Identifying snake sheds, part III

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I noticed that a huge proportion of the hits on this site are for the posts about identifying snake sheds (parts I and II), which I expect is a result of people searching for a key or guide to use to ID a snake shed that they have seen or found. Even though there is some useful information in those other posts, they are written more like detective stories with a particular conclusion in mind, and they certainly aren't comprehensive.

Here, however, I've attempted to put together a more complete how-to guide on how to ID sheds of snakes found in the United States and Canada. One excellent free reference on this subject is an electronic pamphlet by Brian Gray called A Guide to the Reptiles of Erie County, Pennsylvania. Even if you don't live in Erie County, Brian's section on shed snake skins is a very useful guide to many of the common species found in the eastern United States, because it contains many excellent, high-resolution images of the scale characters, and it is organized as a dichotomous key: a series of questions, each with two choices, that inevitably leads to an identification (it's sort of like a choose-your-own-adventure book). Brian's more comprehensive book, The Serpent's Cast, is also an excellent resource, containing images of shed skins that have been painstakingly prepared for viewing the details of the scales necessary for identification to species. Although shed skins that you are trying to identify won't always be that cleanly preserved, often many of the identifying features are still visible.

From Cardwell 2011; viper (top) and colubrid (bottom)

The first thing that many readers will want to know will be whether or not the snake whose shed skin they have found is a venomous species. This distinction corresponds nicely with determining what family the snake is in. In most of North America, there are two families: Viperidae (vipers, which are venomous) and Colubridae (colubrids, which are not¹). The easiest way to distinguish these two families by their shed skins is to locate the sub-caudal scales (the scales under the tail). Colubrids have a double row of scales under the tail, whereas vipers have a single row. This is a pretty invariant character², especially near the anterior part of the tail, and it can help you tell the family of the snake whose shed you've found every time. Coral snakes, which are in the family Elapidae, also have a double row of scales under the tail, but if you think you have found a coral snake shed, post a pic because that's an amazingly lucky find. More about these, and a few other options, later. First, colubrids:

Divided anal scale

Single anal scale

Once you have figured out the family, a second pair of characteristics can help you narrow down which genus of colubrid you might have. These are 1) the texture (smooth or keeled) of the dorsal scales (these are the relatively small scales that cover the snake's entire back and sides) and 2) the condition (single or divided) of the anal scale or anal plate (the scale covering the cloaca). Keeled dorsal scales have a ridge running down the center, whereas smooth dorsal scales have no ridge, like so:

Smooth (left) and keeled (right) dorsal scales

Using these characteristics in tandem should allow you to divide the colubrids in to four groups: single/smooth, divided/smooth, single/keeled, and divided/keeled. These are not taxonomic groups (that is, not all single/smooth snakes are each others' closest relatives), but they are useful for distinguishing genera of colubrids when all you have to go on is the shed skin. All North American vipers have keeled scales and a single anal scale, so these characters are less useful for distinguishing them, but more on these later. Most of the species of North American snake are colubrids (about 80%, or 105 of our 131 species). Here is a quick guide to the colubrids of the US and Canada, by dorsal and anal scale characteristics:

A few genera are split among multiple categories: Gyalopion because G. quadrangulare has a single anal scale whereas G. canum has a divided anal scale, and Opheodrys and Virginia because one species of each has keeled scales and the other has smooth (these are helpfully called Rough and Smooth Green and Earth Snakes, respectively). It's also worth noting that anal scales of Farancia are pretty variable, although your chances of finding a Farancia shed are slim (but see part I).

As you can see, we are using the process of elimination to narrow down the possible candidate species for your shed. A quick look at the range maps in a regional field guide will allow you to cross off about half the genera on the above list, depending on where you live, probably leaving you with 2-6 possibilities. The overall size of the shed can also be of help, although keep in mind that large snakes are born small and that snake sheds stretch somewhat as they are removed. Still, many of the snakes on the above chart reach adult sizes of only 12-24", so they could potentially be eliminated on the basis of size. Width of the ventral scales can help too, because it gives you an idea of body shape, and this does not change as much during the shedding process. However, at this point, the most useful thing to do next is to look at another scale meristic. One that can help you distinguish among the several genera within each group requires counting the dorsal scale rows. Dorsal scales are arranged in rows, the number of which can be counted from left to right, like so:

Three equally good ways to count dorsal scale rows (in C, scale 1 not shown). Modified from K. Jackson (2013)

You'll want to start with the first dorsal in contact with a ventral on one side and proceed over the back and down the other side so that the last scale counted is the dorsal scale in contact with a ventral on the other side of the snake. Although the conventional way (A) is for this to be the same ventral scale as the one your first dorsal scale row was in contact with (that is, count in a ‘V’ shape, as depicted above, so that you are counting all the scales associated developmentally with a single pair of ribs), you should get the same result even if your 'V' is asymmetrical (B), or even if you count in a straight line (C), which can be easier since you don't have to decide where to change direction on the 'V'. Often it doesn't matter, although it's worth noting that in some snakes the number of dorsal scale rows varies along the length of the snake. The best way to guard against this is to count a row in the middle of the body, which is the number meant if only one is given in most keys. More often, you will see numbers of dorsal scale rows given in the format “15-17-15”, indicating the number of dorsal scale rows at three places on the body (in order): the neck, midbody, and a bit (about one head length) before the cloaca.

In North America, you should almost always get odd numbers, and although these numbers can sometimes be fairly variable, combining them with decisions you made above based on the subcaudals, anal scale, dorsal texture, body size, and range should allow you to decide on a genus in almost 100% of cases. Here is a list of the dorsal scale formula ranges for the North American colubrids (remember, it's neck, midbody, and before the cloaca). Where ranges are given in parentheses, species within that genus have differing scale formulas. Where ranges are given without parentheses, there is regional or other variation within one or more of the species in that genus. In a few cases, only the scale row counts at midbody are given.

Knowledge of the number, shape, and relative size of the head scales is usually necessary to distinguish among species within a genus (for example, to tell a Scarlet Kingsnake from a Mole Kingsnake), and unfortunately many sheds are missing their heads or the heads are in poor condition. Other clues can be obtained from pattern, which is often visible in good light, and from counting the total number of subcaudal or ventral scales (impossible if you only have a partial shed). If you have taken your shed to genus and want to send me pictures of the head for help identifying it to species, feel free. I would recommend using your digital camera's macro setting (almost all cameras have one, the symbol is a little flower) to photograph snake sheds. You can also find details of the head scalation of all species of North American snakes in the book Snakes of the United States and Canada by Ernst & Ernst, and much of this information is available online as well. It's often helpful to keep the shed in a Ziploc bag for later reference. I like to write on the bag with a Sharpie the date, location, and tentative ID of the snake.

Non-colubrids

As I mentioned above, all North American vipers have single subcaudals, keeled dorsal scales, and a single anal scale, so these characters are less useful for distinguishing them from one another. However, there are only three genera: Agkistrodon (Copperheads and Cottonmouths), which have no rattles, and two genera of rattlesnakes, Crotalus (which have small scales on the tops of their heads) and Sistrurus (which have large scales on their heads). Telling the different species of Crotalus by their sheds could be tricky, but unless you live in Arizona, there are usually only one or two options in any given location in the US. Size and pattern could also be helpful. Feel free to share pictures (remember to use macro). Copperhead and Cottonmouth sheds can be hard to distinguish, but range, size, and habitat can help, as well as the presence or absence of a loreal scale (the scale on the face between but not in contact with either the eye or the nostril), which Copperheads have and Cottonmouths do not.

Micrurus fulvius

If you live in certain parts of the US, there are a few other snakes that aren't colubrids or viperids whose sheds you might find. One familiar group is the elapids, represented in North America by the Coral Snakes. One species is found in Arizona and New Mexico, and the other in the southeastern coastal plain from Texas to North Carolina. I have never seen a Coral Snake shed, but I would imagine that the highly contrasting, distinctly banded pattern would be easily visible. However, these can also be distinguished by their scale characteristics: Micrurus fulvius has smooth dorsal scales in 15 rows and a divided anal plate, and Micruroides euryxanthus has smooth dorsal scales in a 17-15-15 pattern with a divided anal plate. The other US elapid, the Yellow-bellied Sea Snake (Pelamis platurus, found in the Pacific Ocean off southern California) sheds at sea, so unless you are in very unusual circumstances the sheds will not be found. They have smooth scales with a 39-47, 44-67, 33-46 row formula and a divided anal plate.

Lichanura trivirgata

If you live in southern California or the intermountain west, there are two species of temperate boids, the Rubber (Charina) and Rosy (Lichanura) Boas, whose sheds you could find. Boa sheds are very different from those of other snakes. Boas have small, round dorsal scales that are very numerous - Charina and Lichanura have 32-53 and 33-49 dorsal scale rows, respectively, so you should be able to tell a boa shed by the small size and number of dorsal scales. Rubber Boas have blunt tails and specialized head scales, whereas Rosy Boas have long tails and unspecialized head scales, and their ranges do not overlap. If you live in southern Florida, you might find sheds of Boa Constrictors or Burmese Pythons, which you should be able to tell by their huge size, or any number of other exotic snakes (good luck with those).

Rena humilis

Finally, the southwestern US is home to several species of scolecophidian blindsnakes in the genera Rena and Leptotyphlops. These are tiny and have undifferentiated body scales, meaning that all scale rows around the entire body (including the underside) are the same width. They are iridescent and extremely difficult to count, which has given rise to one of my all-time favorite quotes from a scientific paper: "We castigate the ancient lineage that begat Liotyphlops, for it is obviously the worst designed snake from which to obtain systematic data" (Dixon & Kofron 1983). An additional species, Ramphotyphlops braminus, is introduced in Florida, Louisiana, and Hawaii, as well as in many other locations around the world (it's parthenogenetic and so a really good invader because it only takes one!). Blindsnakes shed their skins in a series of rings rather than in a single piece, and they are so small that any sheds found would be unlikely to belong to any other kind of snake and so fairly easy to identify.

Feel free to comment or email with questions or photographs. Happy herping!

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1 I am making a distinction between North American snakes that are dangerously venomous to humans (vipers & coralsnakes) and those that aren't (colubrids). Although some species of colubrid snake possess deadly venom, such as boomslangs and twigsnakes, these are not native to North America. Other colubrids, including some North American species such as Hog-nosed Snakes (Heterodon), are venomous in the sense that their Duvernoy's gland secretions are toxic to their prey, but are harmless or nearly so to humans. For a very thorough discussion of this issue, check out the book "Venomous" Bites from Non-Venomous Snakes.

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2 Long-nosed Snakes in the genus Rhinocheilus can have a mixture of divided & undivided subcaudal scales.

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ACKNOWLEDGMENTS

Thanks to Brian Gray, Jack Goldfarb, and JD Willson for their excellent photographs.

REFERENCES

Cardwell MD (2011) Recognizing Dangerous Snakes in the United States and Canada: A Novel 3-Step Identification Method. Wilderness & Environmental Medicine 22:304-308. <link>

Dixon JR, Kofron CP (1983) The Central and South American anomalepid snakes of the genus Liotyphlops. Amphibia-Reptilia 4:2-4. <link>

Ernst CH, Ernst EM (2003) Snakes of the United States and Canada. Smithsonian Institution Press, Washington D.C. <link>

Gray BS (2011) A Guide to the Reptiles of Erie County, Pennsylvania. Natural History Museum at the Tom Ridge Environmental Center, Erie, Pennsylvania. <link>

Weinstein SA, Warrell DA, White J, Keyler DE (2011) "Venomous" Bites from Non-Venomous Snakes: A Critical Analysis of Risk and Management of "Colubrid" Snake Bites. Elsevier, Amsterdam. <link>

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Identifying snake sheds

If you have found a snake shed that you wish to identify in the USA or Canada, click here for a guide.

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Often it is possible to identify snakes by their shed skins alone (you can do so too, here). I have had some pretty serious exercise of this skill this week. In the past three days, I have found three fairly fresh snake sheds on islands in the Matanzas River. I am here doing evolutionary biology research on brown anole lizards, which you can read more about here and here, but it’s impossible (for me, at least) not to be on the lookout for snakes and snake-related objects, such as shed skins. Snakes don’t typically hang around the site where they shed very long (don’t worry, I looked), but the sheds themselves have many characteristics that aid in identifying the species they came from.

Two of the three sheds were found right outside the entrance to a gopher tortoise burrow. These burrows are built by the tortoises for shelter, and many other animals, especially snakes, use them as well. They can be as long as 40 feet and as deep at 10 feet. Although many snakes spend a significant amount of time underground, they usually come up to the surface to shed their skin All snakes must do this once in a while, typically every 3 weeks to 2 months, depending on their growth rate and need to heal injuries or slough off parasites.

Entrance to a gopher tortoise burrow

Although snakes shed their skin in a single piece, the shed is very fragile and can be tattered or torn easily by wind or other animals. Most often, though, a shed is either torn apart by the snake as it sheds, or tears itself apart as a result of drying out (it is wet when the snake sheds it off, as a result of lymphatic fluid excreted by the snake’s integument to aid in the shedding process). Sheds are easiest to identify when they are freshly shed, ideally before they have dried out entirely, which only takes an hour or so in the hot Florida sun. Luckily for me, the two sheds I found outside the tortoise burrow had probably been shed only about an hour or so before I found them, so they were fairly intact. Still, one was torn in half, not a big deal from an identification perspective, and the other had suffered some tattering of the head, which is more serious but not fatal.

The first thing to notice is the overall size of the shed, as this can rule out many species if the shed is large, which these were. It’s important to remember that you could be dealing with a juvenile snake, so small overall size does not necessarily mean that one is dealing with a small-bodied species of snake. The shed skin is necessarily larger, both in length and girth, than the body of the snake that shed it, since the shed has to stretch in order to be pulled off the snake. Still, the relative proportions are often fairly similar, and both length and width can be clues to snake body size and shape.

The sheds I collected measured 72” and 60” in length, plus or minus a few inches to account for imperfect measurement of the difficult-to-stretch sheds. The ventral scales at the halfway point of the body were about 1.5” and 1.25” wide, respectively, so I could tell that these were both fairly heavy-bodied snakes, even for their great length. This narrowed down the pool of possible species considerably, but there were still several candidates, including indigo snakes, either of the two large rattlesnakes native to Florida, rat snakes, corn snakes, and pine snakes. Coachwhips also attain these lengths, but are relatively slender.

Other easy clues narrowed the species pool further. The dorsal scales (those covering the top and sides of the body) were mostly smooth, although the middorsal scales of the larger shed showed some slight keels. Keels are when there is a ridge running lengthwise down the center of the scales, similar to the keel of a boat. The texture of the scales (keeled or smooth) is preserved in the shed skin, and can be an important clue to the identity of the shedder. Rattlesnakes, watersnakes, and pine snakes all have strongly keeled scales that feel rough to the touch – I knew that these species were not responsible for the shed when I saw that the scales were smooth in texture. Furthermore, counting the number of rows of dorsal scales, typically halfway between the head and the tail, can give further insight into the identity of the snake. These sheds both had 27 scale rows at midbody.

The larger shed

Patterns are often preserved in shed skins, but without their colors. High-contrast patterns are especially evident, but typically are only clearly visible for a short while after the skin has been shed. These can be important clues, but they are difficult to interpret. Good lighting is often required to make out the pattern of a snake shed. These snakes had slightly different patterns. The larger shed had two dark stripes running longitudinally down the entire body, near the center of the back. The smaller had a similar pattern, but the space between the stripes contained alternating dark and light blotches with dark edges, especially near the head. Neither shed had any evident pattern on the ventral scales.

The head of the larger shed

Between the scale row count and the pattern, I had a pretty good idea what genus these snakes were in. To identify to species, however, is often trickier, because it usually involves examining the scales of the head in detail. This requires 1) having the head to look at and 2) a degree of finesse, because the head is often the most delicate part of a shed. Additionally, the head is often scrunched up inside of the anterior part of the body, as a result of the behavior of the snake during shedding. Extricating the head is not unlike performing a dissection. I have found that it is helpful to photograph the head at every stage of dissection, because you might destroy certain features in pursuit of access to others, or just from even the lightest handling. Also, details are sometimes evident in macro photographs that are not obvious on the shed itself.

The smaller shed

The scales of the head of colubrid snakes each have special names that are determined by their arrangement with respect to the eyes, mouth, nostrils, and to one another. The easiest scales to count are the upper (supra) labials. These are the scales along the upper lip. One or two of the supralabials are typically in contact with the bottom of the eye, which can be an important characteristic. The frontmost scale on the upper lip is called the rostral and is located front and center on the nose of the snake, but there is not much variation among species in the characteristics of the rostral, so it’s not very informative One of these sheds had 8 supralabials on each side of the head, the 4^th and 5^th of which were in contact with the eye; the other was too destroyed to count, especially toward the rostral (where many snakes begin rubbing in order to slough off the shed). Other similarities included two pairs of prefrontal scales (the scales above the rostral and between the supralabials, on the bridge of the nose), a divided nasal scale (the scale behind the rostral that contains the nostril), one loreal (the scale between the nasal and the preoculars), one preocular (the scale behind the loreal and in front of the eye), two postoculars (the scales behind the eye), and a 2+3+3 temporal formula (the numbers of scales in each of the three rows posterior to the postoculars). On the basis of all these similarities, I concluded that these were probably the same species of snake, which I think is Pantherophis [Elaphe] obsoletus quadrivittatus, the Yellow Ratsnake.

A much smaller yellow ratsnake that I caught this week

The size and pattern discrepancies could be solved in two ways. The smaller of the two snakes could have been a corn snake (Pantherophis guttatus), which is what the pattern reminded me more of. However, rat snakes are blotched with a similar pattern as (but different colors than) a corn snake when they are young. Although this pattern fades with age, faint traces of it may be evident even in very large snakes, and these traces may be more evident in shed skins than on the actual snake. It wouldn’t be unusual to find rat and corn snakes cohabitating in the same tortoise burrow, especially because there were only two burrows on this island that I could find. The other explanation is that both sheds came from rat snakes, one male and the other female. Although there are no consistent pattern differences between male and female rat snakes, the slight keels on the middorsal scales of the larger shed are typical of male rat snakes, whereas the completely smooth scales of the smaller shed are consistent with female rat snakes. Furthermore, the larger size of the male is typical of all snakes that have male combat, including rat snakes. Finally, and most convincingly, the tail of the larger specimen made up 17% of the total length, whereas the tail of the smaller specimen, while slightly more battered and harder to measure, only made up about 11% of the total length. This is consistent with sexual dimorphism in tail length observed in nearly every species of snake.

Because both snakes were in the same place at the same time (judging by the condition and likely age of the sheds) and were different sexes, I think it’s more likely that they were the same species, yellow rat snakes, possibly a mating pair. Why they both shed before (or after) mating, I couldn’t say – perhaps the snake version of an after-sex cigarette?

Whereas blotched and unblotched adult rat snakes are possibilities, I have never seen or heard of unblotched adult corn snakes, and in any case, these would have been some big corn snakes, especially the male. They were pretty big even for rat snakes – the record rat snake length is 72” (keep in mind that the measured lengths of the sheds are several inches longer than the lengths of the snakes, because the sheds are stretched out).

The third snake shed I found a few days earlier on the same island, but away from the tortoise burrow, in a sandy area. This was a trickier one, because I only found part of the shed – this time both the head and tail were missing. These are the two most informative sections of a shed, the head for reasons I described above, and the tail because of the ability to tell the sex (given also the total length) and because the subcaudal (under tail) scales can give you information about the family of snakes to which the owner of the shed belongs. All I had to work with were the dorsal and ventral scales, and the limited pattern I could see. I could also tell that this snake was robust, not slender, which eliminated a few species, such as the Racer and Coachwhip, and fairly large, which eliminated several more small-bodied species.

Ventral pattern of rainbow snake shed

The dorsal pattern of this shed was faint, but I thought I could make out a few stripes similar to those of the rat snake, but thinner. I looked closely at the ventral pattern, which was obscured in most places but quite clear in a few spots. The gestalt of the pattern reminded me of something I had seen before, but I couldn’t quite place it at first. I looked more closely at the rest of the shed. The dorsal scales were completely smooth, with no hint of keels. They were in 19 rows, which is an important piece of evidence. I knew that this was not a pine snake, rattlesnake, cottonmouth, rat snake, or kingsnake, all of which have keeled scales, a greater number of dorsal scale rows, or both. Only two large snakes in Florida have 19 dorsal scale rows – the mud snake and the rainbow snake. These are secretive, fully aquatic snakes with specialized diets and cryptic habits; both are rarely seen. You might ask why the shed of a fully aquatic snake would be found on dry land. The interesting thing about mud and rainbow snakes is that they lay eggs, unlike most other species of aquatic snakes. Females of these snakes must come onto land to lay their eggs, much like female turtles. I have never heard that they come to land to shed, but apparently this one did.

Ventral pattern of rainbow snake shed

Based on the habitat and the particulars of the ventral pattern, I think it’s much more likely that this was a rainbow snake (Farancia erytrogramma) than a mud snake. Because the island I was on is in the estuary, close to the ocean, there are almost certainly American Eels in the river, which are the primary food of rainbow snakes. Mud snakes eat giant aquatic salamanders, especially sirens and amphiumas, which do not inhabit salty water – their most common habitat is isolated acidic wetlands, such as Carolina Bays. To find either of these species is a rare treat. To find the shed of one is probably even rarer, although I’m not sure of that. I was pretty excited about this find, because the rainbow snake is one of my very favorite snakes.

Rainbow snake - Photo by JD Willson

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.