Life Science

Identifying Rattlesnakes Using Head Scale Patterns

Rattlesnakes are near and dear to my heart…and I know I’m not alone. Rattlesnakes are dangerous, enigmatic, powerful. It’s a classic recipe for making a fascinating animal. During my early encounters with rattlesnakes in the wild, I discovered that there are a whole host of parameters that people routinely employ in an attempt to identify them.

The first, and probably least reliable, options are color and color patterns. Just pick up a copy of the Peterson Field Guide of Western Reptiles and Amphibians (which I highly recommend as it’s one of my favorite guides) and you’ll instantly see that color and color patterns alone are not good indicators.

Western Diamondback Rattlesnake (C. atrox) coiled in an agitated posture.
Western Diamondback Rattlesnake (Crotalus atrox)
Photo by Clinton and Charles Robertson | Wikimedia Commons | CC-BY-SA

Interspecies differences aside, even two individuals within the same species can often look starkly different making this method a poor option for distinguishing one type of rattlesnake from another. Furthermore, color shades tend to change with the age of a snake and after each molt, compounding the problem of identification based on this method.

Another common method is to use the animal’s size but again, this is not a very reliable means of identification since juveniles tend to be smaller than adults, a gravid female will appear larger than a non-gravid female, etc. There’s just too much variation in terms of size to use it as a reliable marker of species.

Shape, however, is a little more helpful. The shape of the head, for instance, is commonly cited as a way to determine whether or not the snake they’re looking at is indeed a rattlesnake. You’ve probably heard the axiom that a snake with a triangular head is a rattlesnake (or at least some type of viper). This, however, can still get you into trouble since rattlesnakes of the genus Sistrurus actually have slightly rounded heads similar to that of a Gopher Snake (Pituophis catenifir) rather than the triangular head of their Crotalus cousins.

Diagram of the rounded head of a Sistrurus sp snake.
Rounded head of Sistrurus sp. Source: Klauber, 1956
Diagram of the triangular head of Crotalus species snake.
Triangular head of Crotalus sp.; a generalized feature that many people associate with all rattlesnakes. Source: Klauber, 1956

So, if size, shape, and color aren’t good indicators, how are you supposed to figure out what type of rattlesnake is staring you down as it slides across your favorite trail?  My method is to generally start with squamation.

Squamation simply refers to the scale arrangement on a snake and I find it to be the best way of determining the species of rattlesnake I’m attempting to identify. Furthermore, unlike color and size, scale patterns are generally accepted as immutable over the course of the snake’s lifetime and will also not change when the animal is chemically preserved (e.g. museum specimens).

This makes squamation a powerful instrument in any herpetologist’s mental toolkit.  But what scales should you look at? Some of the most useful scale arrangements are found on the head of the snake. Of course, this is a bit problematic because you need to get close enough to see them; however, the ubiquity of zoom lenses on cameras makes this task much safer and easier.

As far as what to look for, I would pay particular attention to the dorsal, distal lateral (toward the nose), and rostral aspects of the head.  Here you’re going to want to study the supraoccularinternasalrostralpreocular, and postocular scales in particular.

There’s a lot of information there and you can probably narrow-down the species of snake fairly accurately just by paying attention to those scales.  From that point, it’s just some fine-tuning and you’re well on your way to making a very reliable guess as to the species of rattlesnake at hand.

Rattlesnake head scale nomenclature diagram.
Head scale nomenclature. Source: Klauber, 1956
Rattlesnake lateral head scale nomenclature diagram
Additional head scale nomenclature. Source: Klauber 1956

Of course, some rattlesnakes are more obvious than others in terms of squamation, such as the sidewinder Crotalus cerastes which is easily distinguished from most other Crotalus sp. by it’s characteristic “horn” supraocular scales, but that’s not to say that squamation isn’t a great option for more overtly similar species as well.

Lateral detail of Crotalus cerastes head scales and supraocular "horns"
Lateral detail of C. cerastes. Note the supraocular “horns”. Source: Klauber: 1956
Squamation differences between two very similar rattlesnake species.
Squamation is a particularly useful tool when comparing two overtly similar-looking species. Source: Klauber, 1956

Squamation is actually so useful, that the renowned rattlesnake expert Laurence Klauber published a set of keys based on the principles of squamation in Volume 1 of his fantastic Rattlesnakes: Their Habits, Life Histories, and Influence on Mankind.

The complete work (Vol. 1 and 2) is hands-down the most complete reference to-date on rattlesnakes, in my opinion, and is a must-have for any rattlesnake aficionado. There is also a cheaper, abridged version available.

So again, while color, size, etc. might get you in the ballpark, it’s really all about the scales.

Important Note and Disclaimer: Do not attempt to approach a rattlesnake and put yourself in danger.  By reading this information you acknowledge that this blog and its author(s) is/are not liable for any injuries, up to and including death, which may result from the use of the information in this post.

This post has been updated from an older version published in 2014.

References and Further Reading

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