Why is there a beaver in my pond?

Have you ever seen something brown and furry swimming around in the water? Maybe you’ve wondered, “what is that?” Or perhaps you just assumed it’s a beaver. In southern Minnesota there are four different animals that meet this description, all of them being brown, semiaquatic mammals.

Before we jump into how to tell them apart, let’s look at what they all have in common. As you might know, mammals are a group of animals that are usually covered in fur or hair and produce milk for their young. Semiaquatic just means something that partially lives in the water and spends the rest of its time on land. Semiaquatic animals will often have certain adaptations such as webbed toes that make swimming easier and the ability to breathe air which make living on land possible.

Let’s meet the contestants!

Minnesota’s four main candidates for that brown furry lump in the water are: the North American river otter, North American beaver, common muskrat and American mink. From there, we’ll split them into two groups. Minks and otters are both what we call mustelids. Mustelids are a group of carnivorous (eats meat) mammal species that usually have more of a long and slender body shape, sort of like a slinky.

Ferrets, weasels and badgers are other examples of mustelids.

Beavers and muskrats on the other hand, are considered rodents.

Rodents are a well-known group of mammals that includes mice, rats, squirrels, gophers and porcupines.

Despite their name, muskrats are more related to hamsters than rats! Although not a rule, many rodents have a rounder or chunkier shape compared to the streamlined bodies of most mustelids which are built to fit in tunnels or narrow spaces for hunting prey. Most rodent diets contain plants, nuts or seeds in some way rather than primarily meat and many like to chew on other things since their teeth never stop growing! Beavers and muskrats are also famous for their construction skills, while otters and minks are more likely to dig a hole to live in or take someone else’s.

The Fearsome Slinkies

For otters and minks, the biggest difference between the two is size. Adult river otters are about the same size or larger than the average house cat, and a mink will be much smaller—only a bit larger than a rabbit. However, besides their size, they do have some other notable differences.

The two species have slightly different colored fur with an otter’s being a sort of tannish brown, and minks are more reddish brown or almost black. When a mink swims, it will stay near the surface with much of its body visible. However, an otter will stay mostly underwater with only part of its head above the surface. An otter also has a large nose, almost resembling a dog’s, which creates a U-shaped wake on the surface when it swims. A mink’s face is much sharper with a smaller nose, making more of a V-shaped wake.

Although both species are semiaquatic, they spend different amounts of time in the water. River otters are very comfortable swimming and catch most of their food in water. Minks, on the other hand, usually spend much more time on land than an otter. While minks enjoy eating fish and other aquatic animals when available, small mammals make up a large portion of its diet. Because of this, it is not necessarily a requirement for minks to live near water to thrive.

When swimming, most of the mink’s body is visible, where, the head is most visible for the otters.

World’s Furriest Architects

Most people would probably say that muskrats and beavers are much harder to tell apart than otters and minks, as they have very similar fur colors, habitats, shapes and diets. Once you know what to look for though, they are much easier to identify. Just like with otters and minks, size is one of the main differences between beavers and muskrats. Muskrats are similar in size to a gray squirrel, and beavers would be about the size of a medium-sized domestic dog.

In the water, it might be more difficult to tell exactly what size something is, so try looking at the relative size of its head compared to the body. Muskrats have bigger heads relative to their size, their body only being about twice as long as their head. Beavers have a much smaller head in comparison with a much longer body. If that doesn’t work, try the tail. Beaver tails are flat like a pancake and used to slap the water as a form of communication between them. Muskrat tails are long and skinny like a rat’s except with flat sides and will move around while in the water to help with swimming. Lastly, if you can’t see the body or the tail, check if it has ears! Beavers have very visible ears, while muskrats are covered by fur.

Although their diets are similar, beavers tend to eat a lot more woody plants especially in colder months when food is scarce. Beavers are herbivores, which means they will only eat plants. While muskrats mostly eat aquatic plants, they will rarely eat small animals and are considered omnivores. They aren’t known for their building capabilities like beavers are, but they will still build huts out of aquatic plants such as cattails. In the winter, muskrats will also build “push-ups.” This is where they chew a hole in the ice and push a pile of plants up through it, so the hole doesn’t refreeze. Muskrats will use this push-up as a place to rest and breathe while looking for food underwater in the winter.

Of course, beavers are famous for building dams which are used to change the flow of water. In addition to the dams, beavers will also build separate lodges which have an underwater entrance. These lodges are used as a safe place to live and store food for the winter. Beaver dams and lodges are both built of mostly woody materials rather than the cattails and reeds muskrats prefer. Since beavers are larger than muskrats, their lodges and dams will also be bigger. So, if you see something carrying a log or large stick in the water, it’s probably a beaver!

Put that knowledge to the test!

Now that you’ve learned about the brown semiaquatic mammals of southern Minnesota, it’s time to test your knowledge! Get outside on the refuge’s 50 miles of trails and see if you can identify any of them. Most of these animals have been seen near Old Cedar Avenue, Bass Ponds, Louisville Swamp or Blue Lake at Wilkie, so those are some good places to start! 

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