There are two genera of pit vipers (Crotalinae) with rattles – genus Crotalus, with 51 species, and Sistrurus, with only three species. And they can all be found in the New World (North and South America).
Most of the Rattlesnakes that you’d think of come from the genus Crotalus. It includes Western and Eastern Diamond-Backs (Crotalus atrox & adamanteus), Mojave Rattlesnakes (Crotalus scutulatus), Sidewinders (Crotalus cerastes), and Timber Rattlesnakes (Crotalus horridus). While the smaller Pigmy (Sistrurus miliarius) and Massasauga Rattlesnakes (Sistrurus catenatus (Eastern) & targeminus (Western)), belong to the genus Sistrurus.
There are a lot of misconceptions about Rattlesnakes. One being that you can tell the snake’s age by how many rattle segments they have.
Actually, Rattlesnakes are born with a first rattle segment, called a button (as seen in the image to the right). At first the button cannot make any sound and some biologists think it may be used as a lure.
Every time the snake sheds it gains another segment. It may take multiple sheds to get a fully developed segment, though. Snakes shed more often when they are young and slowly decrease the frequency of shedding as they age. Also, as the snake moves around, older parts of the rattle will likely fall off. In my illustration you can see how the segments interlock loosely enough to make noise.
Rattlesnakes aren’t the only snakes that rattle, though. Just about any kind of snake you’d encounter will shake its tail when it’s nervous. It is likely that the evolution of tail shaking may have effected many species of snakes around the same time, and was only specialized in rattlesnakes. In other words, when a non-venomous or venomous snake without a rattle shakes its tail, it is not trying to imitate a rattlesnake, because that habit evolved before the rattle did.
This beautiful Mojave Rattlesnake (Crotalus scutulatus) I found in Inyokern California. It is doing a great job of demonstrating a defensive posture and rattle.
I hope this helps you appreciate these fascinating animals. If you’d like to learn more about some strange defense strategies in snakes, you can read about the genus Heterodon here.
Sources: ResearchGate, Wikipedia, Britannica