The iconic rattlesnake, endemic to the new world, so impressed early colonists that its likeness adorned books, flags and crests that were sent back home. According to a study by Mathew P. Rowe, et al., published in "Biology of the Vipers," the rattle may be the most extensively studied anatomical feature among snakes. It's not only rattlesnakes that shake their tail, though; many pit vipers and a number of harmless colubrid snakes engage in the behavior as well.
Purpose of Tail-Shaking Behavior
As tail-shaking behavior is so widespread, it may serve slightly different functions for different species. Nevertheless, Harry Greene asserted three possible goals of tail shaking behavior in a 1973 study published in the “Journal of Herpetology.” The goals listed were all defensive: diverting the attack to the tail, confusing the predator and inhibiting an attack by advertising their venomous bite. Tail-shaking behavior and morphology is most developed in rattlesnakes, but a rattle is not necessary to produce a buzzing sound, as many snakes simply vibrate their tail against the leaf litter to produce a loud buzz.
Rattlesnakes represent the pinnacle of tail-shaking evolution. The snakes’ namesake appendage is formed from a collection of horny, interlocking segments that are loosely attached to one another. Born with a single, rounded rattle segment called a button, rattlesnakes add a new segment each time they shed their skin. Segments of the rattle break periodically, and longer rattles do not necessarily produce louder sounds. When the apparatus is shaken at high speed, a buzz or rattling sound is created; rattlesnakes have three sets of dedicated, tail-shaking muscles in their tail. Two of the muscle groups pull the tail to each side, and a third set of muscles allows the ventral surface to be aimed in either direction.
Other Pit Vipers
Rattlesnakes aren’t the only dangerous snakes to shake their tail; several other, closely related pit vipers exhibit the behavior. Cottonmouths (Agkistrodon piscivorous), copperheads (Agkistrodon contortrix), cantils (Agkistrodon bilineatus) and terciopelos (Bothrops asper) all shake their tails when frightened. Bushmasters -- the world’s largest pit vipers -- take tail shaking further and have evolved spine-like scales at the tip of their tails. This facilitates the tail shaking, and creates a louder sound than if no spines were present.
Rat, King and Gopher Snakes
Rat (Pantherophis sp.), king (Lampropeltis sp.) and gopher snakes (Pituophis sp.) and other closely related species are well-known for shaking their tail when confronted with a predator. It's not clear whether tail-shaking behavior in itself dissuades predators, or if it serves as a form of Batesian mimicry. In Batesian mimicry, a harmless or palatable species mimics a dangerous or distasteful species through behavior or morphology to protect itself. None of these species have well-developed tail-shaking muscles and cannot keep up the display very long.
Loss of the Rattle
Santa Catalina Island rattlesnakes (Crotalus catalinensis) have lost their rattles over time. It is thought that because these snakes primarily stalk birds at night in the trees, a functional rattle may unintentionally make noise and scare off the birds. These snakes have no large predators on the island, so the rattle is not necessary, though they will still vibrate their tails when threatened. Additionally, the tiny pygmy rattlesnakes (Sistrurus miliarius) are in the process of losing their rattles. These small snakes, which are somewhat reluctant to rattle at predators, are only able to conjure a high-pitched, barely audible buzz -- hardly a deterrent to large predators.
- Copeia: Effects of Some Ophidian Tail Displays on the Predatory Behavior of Grison (Galictis sp.)
- Biology of the Vipers: Rattle Loss in Pygmy Rattlesnakes (Sistrurus Miliarius): Causes, Consequences and Implications for Rattle Function and Evolution
- Toledo Zoo: Bushmaster
- Rigorvitae: A Serpent's Tail
- CaliforniaHerps.com: Snake Behavior
- United States Army Public Health Command: Venomous Reptiles of Central America
- San Diego Natural History Museum: Crotalus Catalinensis