Their tails are modified into rattles–a congregation of segments of old shed skin. When a rattlesnake vibrates its tail, a loud buzzing noise is produced that is actually a warning signal for enemies. They have very strong neurotoxic venom and a bite should not be underestimated even from a small snake such as the sidewinder–it is always a medical emergency.

Rattlesnakes are similar in the respect that they all bear live young. Rattlesnakes are usually found in arid regions from Canada to South America. Some of the best known species in the United States are the timber rattlesnake, Crotalushorridus; the eastern and the western diamondbacks, Crotalusadamanteus and Crotalus atrox; the last two being the first and the second largest snakes in the U.S.A. Crotalus adamanteus is also the world’s heaviest snake- reaching over 34 pounds, and it has been recorded to reach 8 ft in length. Its venom is not particularly toxic. It has been estimated to be 10 times less toxic than the venom of the Indian cobra. However the aggressive disposition and the large venom output make this snake highly dangerous. Annually, in the U.S.A. most snakebites are from the western diamondback, Crotalus atrox.

Some species like the tropical or South American rattlesnake, Crotalus durissus have venom with a particularly strong neurotoxic effect–stronger than that of the other species.