Ontario rarely had sightings of opossums in the 1960s, but now have them regularly, some speculate that this is likely due to global warming causing winters to be warmer. Some people speculate the expansion into Ontario mostly occurred by opossums accidentally being transferred across the St. Lawrence, Niagara, Detroit and St. Clair rivers by motor vehicles or trains they may have climbed upon. As the opossum is not adapted to colder winters or heavy snow, its population may be significantly reduced if a colder winter with heavier snow occurs in a particular northern region. The Virginia opossum was not originally native to the West Coast of the United States. It was intentionally introduced into the West during the Great Depression, probably as a source of food, and now occupies much of the Pacific coast. Its range has been expanding steadily northward into British Columbia, Canada. Description Virginia opossums can vary considerably in size, with larger specimens found to the north of the opossum's range and smaller specimens in the tropics. They measure long from their snout to the base of the tail, with the tail adding another . Males are slightly larger, with an average body length of with an average tail length of , while females are long with a tail. Weight for males ranges from and for females from . Their coats are a dull grayish brown, other than on their faces, which are white. Opossums have long, hairless, prehensile tails, which can be used to grab branches and carry small objects. They also have hairless ears and a long, flat nose. Opossums have 50 teeth, more than any other North American land mammal, and opposable, clawless thumbs on their rear limbs. Opossums have 13 nipples, arranged in a circle of 12 with one in the middle. The dental formula of an opossum is . No other mammal in North America has more than 6 upper incisors, but the Virginia opossum has 10. Perhaps surprisingly for such a widespread and successful species, the Virginia opossum has one of the lowest encephalization quotients of any marsupial. Its brain is one-fifth the size of a raccoon's. Tracks Virginia opossum tracks generally show five finger-like toes in both the fore and hind prints. The hind tracks are unusual and distinctive due to the opossum's opposable thumb, which generally prints at an angle of 90° or greater to the other fingers (sometimes