Monday, April 2, 2012

Natural History of Neck-banded Snakes

The following article is a guest post by Andrew Durso. I would like to feature more guest posts here in the future; please contact me if you're interested in contributing. Want to see more posts from Andrew or other potential future guest bloggers? Encourage them by letting them know what you think in the Comments.

Andrew Durso currently lives in Logan, Utah, and is a Ph.D. student at Utah State University researching the physiology, ecology, and behavior of toad-eating snakes. Originally from North Carolina, he received his B.Sc. degree in Ecology from the University of Georgia, where he continually allowed the Auburn University Bioblitz team to triumph in herping competitions, in order not to injure their already low self-esteem (examples here and here). More recently, he completed a M.Sc. in Biology at Eastern Illinois University, studying methods to indirectly monitor snake diets. Andrew enjoys herping, bird-watching, cycling, and cooking. He hosts a blog on snake natural history at http://snakesarelong.blogspot.com/.


Also called Shovel-toothed Snakes or Skink-eaters, the members of the genus Scaphiodontophis are among the most unusually-patterned snakes. The one below should probably be called a neck-and-tail-banded snake, although I admit that's a little cumbersome.


As you can see, these snakes' patterns are just ridiculous. It looks like someone erased the middle third of a milksnake, or like a milksnake and a brownsnake had a love affair. But these snakes aren’t genetic freaks. There are likely some benefits to their unusual patterning, for reasons we can only guess.

Well, we can do a bit more than guess. In a 66-page paper published in the scientific journal Biological Journal of the Linnean Society, Jay Savage and Joe Slowinski examined the morphology (shapes and sizes), taxonomy (evolutionary relationships), variation, coloration, and behavior of Scaphiodontophis, and reached some very interesting conclusions.

Before we get into their findings, let us first describe their basic biology. Snakes within Scaphiodontophis are non-venomous, active during the day, and can most often be found within the leaf-litter of humid evergreen forests. They are unusual among snakes in North and South America because they have hinged teeth. It is thought that this tooth structure is useful for eating skinks and other hard-bodied prey. 
Eastern Slender Glass Lizard, Ophisaurus attenuatus
The tail of adult Scaphiodontophis is an incredible 39 to 49% of the total length. In other words, a 1-foot long snake could be about six-inches of tail. These tails have some other unusual features. Although they are disproportionately thick, they are surprisingly fragile, breaking off fairly easily. Just like many lizard and salamanders, Scaphiodontophis snakes probably drop their tails as an antipredator device. If a predator is distracted by a detached (but still wriggling) tail, the animal has a good chance of escaping. These behaviors remind me of the glass lizards of the southeastern USA (genus Ophisaurus), whose tails can be a whopping 50-75% of their total length and also detach fairly readily!

Now that we have a basic understanding of these snakes, let’s return to what I feel is their most unusual feature: their patterning. What is up with the coloration? At first glance, these snakes look like just another coral-mimic. Many snakes, such as Scarlet Snakes, Cemophora coccinea, are thought to look like venomous coral snakes because predators are less likely to try to feed on them. But that explanation isn’t completely satisfying for Scaphiodontophis. A closer look reveals pattern diversity that does not precisely correspond completely to any coral snake patterns. Individual snakes can have a pattern of tricolor monads (which are red bands separated by light-black-light), tricolor dyads (red separated by black-light-black), or even tricolor triads (red separated by black-light-black-light-black) or tetrads (red separated by...you get the idea). These bands sometimes extend along the entire body, are sometimes just on the front end, and are sometimes on the front and back ends but not the middle, as in the picture above. To make matters even more complicated, the black and yellow bands are sometimes offset so that they alternate with one another on the two sides of the body. Also, as if the "the extreme variability" (as described by Savage and Slowinski) wasn’t enough to confuse things, one snake’s patterning can change as the animal ages
!

In the 1950s and 1960s, biologists observed that several different Scaphiodontophis snakes that had previously been considered separate species could actually be found in the same places and were breeding with each other. That was when everyone realized that what they were using to distinguish between species probably wasn't accurate. This discovery led scientists to reclassify these animals as different species based solely on their based on neck pattern and coloration. This may have seemed like a good idea at the time, but the result was that populations of animals living far away from each other were considered the same species. Specifically, animals in Panama/Columbia and northern Central America were all considered S. annulatus, while everything in between those two areas was considered another species, S. venustissimus . This doesn’t make a lot of sense but it was the best we could do with the limited information we had. Because specimens of many neotropical snakes are still hard to come by, it is difficult to identify where the range of one species ends and another one begins.

Anyhow, Savage and Slowinski sorted all the existing Scaphiodontophis descriptions and synthesized the known information, as well as new information they collected on their own, and concluded that the situation wasn’t as complicated as everyone thought. They said that instead of two (or many) species, there really there was just one species with highly variable coloration patterns. In fact, their color patterns could comprise an entire new semaphore alphabet. 

Towards the end of their section on variation, the authors conclude that "No two Scaphiodontophis have the same coloration when head, nuchal (neck), body and tail patterns are considered in combination," which is pretty cool when you think about it. That pattern of these snakes is like a zebra’s stripes or a human fingerprint. A detailed system of notation for describing the color pattern of an individual Scaphiodontophis was developed by Savage and Slowinski, which for one snake looks like: wZ/B-6D-22/2M-S/2M/u+. Seriously.


So what explains all this pattern diversity? Well, the first question should probably be: why isn't the whole body banded? We have a partial answer to this question. This snake is an ambush predator, and it's typically found sitting motionless with the front of its body exposed and the posterior remainder buried in the leaf litter. At first, I didn’t buy that explanation, but check out this picture below, apparently taken in captivity, of a Scaphiodontophis in "typical diurnal posture".



Photo from Henderson (1984)
This apparently unique juxtaposition of behavior and microhabitat might be responsible for differences in the pattern on the snakes' bodies. That is to say, it is likely advantageous for the exposed part of the body to bear a banding pattern to mimic a coral snake because this would deter predators. However, this still doesn't explain why there is a disadvantage of having a banded back or middle body - but likely there is one, or else the entire snake would look banded (it is worth noting that there are a few populations in Nicaragua, Costa Rica, and Panama that are completely banded).

The inconsistent patterning could be because producing bright red, yellow, and black pigments is energetically expensive, but coral snakes and many mimics seem to do it just fine. I think it's more likely that the regional variation in patterning is due to regional variation in predator behavior, but that idea requires rigorous testing before we can conclude anything with confidence.

Like an Inflexible Chameleon
Studies have shown that coral snake mimics don't have to match the exact pattern of a coral snake very closely to convince predatory birds to avoid them. One way this has been tested was by puting out snake models in the forest. Different models had different color patterns: some looked like coral snakes, some didn’t, and some contained a mix of patterns. Scientists then counted which models predators tried to attack. They found that when predators see a model with a mix of patterns, they will direct attacks at the unbanded portion of a model. This could explain the partially-banded patterns we see in actual Scaphiodontophis snakes. So, although we may have some clues as to why these snakes look the way they do, if anything is to be learned from the history of research on Scaphiodontophis, it is that nothing about these incredibly interesting snakes is as simple as it seems. 




Thanks to photographers Silviu Petrovan, Mauricio Rivera, Pierson Hill, Jaroslav Vogeltanz, and Jairo Maldonado for the use of their photos, and to Dave Steen for inviting me to guest write this article.
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Want to learn more? Check out the following scientific articles



SAVAGE, J., & SLOWINSKI, J.B. (1996). Evolution of coloration, urotomy and coral snake mimicry in the snake genusScaphiodontophis(Serpentes: Colubridae) Biological Journal of the Linnean Society, 57 (2), 129-194 DOI: 10.1006/bijl.1996.0010

SAVITZKY, A. (1981). Hinged Teeth in Snakes: An Adaptation for Swallowing Hard-Bodied Prey Science, 212 (4492), 346-349 DOI: 10.1126/science.212.4492.346
Henderson, R. 1984. Scaphiodontophis (Serpentes: Colubridae): natural history and test of a mimicry-related hypothesis. Special Publication, University of Kansas Museum of Natural History 10:185-194.

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