The vipers are venomous snakes found worldwide except in Australia, New Zealand, Ireland, Antarctica, some islands, and from the tropics to above the Arctic Circle. They are carnivorous, using their venom to paralyze and kill prey. They also use it defensively but may conserve venom by delivering a dry bite. All have a pair of hollow or grooved fangs that fold back into the roof of the mouth. When striking, vipers can open their mouths nearly 180 degrees, erecting the hinged fangs. Powerful muscles inject venom into the target.
These sturdy-bodied snakes have short tails and triangular heads that are distinct from the neck. They have keeled scales, slit-shaped pupils, are usually nocturnal, and most give birth to live young. In fact, the term viper comes from Latin vivo (living) and pario (give birth).