University of Vermont zoologist Bill Kilpatrick begged his students for years to bring him dead opossums.

It's not that the genial former Texan has any particular love of opossums. It's that the cat-sized animal has been slowly making its way north through the Champlain Valley, and Kilpatrick kept tabs on its migration, in part, by tracking road-killed animals. Kilpatrick suspects two factors for the migration: global warming and increased availability of food.

They are very opportunistic, and my guess is that global warming has something to do with pushing them farther north,'' Kilpatrick said.

Animals migrate and expand their range for many reasons. The cormorant population on Lake Champlain has exploded in the past decade, for example, because more birds are thriving in the southern United States, where they spend the winter. Global warming might become another reason why animals move.

Scientists believe human activities are causing the Earth's climate to warm. Along with rising sea levels and shifts in ecosystems, scientists predict that animals, too, will respond to the change by moving north.

While there's no way to prove that is what's happening with opossums, it's clear they've moved up the Champlain Valley, Kilpatrick said. By 1988, the opossum arrived in Kilpatrick's back yard. That's when the first one was found in Burlington. By 1995, opossums had migrated all the way to Montreal.

Opossums are not animals equipped for the cold of Vermont winters: Their coats are thin; their tails are naked; and they've got big, bald ears and long snouts. Yet somehow, they've managed in the past two decades to survive and expand northward. Many of the animals show signs of frostbite, but they survive nonetheless.

Giant mutant rats

Dave Richmond in Essex calls himself ''Mr. Fix-it.'' Among the odd jobs he does is trapping and relocating animals. He remembers when opossums began showing up in the Champlain Valley; people routinely called him about the strange animals with long tails prowling in their back yards.

When I first saw them, I thought for certain someone had a

pet and let it go,'' Richmond said. ''Now they're getting pretty common.''

Richmond estimates he receives only two or three calls a year from terrified residents. People are beginning to recognize them,'' he said.

Scott Darling, chief wildlife biologist with the Vermont Fish and Wildlife Department, recalls when a woman from Rutland called about giant mutant rats on her front porch.

She was in a real panic,'' he said.

Darling agrees with Kilpatrick that warmer winter temperatures are enabling the opossum to push north. And, he says, the suburbanization of Vermont, in which forests and farm fields are converted to people's back yards, creates a perfect habitat for opossums. They'll eat dog food, garbage, bugs, anything they can find, Darling said.

Darling isn't worried that the opossum will turn into a nuisance species, such as the cormorant. He recognizes, however, that if numbers increase, they might begin to have an effect on songbirds that nest in places where opossums can steal the eggs to eat.

It's an intriguing animal,'' he said.

Polly the Possum

Carol Winfield is a certified wildlife rehabilitator in Westford. Her state and federal licenses permit her to care for injured or orphan animals. Part of her self-appointed task is to educate Vermonters about the kinds of animals around them.

That's where Polly the Possum comes in.

A Virginia native, Polly was part of an orphaned litter that wasn't going to survive in the wild. Wildlife rehabilitators had to feed Polly with a bottle long after her littermates could forage for themselves; they gave Polly to Winfield because they didn't think she would survive in the wild.

Now the 2-year-old female opossum travels around with Winfield to schools where Winfield talks about wild animals. She thinks the fact that opossums are moving north is fascinating but worrisome, too.

These critters never survived this far north before,'' she said. ''I think that's pretty indicative of climate change.''