The Rattlesnakes, an Overview
Classification, Range, Reproduction, Venom, Sensory Pits and Rattles
Nov 11, 2008
Close rattlesnake relatives include copperheads, water moccasins, palm vipers, eyelash vipers and a number of other venomous snakes native to North and South America. The family to which the rattlesnakes belong, the Viperidae, contains 272 species, including the well-known Gaboon viper, Bitis gabonica and the saw-scaled viper, Echis carinatus.
Range
Rattlesnakes are confined to the Western Hemisphere, and reach their greatest diversity in the American Southwest and Mexico, with 13 of the 33 known species being found in Arizona alone. Only 4 species – the Eastern diamondback, Crotalus adamanteus, the timber rattlesnake, C. horridus, the Eastern massasauga, Sistrurus catenatus catenatus and the pigmy rattlesnake, S. miliarius – range east of the Mississippi River, while South America is home to but 2 species, the neo-tropical rattlesnake, Crotalus durissus and the Uracoan rattlesnake, C. vegrandis.
Heat-Sensing Pits
All rattlesnakes possess a highly sophisticated organ (“pit”) that detects the infra-red rays, or heat, produced by birds and mammals. Located between the eye and nostril, this organ is far more sensitive than the heat receptors that have evolved among another group of snakes, the boas and pythons.
The arrangement of the heat receptors within the sensory pits are recreated in the brain and integrated with visual information received there. The pits may thus be considered more “imaging devices” than mere heat receptors, and likely provide detailed information concerning the size and shape, as well as location, of endothermic animals. Aided by these unique organs, rattlesnakes and other pit vipers are able to hunt and escape predators in complete darkness.
The Rattle
The rattle, unique among snakes, is composed of a series of loosely-connected segments. Specialized muscles in the tail vibrate the rattle to produce the characteristic warning sound. A new segment forms and is added at each shed. Old segments often break off and, as a snake may shed from 1 to 4 or more times yearly, or not at all, the number of rattle segments is not an indication of age.
The Santa Catalina rattlesnake, Crotalus catalinensis, confined to the island of the same name (off Baja California, Mexico) is the only species of rattlesnake that does not have a rattle. A number of non-venomous snakes vibrate their tails among dead leaves when disturbed, perhaps in imitation of rattlesnakes. Contrary to popular belief, rattlesnakes do not always sound a warning before striking.
Venom and Hunting Strategies
Vipers and pit vipers have evolved long, hinged fangs that fold back against the roof of the mouth when not in use. Venom is injected with a single bite, in the manner of a hypodermic needle.
Most species are “ambush predators” that lie in wait for a meal near rodent runways, den entrances and other promising hunting sites. Rattlesnakes retire after biting a potential meal, following the scent trail once the stricken animal has expired. This strategy spares them the injuries that can be inflicted by upon snakes such as cobras, which must hold on while injecting venom.
Venom toxicity and composition varies among subspecies and even between populations of the same rattlesnake subspecies. Rattlesnake venom is often particularly effective on the most frequently-taken prey species, and evolves over time in reaction to immunities acquired by such animals.
All rattlesnakes bear live young. Females of some species remain with their offspring until the first shed (9-14 days after birth). The black-tailed rattlesnake, Crotalus molossus, has been shown able to recognize siblings even after a 2-year separation.
Sources:
Campbell, J.A. and ZW. Lamar. Venomous Reptiles of Latin America. Sacramento: Comstock Publishing, 1989.
Mehrtens, J.M. Living Snakes of the World. New York: Sterling Publishing Co., Inc., 1987
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