They're fierce. They're furry. They're cousins to the weasel. And they could be coming to a suburb near you.

According to a new study, fishers are starting to expand their range in places in the northeastern United States where they haven't lived for some 200 years, including places near people. And their range isn't the only thing that's growing: Recent examinations of eastern fisher skulls show that the predators are becoming bigger than their counterparts out West.

What's a fisher? Well, the first thing to know is that it doesn't eat fish. The name "fisher" is thought to have come from early American immigrants who noted the animal's resemblance to the European polecat, which was also called a "fitchet," "fitch," or "fitchew."

Fishers are long-tailed carnivores in the family Mustelidae. Sometimes called the weasel family, it also includes badgers, otters, and minks. The fisher is found only in Canada and the northern U.S.

In size and attitude, it's somewhere between a domesticated ferret and a wolverine.

"Maybe a little closer to a wolverine," says Scott LaPoint.

A postdoctoral researcher at the Max Planck Institute for Ornithology, LaPoint is the lead author of a new paper published in Animal Conservation that details how these medium-size mustelids have begun to retake their old territory after centuries of persecution. (The research was funded in part by the National Geographic Society's Waitt Grants program.)

"[Fur] trapping and forest clearing practically wiped fishers out of the U.S.," says LaPoint, "but [they] are now reclaiming much of their historic range" across the northern U.S. and down the Appalachian Mountains.

In the eastern U.S., LaPoint found, fishers have now expanded their geographic range by 119 percent since about 1900. One male was even spotted prowling the streets of the Bronx earlier this month—which means the city's rats, squirrels, and pigeons should be on high alert.

Mustelids in the Middle

Fishers are what biologists call mesopredators, or middle predators. That means they're a link below apex, or top, predators in the food chain. Adult females weigh about 4 pounds (1.8 kilograms); adult males tip the scales at about 12 (5.4 kilograms). Full-grown males can be nearly four feet (1.2 meters) long, though a lot of that is tail.

Fierce and lithe, fishers can pursue prey into tight spaces like underground tunnels or the passages some creatures create beneath winter snow. After they chase down hares and squirrels, they dispatch them with a quick bite to the back of the neck.

Sharp, partially retractable claws let fishers scale trees like a cat. And their hind paws can rotate nearly 180 degrees, allowing them to come back down a trunk headfirst. This last trick is what makes fishers one of the few predators that can take down a porcupine.

Dance of Death

With a nearly impenetrable coat of 30,000 quills, each tipped with a microscopic barb that sticks in flesh like a whaling harpoon, porcupines don't have much to fear from predators large or small. But every defense has its weakness.

"The fisher attacks the head, because the head has no quills," says Uldis Roze, a professor emeritus of biology at Queens College at the City University of New York and a porcupine researcher for more than 35 years. "Of course, the head is crucial to the porcupine. No head, no porcupine."

To avoid the most dangerous parts of a porcupine—the neck, back, and tail—a fisher dances circles around its prey. Each time the low-to-the-ground fisher gets a look at the porcupine's face, it lunges forward and strikes. Repeated bites to the face disorient the victim, cause bleeding, and eventually send it into shock. The fisher then rolls the porcupine over and starts eating through its stomach, neatly skinning the kill to avoid the still dangerous quills.

The process is neither quick—Roze says the lethal dance alone takes 30 minutes to an hour—nor pretty. Often the fisher scalps the porcupine in the process. Roze and a colleague once happened upon a fisher moments after a kill and found a porcupine carcass completely beheaded.

When a porcupine is cornered, it will sometimes jam its vulnerable head against the base of a tree. Any other predator would be at an impasse, facing a wall of quills on every side. But the fisher's special ankles mean it can hop on the trunk and start attacking from above.

The Ecology of Fear

As fishers have been pushing back East, they've also been getting bigger. LaPoint's research evaluated specimens from across the U.S. and found that eastern fishers now have larger skulls than those examined in the American West and Midwest.

This is what biologists call directional selection. In this case, a random mutation of genes produces a few fishers with slightly larger skulls. That may allow them to take down their prey more efficiently or target larger prey. Over time, as the bigger-skulled fishers keep passing on their genes, the new genes become the norm.

According to one of LaPoint's co-authors, Roland Kays, the lab director at the North Carolina Museum of Natural Sciences and a professor at North Carolina State University, "Finding that fishers are evolving a larger body size and that they're eating larger prey really shows that they're starting to move into the larger predator space."

Around the same time that fishers were disappearing, aggressive antipredator campaigns sought to kill off wolves and mountain lions. As this study and others illustrate, we're still trying to understand the effects.

Kays says the absence of apex predators—and the effect it's had on fisher size and range—constitutes an interesting ecological phenomenon.

"We call it the ecology of fear," he says. "If you have a predator that you're afraid of, you use the landscape in different ways."

For a fisher, that would mean avoiding risky places like clearings or foraging only when it knows that apex predators won't be around. But no predators means no pressure—or competition.

Fisher Kings

Whether they're roaming around the Bronx or figuring out ways to cross highways, fishers are thriving in part because they're highly adaptable. Human development presents certain opportunities, such as backyard bird feeders, farmers' fields, highway roadkill, and neatly groomed parks. "Think of all those naive squirrels!" says LaPoint.

"Fishers have been able to recover right under our noses," says Kays, who has fitted more than a dozen with high-resolution GPS collars and followed them all over suburbia. "A lot of people don't realize how common they are, or that they're even in the forests around their houses, because they keep this low profile."

Their comeback, he adds, "shows the resiliency of nature, and the ability of animals to adapt to modern conditions and make the best of it. [Fisher's are] a good conservation story."