer from predators.
96That's all for this week's question.
97Thanks Mateo for asking it.