Life is Short but Snakes are Long is two years old this month!

Wednesday, April 4, 2012

Identifying snake sheds



Often it is possible to identify snakes by their shed skins alone. 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 4th and 5th 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

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

4 comments:

Michael said...

I really like the title of your post and I certainly agree with you. In US I believe the Florida have the most kinds of the snakes including some of the largest snakes in US. There are more than 40 types of snakes that can be found in Florida and many of these Florida snakes can grow very large.

Andrew Durso said...

Hi Michael,

Thanks for the compliment! Florida has 58 snake species, including non-natives, but in fact the state of Texas has more (83), on account of having both a larger land area and a great diversity of ecoregions. Florida and Teaxs both are home to the eastern indigo snake (Drymarchon couperi), the largest snake in the US.

Unknown said...

I recently moved to Port St. Lucie FL and I have seen a Gopher Tortoise in my yard, (I think he is living under my deck). Last week I found a snake shed inside my lanai between the tracks for hurricane shutters. It measures 30 inches with accordianed skin of 2 inches. Can you help me identify what snake this might be? My property is 1/2 acre but I am in a residential area. Also wondering if the Gopher tortoise might be undermining my house!

Andrew Durso said...

Sure, I'll be glad to help! But we probably won't be able to arrive at an answer without more details. At that size, there are a half-dozen species it could be. Can you email me some well-focused photos of the shed, preferably of the head and under-tail regions? (amdurso@gmail.com)

Tortoise burrows are unlikely to cause damage to your foundation - in fact, once he realizes he's digging underneath something like a house or road, he'll probably relocate.