If you are to encounter a black snake that quickly slithers away from you, or becomes aggressive, the chances are that the snake is actually a black racer. If you then encounter a black snake that freezes in its place, or slithers away from you slowly, then that is probably a black rat snake.
Rat snakes are medium-to-large constricter snakes that are found throughout most of the northern hemisphere. They will feed primarily on rodents and birds, and in some cases can occupy the top levels of the food chains. This is due to the fact that some of these snakes can grow to 10 ft long. Many species make attractive and docile pets, though naturally they can’t just be taken from the wild. Other snakes can be skittish and aggressive, but their bites are seldom serious.
As with most of the colubrids, rat snakes do not pose any threat to humans. Rat snakes were long thought to be completely nonvenemous and safe, but it has been seen recently that the snakes (the Old World species) can actually possess a small amount of venom. This small amount is so insignificant that it does not pose any threat to humans at all.
Previously, most rat snakes had been assigned to the genus Elaphe, but since, many of them have been renamed after the mitochondrial DNA analysis that was performed in 2002. So remember, Rat snakes are non-venomous, but should not be mistaken for other more dangerous snakes that can be found in the wild.