Here is Rod MacIver talking about the stamina, intensity, and loyalty of beavers.
Lately I’ve been walking down to the beaver flow near my cabin just as the sun is setting behind a nearby hill. It takes about fifteen minutes to walk down there. No matter how quiet I am, the beavers see me approach. I bring my Black Lab, Nemo, with me, and we find a fallen tree about six feet from the water’s edge to sit on and enjoy the evening.
Before they got used to me, as I approached, the beavers would start swimming around in circles and whacking their tails on the surface of the water. After a few days of that, they seemed to get comfortable with us sitting beside their pond. One of the adults—there are two large beaver lodges on this particular pond—would swim towards me, careful to keep a large tree between us. I couldn't see the animal itself unless I peeked around the tree, although I could see the circles of wake created as the beaver swam through the water. Everything is fine unless Nemo steps into the water. If he does that, bedlam breaks out. Tails start whacking all over the pond and a few seconds later, there are no beavers to be seen.
Beavers have been known to swim as far as half a mile underwater. They can hold their breath for twelve to fifteen minutes, and they have transparent eyelids which protect their eyes when under water. Like birds and reptiles, beavers have a single lower body opening that accommodates their urination, bowels and reproductive organs. This opening also contains castor glands that secrete an oil used for waterproofing their fur, marking territory and attracting mates. Most beavers are monogamous and mate for life.
Beavers make ponds out of streams and dead trees out of living ones, and in the process create havens for everything from Northern Harriers to wood-eating insects to salamanders to river otters. Basically they create highly diverse little ecosystems.
The thing that has always amazed me most about beavers is their practice of setting off across long distances—distances that sometimes include ranges of hills—looking for new ponds when the pond that they grew up on becomes over-populated. That is a feat of courage—to set off at a waddle pace, more or less defenseless, through hostile territory, with no idea if a pond or river is a mile away or fifty miles away. Being the water creatures they are, maybe they do have some way of sensing water. Or maybe they just know if they walk uphill long enough, sooner or later they will start going downhill, and sooner or later when headed downhill, water is found. In that way, they colonized most of forested North America.