From the Contact Station October 2019

Beaver, Nutria or Muskrat?

Among the many mammals we have on the refuge, most often seen are the nutria who can be confused with our beaver and the common muskrat.  All three of these mammals are part of the order Rodentia, which comprises about 40% of all mammals around the world.  Rodentia are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws.  That is one of the features you can clearly see as you observe these animals in the refuge.

Beavers are in their own family, Castoridae, while Nutria and Muskrat are in Myocastoridae and Muridae, resp.

All three species are dark brown in color and semi-aquatic with their large front teeth that are yellow to orange in color. But how do you know which one you are looking at?  The first clue is that nutria are everywhere, but that is not being very scientific about this.  However; size, tails and whiskers can help you tell them apart.

The beaver, Castor canadensis, as the largest of the three, can weigh 45 pounds or more as an adult. Their tails are large, broad and flat, which they smack loudly on the water to signal alarm.  Beavers measure about 50 inches long, including their 12-inch tail. Their subtle black whiskers can only be viewed from very close.

Beavers create lodges from mud and sticks. They are active at night so less likely to be seen during the day on the refuge.  Our refuge biologist estimates there are probably 12 families around our 5000 acres.  You can see a lodge out in the water past signpost 9 and on the lake behind the Plankhouse at the Carty unit.  We see evidence of their nightly activity from the gnawing of trees. Our new plantings are caged in some areas to give them a chance to grow without beaver interference.  Beavers like to crease quiet water so they will push saplings into the pump areas creating job security for the staff that has to come remove the beaver supplied barriers every day.

The nutria, Myocastor coypus, is a native of South America where it is called coypu.  They were introduced into Oregon in the 1930s to be farmed for fur.  That didn’t work out so well and many were released in the late 1940s.  Being very prolific and mobile, they have worked their way up through western Oregon and into our area.

A large adult can nutria can weight up to 15 or 20 pounds.  Their tails are heavy and rat-like, being round and thinly covered in bristly hairs, that trail straight behind when swimming. Sometimes the body end of the tail humps out of the water as they feed.  The body is about 30 inches long, including a 10-inch tail.  Their white whiskers, at 3 to 5 inches, protrude from either side of their nose and can be easily seen from a distance as a distinguishing characteristic from the beaver and the muskrat. Sometimes the fur can be light tan to white. You may have seen our white nutria; she is quite popular with the kids.

Nutria often live in burrows in the dikes unfortunately and make filling adjacent ponds challenging when the burrows penetrate through.  In marshes they create flat, nest-like platforms of dead vegetation for feeding, loafing, grooming, and birthing.

Our common muskrat, Ondatra zibethicus, is the little guy here.  They generally weigh 2–5 pounds and can be confused with the large number of baby nutria we see.  They are crepuscular, being most active at dawn, dusk and at night.  Their long, flattened, ribbon-like tail can be seen whipping snake-like behind them when swimming in the water.  A very different movement from the nutria.  They can stay underwater for up to 15 minutes as they search for food.  They are 20 inches long, including a 9-inch tail. They have subtle black whiskers which can only be viewed from very close.  Muskrats create feeding stations from mud and vegetation where they can eat their aquatic vegetation without worrying about predators.

But there are problems with our Rodentia friends. Nutria are invasive. They easily out-compete the native muskrat which has similar food habits.  And, because they are in big numbers and eat a lot of the aquatic and semiaquatic vegetation, they can have a negative impact on our wintering waterfowl.  They put holes in the dikes for their burrows.  And, they are very prolific.  From time to time, the population is suppressed by shooting, which was done this summer.  Unfortunately, sometimes they dive in the water when shot and then cannot immediately be found to be collected.  That happened this summer.  When it got hot, the dead animals filled with gases and floated to the surface several days later.  In that amazing circle of life, our predators and scavengers have had a good summer for all of that.

So, to get back to the initial question.   Does the animal have big white whiskers and a rat tail that follows straight behind when swimming?  It’s a nutria.  Is it a small animal with a tail snaking behind it in the water and no discernable whiskers?  It’s a muskrat.   Is it dark and I shouldn’t be on the refuge at this time and that is a big rat with, wait, a flat tail?  The Ridgefield police won’t be happy with you when you call them to let you out of the gate no matter your story about the beaver you just saw.

Cranes, Swans and other waterfowl are coming in.  Enjoy fall and the coming of winter visitors.

-Susan Setterberg, Contact Station Volunteer & Board Past President