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Squamation: How to identify rattlesnakes using scale patterns

Western Diamondback Rattlesnake (C. atrox). Photo Credit: Clinton and Charles Robertson (Wikimedia Commons) Western Diamondback Rattlesnake (C. atrox). Photo Credit: Clinton and Charles Robertson (Wikimedia Commons)

Rattlesnakes are near and dear to my heart. And I know I’m not alone. People generally love a good mystery when it comes to the natural world, and the enigmatic rattlesnake doesn’t disappoint.

During my early encounters, I inevitably began to attempt to distinguish one species from another and found that there are a whole host of parameters that people routinely employ.

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

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. I’m sure you’ve all 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.

Rounded head of Sistrurus sp. Source: Klauber, 1956
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 is one supposed to figure out what type of rattlesnake is staring you down right now from it’s favorite basking spot?  My favorite option is something called 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 snake I’m attempting to identify.  Furthermore, unlike color and size, scale patterns are generally accepted to not change over the course of the snake’s lifetime and will not change when chemically preserved (in the case of museum specimens).

This makes squamation a powerful instrument in any herpetologist’s mental toolkit.  But what scales should you look at? Probably 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 availability of zoom lens digital SLR’s and even point-and-shoots cameras makes this task much safer and easier.

As far as what to look for once you have a good view of the head, 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.

Head scale nomenclature. Source: Klauber, 1956
Additional head scale nomenclature. Source: Klauber 1956

Of course, some rattlesnakes are more obvious that 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 C. cerastes. Note the supraocular “horns”. Source: Klauber: 1956
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: Do not attempt to approach a rattlesnake and put yourself in danger.  By reading this information you acknowledge that this blog and it’s 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.


References and Further Reading

Klauber, L. M. (1939). A statistical study of the rattlesnakes. San Diego Society of Natural History.

Klauber, L. M. (1943). “The correlation of variability within and between rattlesnake populations”. Copeia. 1943 (2): 115–118.

Klauber, L. M. (1956). Rattlesnakes (Vol. 1). Univ of California Press.

Klauber, L. M. (1972). Rattlesnakes: Their Habits, Life Histories, and Influence on Mankind (Vol. 1). Univ of California Press.

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