Ms. Zabel's fourth-grade students at Hawthorn Elementary North in Vernon Hills asked: "Why does the nutria have orange teeth?"

Rodents tend to be unpopular animals. Nutria are no exception. They are very large, weighing up to 18 pounds, and have protruding orange teeth. These mammals have a voracious appetite for marsh plants and are wreaking havoc on the wetlands along the Gulf Coast, Atlantic and Pacific Northwest. Other names for these furry creatures are coypu, coypu rats and swamp beavers.

Hunters imported nutria to the United States in the early 1900s for their furry pelts. When the fur business flopped, the nutria populations exploded. Their huge incisors - the front teeth - make them efficient weed-eaters.

"People sold them as weed cutters," said Jay Petersen, curator of mammals at the Brookfield Zoo. But the ever-booming population made nutria a menace because they plow down wetland plants and in the process alter bird nesting, fish and insect habitats.

Research Ecologist Rebecca J. Howard said iron, an essential mineral that helps keep blood healthy, is responsible for the orange-colored teeth.

"The enamel - the hard covering of the tooth - includes an iron-containing pigment that becomes more orange as the animals age. This pigment is found in several species of rodents, but is absent in most other mammals, including humans."

Like all rodents, nutria have front teeth that keep on growing throughout their lives, a process called aradicular. Chewing keeps the teeth honed and sharpened.

"They are constantly worn down by their feeding and gnawing activities," Howard said.

Nutria have an unusual mouth. They can close their lips behind their teeth, Peterson said.

"There's a gap between their molars and incisors. Their lips can seal out water."

The nutria population here in the United States is out of control. Jacoby Carter at the USGS Wetlands Research Center in Lafayette, La., said, "To my knowledge the only way to control nutria population is through predation. That predation can be by people hunting them or it can be by wildlife. Humans can be very effective at reducing nutria populations by hunting. In their homeland of South America nutria have been hunted so intensively that the main concern is to not kill too many of them."

In Louisiana, Carter said, alligators prey on nutria. Large dogs also like to hunt them.

Brookfield's Petersen said invasive species like insects and aquatic life can really create huge environmental problems.

"Big mammals, like nutria, are probably the least harmful of the invasive species," he said.