Rat snakes are medium to large snakes that can be found mostly in the northern hemisphere. They feed primarily on rodents and birds and, with some species being over 3 m (10 feet), they are on the top levels of some food chains. Many species make attractive pets and one, the corn snake, is one of the most popular reptile pets in the world. Other species can be very sensitive and sometimes aggressive, but the bites are not dangerous. Rat snakes were long thought to be completely nonvenomous, but recent studies have shown that some do have harmlessly small amounts of venom in them. When frightened, rat snakes may freeze, which is why a lot has been killed in traffic.