The Crotalinae, or crotalines, are a subfamily of venomous vipers found in Asia and the Americas. The groups of snakes represented here include rattlesnakes, lanceheads and Asian pitvipers. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head. Currently, 18 genera and 151 species are recognized. These are the only viperids found in the Americas.
Although a few species are highly active by day, most are nocturnal, preferring to hunt when their favored prey are also active. Many temperate species will congregate in sheltered areas or dens to overwinter, though most species do not mass together in this way.