Illinois has 39 snake species within its boundaries, but the majority of snakes live in the southern region of the state. The northern region of Illinois has colder temperatures than the south, due to the winds from Lake Michigan. Snakes in North Illinois have a higher tolerance for colder temperatures than southern Illinois snakes. Also, some of North Illinois' snakes are threatened due to the urban growth and water degradation in the Chicago and Rockford areas.
Watersnakes is the term for snakes who spend the majority of their time in or near freshwater lakes, swamps and rivers. This type of snake is often mistaken for a venomous snake with similar scale patterns. Northern watersnakes resemble cottonmouths, while diamondback watersnakes look like diamondback rattlesnakes, but do not have a rattles at the end of their tails. Other watersnakes found in Illinois includes southern watersnake. In North Illinois, watersnakes live in the Mississippi River, Rock River and the lakes region near North Chicago.
According to the Illinois Department of Natural Resources, Illinois is home to four venomous snake species – cottonmouth, copperhead, timber rattlesnake and eastern massasauga. The only venomous rattlesnake in North Illinois is the eastern massasauga, which lives in the region's freshwater rivers and lakes; the eastern massasauga is also endangered in the state of Illinois. All of Illinois' venomous snake are pit vipers; they received this name due to the physical “pit” between the snakes' eyes and nostrils. Venomous snakes kill their prey by biting them with their fangs, which contain venom that paralyzes the prey.
North Illinois is home to several snake species on the state's threatened list. This list features snakes who are not yet endangered, but their population numbers are declining. Threatened snakes in North Illinois include Kirtland's snake, western hognosed snake and lined snake. Kirtland's snake lives in grassy areas and lakes near the greater Chicago area and are threatened due to habitat loss and urban development. Western hognosed snakes dwell in the sandy prairies of Northwest Illinois; this snake is threatened from loss of habitat and pollution.
Certain snake species have a higher degree of cold-tolerance than other snakes. This means they are able to withstand colder temperatures; thus, these snakes do not have to hibernate as long as other snake species. Illinois' two garter snakes – plains gartersnake and common gartersnake – belong to this category. The plains gartersnake wakes up during its hibernation period, while the common gartersnake – one of the most wide-ranging snakes in the United States – wakes up from hibernation in March, over a month earlier than most snake species.