I saw a muskrat this spring along the north branch of the Chicago River between Irving Park Road and Montrose Avenue. A chain-link fence there separates the steep, wooded riverbank from Homer Park. The muskrat was on the river side of the fence.

Mention animals living in the Chicago River and people usually respond with creature-from-the-black-lagoon jokes, but as far as my superficial examination could tell, this was a perfectly healthy individual. He or she looked sleek and plump and showed no signs of ghastly mutations or horrid tumors.

I’m sure the Metropolitan Water Reclamation District would claim credit for the presence of this rodent in the Chicago River. The Deep Tunnel Project has improved water quality somewhat in the river, so they could be right. But I never looked for muskrats in the river before Deep Tunnel, so I can’t be sure they weren’t there all along.

Muskrats are both adaptable and prolific, so they might have either survived low water quality or quickly colonized any bad water that suddenly got better. Illinois is one of the most muskrat-rich states in the union, ranking tenth in the number of pelts taken each year by trappers. Few of our pristine wetlands remain, but the state is crisscrossed with drainage ditches dug to keep farmland dry and the adaptable muskrats have found the ditches to be quite suitable as homes. And we shouldn’t be surprised to find them in the city; the Field Museum has some specimens that were taken in Jackson Park.

The most obvious signs of the presence of muskrats are the houses they build in marshes, low domes as much as eight feet across that rise two to three feet above the water. They are built of assorted vegetation: rushes, cattail stalks, and other plant parts.

You can tell them from the similar houses built by beavers both by their overall size–beaver lodges tend to be larger–and by the plants used to build them. Beavers use lots more wood: branches, logs, twigs. Muskrats use more herbaceous–that is, nonwoody–material. Both rodents place the entrances to their houses underwater. A slanting tunnel leads upward from the subaqueous doorway into a chamber above water level.

In drainage ditches and channelized rivers like the north branch of the Chicago, there are no shallows or backwaters where muskrats can build, so they adopt a different strategy. They dig into the banks, excavating a tunnel that slants upward from an underwater entrance to a dry chamber above the water level. Beavers do the same thing.

A typical tunnel slants upward at about a 45-degree angle until it reaches about five feet into the bank. There it widens into a chamber measuring about two feet by three feet, often lined with cattails or sweet clover.

Muskrats are fairly large for rodents. An average animal might be two-and-a-half feet long including the tail. They can weigh as much as four pounds. In addition to trapping them for furs, people also eat them. As meat, they are usually referred to as “marsh rabbit,” the idea being nobody would want to eat something called a rat.

Aside from humans, minks and raccoons are their major enemies. Both predators dig into muskrat houses to get at the animals inside–particularly the helpless young. I know we have lots of raccoons along the north branch, but I don’t know about mink. The species is common in northern Illinois, but I don’t know if the Chicago River is clean enough to support them.

Muskrats are mostly vegetarian, although they do sometimes eat clams, crayfish, and other small creatures. Unlike beavers, they stick to the softer parts of plants. They aren’t given to stripping bark from trees. The tuberous roots and tender young shoots of cattails are special favorites.

Feeding platforms are another sign that muskrats are about. The platforms look much like the houses, except they are likely to be littered with the gnawed remains of cattail tubers and the empty shells of crayfish.

Muskrats are beautifully adapted to a life in the water. Their sleek, waterproof fur keeps them warm and dry. Their naked tails are laterally compressed and serve them as rudders. Their hind feet are partially webbed, and they can swim forward or backward or hang motionless in the water with only their eyes and nostrils above the surface. They can stay underwater for long stretches. One investigator filmed an individual that stayed down 17 minutes, came up for three seconds to take a breath, and then went back down for another 10 minutes. Their incisors are exposed even when their mouths are closed, so they can chew underwater.

They tend to be solitary. Those houses and burrows probably hold either one lone adult or a mother and her young. The young are weaned after about a month and driven away shortly afterward. A study of Illinois females concluded that two litters a year was the state average, with each litter averaging three to four young. Other studies have found as many as 11 young in a litter and as many as five litters in a year. Fights are common between adults defining their territories, especially during the breeding season. Beavers, by contrast, are quite social. A whole family shares a beaver lodge, and young animals may stay with their parents for as long as two years.

In prime habitat, muskrat populations can build to amazing levels. A study at Rice Lake–a backwater lake on the Illinois River near Peoria–found 75 houses in an 11-acre patch of reeds and cattails. Trappers took more than 2,000 animals from the 1,000-acre lake in some years. Another study of Illinois drainage ditches found as many as 60 animals per mile of ditch.

Animals present in those kinds of numbers can have a major effect on their environment. Muskrats play a big role in maintaining open water areas in marshes simply by eating cattails. Their houses also provide nesting sites for black terns, Canada geese, common moorhens, and other birds.

Prolific animals with dense, durable, waterproof fur are very valuable, and as many as ten million are trapped in the U.S. every year for the fur trade. Thirty years ago a mammalogist calculated the total market value of muskrat-fur coats and other items to be $100 million annually, though that number includes the costs of processing, tailoring, and selling.

The money to be made on muskrat fur attracted European attention early this century. In 1905, a few animals were released in Czechoslovakia in hopes that they would establish themselves and create the means for a profitable business. The rats quickly spread north and east into Poland, Russia, and Finland and west into Germany. Meanwhile others were imported into France and England. The French animals were kept as captives on fur farms, but the animals’ solitary habits created problems. Crowded into pens on the farms, the muskrats started fighting. Since the rats couldn’t get away from each other, the struggles got quite protracted. All the battles were damaging the pelts, so the French tried larger pens maintained in semiwild conditions. The muskrats quickly escaped from these and established a truly wild population.

Muskrats adapted well to marshes in Finland and Russia, but in the more densely populated regions of Europe they became serious pests. Their burrows undermined bridges, roadways, and dikes. And whenever the supply of aquatic plants grew low, the muskrats started to eat crops.

By 1932, Poland had enacted a law forbidding the transport of muskrats on pain of six months in the slammer. As early as 1917, the Germans tried to halt the advance of the muskrats by hiring trappers. Trappers in Britain managed to extirpate the animals after a five-year campaign that cost almost thirty dollars–in 1937 currency–per rat. But the Germans eventually gave up their efforts after they discovered that many trappers were carefully leaving a few muskrats alive in their territories to keep the bounties coming. The trappers seemed to think it’s OK to kill muskrats, but don’t kill the job.

As a lesson in the unpredictable outcome of animal introductions, the muskrat is a beauty. A successful addition to the local fauna in Russia and Finland, it became a pest in Germany, France, Holland, and England. And in Japan, where muskrats were also introduced, they occupied a small area around Tokyo and never expanded their range into the rest of the country.