Can You Tell a Rattlesnake's Age by Its Rattle? A Common Myth

Where the “one rattle per year” idea comes from

The myth is easy to understand. The rattle grows by adding one ring at a time, much like the growth rings in a tree, so it seems natural to count them as years. Older adult snakes also tend to have longer rattles than newborns, which seems to support the idea.

But the rule breaks down as soon as you look at how a rattle actually grows. Herpetologists have known for at least a century that rattle segments are tied to skin shedding, not to the calendar. The pioneering rattlesnake researcher Laurence Klauber examined large numbers of rattles and showed that segment counts are a poor guide to age.

How a new segment forms

A rattlesnake is born with a small cap at the tip of its tail called a prebutton. At its first shed, usually within about one to two weeks of birth, this is replaced by the button, the first permanent segment. After that, every time the snake sheds, the old skin at the tail tip stays behind as a new hollow ring of keratin locked onto the one before.

Each shed pushes the older segments farther back, so the newest segment is always next to the tail and the oldest, the button, is at the very tip. To see how these rings make the warning sound, read why rattlesnakes rattle.

How often rattlesnakes shed

The key problem for age-counting is that shedding is irregular. How often a snake sheds depends on:

• Age and growth. Young snakes grow fast and shed more often, often two to four times a year.
• Food. A well-fed snake grows faster and sheds more often than a hungry one.
• Climate. In warm regions with a long active season, snakes have more time to grow and shed. In cool northern areas, snakes may spend half the year in dens and shed only once or twice a year.
• Injury and health. Damage to the skin can trigger extra sheds.

So two snakes of exactly the same age can carry very different numbers of segments. A well-fed young western diamondback rattlesnake in the warm Southwest may add several segments a year, while a timber rattlesnake in the cool Northeast adds far fewer. For more about the shedding process itself, see why snakes shed their skin.

Why segments break off

The rattle is made of dead material that cannot repair itself, and it takes a lot of wear. Rattlesnakes crawl through rocks, cactus, brush and narrow burrows, and the end segments gradually crack and snap off. Rattlesnakes often hold their tails slightly raised when moving, which helps protect the rattle, but losses still happen.

Long rattles are especially fragile, because the far end swings more and catches on things. As a result, wild adults usually carry a string of roughly 6 to 12 segments, whatever their age. Captive snakes kept in smooth enclosures sometimes keep much longer strings, which can give a misleading idea of what is normal in the wild.

When rattles do give a clue

There is one situation in which the rattle is useful. If the button is still present and the string tapers evenly toward it, no segments have been lost yet. In that case the number of segments matches the number of sheds since birth. Because researchers know roughly how often young snakes of a population shed, they can then give a rough age for juveniles.

Rattle segments also hold a record of growth. Each new ring is sized to the tail at the time it formed, so a rattle that stays the same width for several segments suggests a snake that has stopped growing much, while steadily widening segments suggest a growing young snake. This is still an estimate, not a precise count of years.

How scientists estimate a rattlesnake’s age

For accurate ages, researchers rely on long-term field studies:

• Mark and recapture. Snakes are marked, for example with a tiny tag under the skin, ideally when they are born. Each time a marked snake is found again, its exact age is known. Long-running studies of timber rattlesnakes in the northeastern US have followed individuals for decades this way.
• Growth curves. By measuring many known-age snakes, scientists can describe how fast a species grows and roughly estimate the age of a snake from its length, at least for younger animals.
• Bone growth layers. In some reptile studies, thin slices of bone show annual growth rings, similar to tree rings. This method, called skeletochronology, can be used on snakes that have already died.

These studies show that many rattlesnakes can live well over 10 years in the wild, and some timber rattlesnakes have been documented at 20 years or more, far older than their rattles would suggest.

The bottom line

A rattle is a record of sheds, minus whatever has broken off. It can hint that a snake is young, but it cannot tell you how old an adult is. If you see a rattlesnake, there is no need to get close enough to count anything: give it plenty of space and let it move on. If someone is bitten, call 911 and seek emergency medical help immediately.