True vipers eat small animals, including lizards, mice, insects, voles and birds. Pit vipers, some of whom live near water, also eat these animals, as well as frogs and fish. Very large vipers have been known to eat animals as large as porcupines or antelopes.
Most vipers are well camouflaged and can find their prey by lying in wait on the ground or, uncommonly, in a tree. When the prey comes near, they strike and envenomate it. Generally, they do not chase their prey but wait for the venom to overcome it. Then the viper eats the prey.
The fangs of true vipers are so long that they have to fold them away against the roof of the mouth when they're not in use. The fangs also have channels down the centers to deliver the venom. There is evidence that the snake knows just how much venom to deliver depending on the size and species of the prey.
Pit vipers have heat-sensing pits in their heads to help find their prey. Many pit vipers, such as the cottonmouth and the copperhead, are found in the New World, though others such as the temple viper dwell in Asia. Vipers are found in every continent except Australia and Antarctica.