Some biologists believe that beaver-like animals the size of black bears once roamed North America. A variety of evolutionary forces may have shrunk them to the 40– to 100– pound range, but the continent's largest rodents have still earned a jumbo reputation for destruction. And that often makes them unwelcome critters on the block.

In the 19th and early 20th centuries, zealous trappers nearly wiped out Castor canadensis, which was valued both for its lustrous pelt and its castoreum, the substance found in its anal sac and used to make aspirin and in perfume. But efforts to reintroduce and protect beavers were successful and by the 1940s, they were once again commonplace in every state except Florida.

Beavers are a "keystone" species, which means they modify their environment for the benefit of the surrounding ecosystem. These modifications, in the forms of dams, are unfortunately also what wreaks havoc with roadways, cellars, and culverts.

In attempts to control beaver damage in the Northeast, citizens and state conservation specialists fed up with dam-related flooding have been working overtime on solutions. There was the controversial late-night, state-park-sanctioned shooting of two misbehaving beavers in New Jersey last year. There's Knowlton Township, a New Jersey town with an "if you can't beat 'em, join 'em" attitude, which now puts on an annual celebration of its beaver residents. Many counties have chosen to extend beaver-trapping season regularly or install beaver-proof pipes to help drain flooded areas.

While the human-beaver relationship is complicated, the animal itself is a marvel. Back in 1913, naturalist Enos Mills wrote: "The beaver is a persistent practicer of conservation… useful, skillful, practical, and picturesque."

The big, busy rodent is a nocturnal herbivore, consuming a bounty of greens on spring and summer nights, with a preference for water lilies. Come autumn, its attention turns to trees, which the beaver uses for food and shelter. A connoisseur of the soft underbark of willows, birch, and poplar, it will use its famous incisors to gnaw down these smaller trees and store them for wintertime meals. Bigger trees form the dams that create ponds and wetlands. Deeper water allows the awkward-on-land rodent to travel more easily and gracefully. And the benefits of dams are far-reaching: They create greater biodiversity, provide water for area wildlife, and even mitigate floods, because the capacity of the wetland to hold groundwater is increased.

The dams also isolate the beavers' lodges, structures crafted from mud and sticks with a clever system of underwater entries, which require skillful navigation and thwart approaching predators. And since beavers don't hibernate, when snow piles on the lodge, it provides a toasty layer of insulation for both them and their food supply.

While highway departments may curse the creature, the beaver remains the state animal of New York and the mascot of MIT and Babson College. A now-famous one, "José," made his way to New York City via the Hudson River in 2007 and was given a hero's welcome. No beavers had lived in the Big Apple (whose official seal bears the image of, you guessed it, a beaver) since the early 19th century.