Lizards of glass

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I love snakes, but I am also partial to other limbless animals, especially limbless vertebrates. Among the salamanders, sirens and amphiumas are some of my favorites. Caecilians are also pretty cool. And in the sister group to the snakes, it’s hard to pass up the many species of legless lizard. Snakes are pretty much just legless lizards too, since they evolved from lizard ancestors, but they have undergone a spectacular evolutionary radiation into over 3,000 species in over 20 families. Elsewhere in the lizard family tree, limblessness has evolved many times, usually in concordance with a fossorial or burrowing habit.

The southeastern US is home to four species of legless lizard, called glass lizards, in the genus Ophisaurus, which means snake-lizard (ophi for snake, and saurus for lizard, as in dinosaur). They are not closely related to snakes, nor to any other species of southeastern lizard, but rather to the alligator lizards of the Pacific northwest, genus Elgaria. They can be differentiated from snakes by several characteristics. Chief among these are the presence of external ear openings and of moveable eyelids, both of which snakes lack. Instead, the eyes of snakes are covered by a hard, clear scale known as a spectacle. Snakes cannot hear, although they are very sensitive to vibrations of the ground and air, so they can detect most airborne sounds almost as if they are feeling them. Our sense of hearing (and the glass lizards’) is a mechanical sense also, so it’s not really that different.

In addition to these subtle features, the glass lizards possess a longitudinal groove that runs down each side of their body, which snakes lack. This groove allows the body wall to expand and contract as they breathe, digest food, and reproduce. Snakes have solved this problem in two different ways. One is to reduce or stagger the internal organs so that they fit better into a cylindrical body. Many of the paired organs, such as kidneys and gonads, are situated one in front of the other, and some are no longer paired, such as the single left lung. Snakes also have extremely stretchy skin that allows considerable distortion of the body after they have eaten a large prey item, but glass lizards have two lungs and are quite rigid. You can feel the stiffness of a glass lizard when you pick it up – but when you do, be careful! They aren’t called glass lizards for nothing. Like many lizards, glass lizards can break off, or autotomize, their tails, which can serve to distract a predator in pursuit, which may choose to attack the writhing tail while the body slinks away inconspicuously. In some glass lizards, the tail may be as long as or longer than the body, so the effect can be quite dramatic.

This week in Florida, I have been lucky enough to stumble upon two of the four species of glass lizard. The four species can be hard to distinguish, and until the 1950s were lumped together into a single species. Juvenile glass lizards can be particularly tricky, but luckily for me both of the lizards I saw were adults. The first was caught by friends the day before I arrived, when it was found swimming in an estuary of the Matanzas River. This is unusual habitat for a glass lizard, which are usually found in pine flatwoods and other grassy uplands. However, we determined that this species was the Island Glass Lizard, Ophisaurus compressus. As you might imagine, these are often found on islands and in coastal dunes, and they must get there somehow.

Island Glass Lizard - Ophisaurus compressus

You can tell this species from the others in several ways. One is to closely scrutinize the scales of the head. All glass lizards have a ring of small scales around the eye. In the island glass lizard, these are directly adjacent to the upper labial (lip) scales, whereas in the other common species of glass lizard, they are separated by an additional row of scales, called the lorilabials. A smaller species, the mimic glass lizard, also has scalation similar to the island glass lizard, but we are outside of its range and it does not grow as large as this individual. It is called the mimic glass lizard because it is so similar in appearance to the island glass lizard that it can only be reliably distinguished by size, range, and scale counts. It was only described in 1987, when a curator at the North Carolina State Museum of Natural Sciences noticed that some small glass lizards in his collection were subtly different from the three described species. The mimic glass lizard is found in recently burned pitcher plant bogs and other mesic flatwoods.

Closeup of Island Glass Lizard, showing lack of lorilabials

Closeup of Eastern Glass Lizard,
showing lorilabials between perioculars and supralabials

The other glass lizard was found on a small island. This was an Eastern Glass Lizard, Ophisaurus ventralis. The lorilabials tell us that this is not the island glass lizard, but there is still a fourth species, the slender glass lizard, that we must differentiate. The color pattern can help us with this identification. Eastern glass lizards are checkered over the entire dorsal side of the body (above the groove), whereas slender glass lizards are striped both above and below the lateral groove. This species is a habitat generalist that can be found widely throughout the coastal plains of the Atlantic and Gulf coasts, from North Carolina to Louisiana. Slender glass lizards in the southeast are more closely associated with the imperiled dry uplands dominated by turkey oak and longleaf pine, ever more rare habitats that support a great variety of supremely interesting herpetofauna.

Eastern Glass Lizard - Ophisaurus ventralis

Unfortunately for the eastern glass lizard, its tail broke as I was photographing it. I was able to recover both the tail and the body of the lizard, and you can see how the muscles of the broken segment of tail are pulled out of the tail segment attached to the body, leaving deep cavities. The tail has a store of ATP, so it can thrash about for a few minutes. In the time between the break and the video I took, above, I was able to capture the body of the glass lizard, put it into a bag and tie the bag shut, and switch my camera to video mode, so the thrashing had already decreased in vigor by about half, believe it or not.

Broken tail segment with muscles protruding

Cavities left in tail segment still attached to body

Kind of sweet, kind of disgusting.

So there is your quick guide to identifying the glass lizards of the southeast. It can get tricky with the juveniles, as I said, so pay close attention to detail and take lots of photos of ones that you find. Handle them carefully! Feel free to post them here for aid in identification.

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.