Why Do Rattlesnakes Rattle? How the Rattle Works and Warns

A warning sign for big animals

A rattlesnake’s main enemies are not only predators such as hawks and coyotes. Large grazing animals, from bison and deer to horses and cattle, can crush a snake simply by stepping on it. For a snake that weighs a couple of kilograms, being trampled is as deadly as being eaten.

The rattle solves this problem. Its sound is a loud, unmistakable “keep away” signal that many mammals, including people, react to instinctively. It is an example of what biologists call aposematism: a warning display that advertises danger so that both the snake and the other animal avoid a costly fight. Striking and injecting venom is risky for the snake too, because venom takes energy to make and a fight could injure it.

Rattling is only one step in a series of defenses. A rattlesnake’s first choice is usually to stay still and rely on camouflage, or to slip quietly away. It rattles when it decides that hiding has failed. That is why many snakes stay silent as people walk past; see do rattlesnakes always rattle before striking?.

How the rattle is built

The rattle is a chain of hollow segments made of keratin, the same protein as fingernails. Each segment is shaped so that it fits loosely over the one in front of it, a bit like a stack of nested cups with lips that catch on each other. The segments are not alive and contain no nerves.

At birth, a rattlesnake has only a small cap at the tail tip called a prebutton. When the baby sheds its skin for the first time, about one to two weeks after birth, this is replaced by the “button,” the first permanent segment. Every time the snake sheds after that, the skin at the tail tip is left behind as a new segment, pushing the older ones back.

Because a snake can shed several times a year and the end segments regularly break off, the length of the rattle tells you little about the snake’s age. You can read more in can you tell a rattlesnake’s age by its rattle? and in why snakes shed their skin.

How the sound is made

The rattle itself has no moving parts of its own. Instead, special “shaker” muscles in the snake’s tail contract back and forth very rapidly. Each twitch knocks the loose segments against each other, and because they are hollow, the clicking blends into a continuous buzz or hiss.

These tail-shaker muscles are among the fastest-contracting muscles known in any vertebrate. Their speed depends on temperature, since rattlesnakes are cold-blooded. In western diamondbacks, the rate rises from roughly 20 cycles per second in cool conditions to about 90 per second in a warm snake. The muscles are packed with energy-producing mitochondria and are built so efficiently that a snake can keep rattling for hours without tiring.

Why the pitch suddenly changes

A study published in 2021 in the journal Current Biology looked closely at how western diamondback rattlesnakes respond to an approaching object. As the object moved closer, the snakes rattled faster. Then, at a short distance, they abruptly jumped to a much higher rattling frequency, roughly 60 to 100 cycles per second.

In tests with human listeners in a virtual environment, this sudden jump made people think the sound was closer than it really was. The researchers suggested that the high-frequency mode works as a kind of acoustic trick, making a threat believe it is about to step on the snake and encouraging it to back off sooner.

The finding shows that the rattle is not a simple alarm with one setting. Rattlesnakes adjust the signal as a situation develops, which helps explain how such a simple structure can be so effective.

How the rattle evolved

Rattles exist only in rattlesnakes, a group of about 30 or more species in the genera Crotalus and Sistrurus found from southern Canada to Argentina. But many other snakes vibrate their tails when threatened. Copperheads, cottonmouths, rat snakes and kingsnakes all do it, and in dry leaves the result can sound surprisingly like a rattle.

Scientists think tail vibration came first, as an ordinary defensive behavior, and that the rattle evolved later as a way to amplify it. A 2016 comparison of many viper and colubrid species found that the closer a species is related to rattlesnakes, the more its tail-vibrating behavior resembles theirs in speed and duration. Even a small early rattle would have made the warning louder and given those snakes an advantage.

At least one rattlesnake has gone the other way. The Santa Catalina Island rattlesnake (Crotalus catalinensis) of Mexico has segments that do not interlock and fall off, so adults have no working rattle.

Myths and facts about rattling

• Myth: a rattlesnake rattles to attract or freeze prey. Fact: hunting rattlesnakes are silent ambush predators; the rattle is purely defensive.
• Myth: the rattle is made of bone. Fact: it is dead keratin, like fingernails, and contains no bones or nerves.
• Myth: a rattle means an attack is coming. Fact: rattling usually means the snake wants you to leave. Back away slowly and it will usually do the same.
• Myth: rattlesnakes always rattle first. Fact: many stay silent, and a snake that is stepped on may strike with no warning.

The eastern diamondback rattlesnake, the largest species, is famous for both habits: some individuals rattle from many meters away, while others lie quietly while hikers pass.

What to do if you hear a rattle

A rattle is a useful warning, so take it seriously. Stop, look carefully to find where the sound is coming from, and then move slowly away, giving the snake plenty of space to leave. Never try to catch, move or kill a rattlesnake. If someone is bitten, call 911 (US) and seek emergency medical help immediately.