Pit vipers that warn with a castanet of keratin
Rattlesnakes of genera Crotalus and Sistrurus are New World pit vipers that shake interlocking tail segments as a predator deterrent. Some thirty-six species and sixty-five to seventy subspecies range from Argentina to Canada; Crotalus recalls Greek castanets, Sistrurus a rattling tail.
All 36 species live in the Americas, from southern Canada to central Argentina, and most favour dry country, concentrated in the American Southwest and Mexico; only four occur east of the Mississippi and two in South America. Their likely ancestral home was the pine-oak forest of Mexico's Sierra Madre Occidental. The giant of the group, the eastern diamondback, reaches about 2.4 metres. Many species cling to open, rocky ground, where crevices provide shelter and abundant rodents and lizards.
They ambush or burrow-hunt mice, rats, rabbits, ground squirrels, prairie dogs and ground-nesting birds. Facial pits sense body heat out to roughly 30 centimetres, working like a pinhole camera as infrared warms a membrane at the back of each pit. Rod-rich eyes suit darkness, though vision is sharper by day, and cone cells allow some colour perception, but without a fovea the images are never crisp. Tongue-flicking delivers scent to the Jacobson's organ, while the lack of outer ears leaves them relying on ground vibrations. A rattlesnake needs at least its own weight in water each year, and desert snakes flatten their coils to catch rain, sometimes clustering into a living mat.
Venom glands at the back of the upper jaw feed hinged fangs that fold away when idle. Newborns arrive with working venom, and adults replace fangs every six to ten weeks, with at least three spare pairs waiting. The venom, a cocktail of five to fifteen enzymes plus metal ions, lipids and peptides, mainly destroys tissue and disrupts clotting, although the tiger rattlesnake and some Mojave rattlesnakes add a paralysing neurotoxin.
Young snakes are eaten by ravens, roadrunners, coyotes, raccoons and even Formica ants. The common kingsnake shrugs off the venom and hunts adults, which smell it coming and arch their bodies defensively instead of coiling to strike. Rattlesnakes cause most snakebites in North America yet seldom kill if bites are treated quickly, while habitat loss, poaching and extermination drives threaten many populations.
Source: Rattlesnake
