Thursday, April 23, 2015

Two Species of Cottonmouths? This Scientist Says Yes!


    Dr. Frank Burbrink wants you to stop watching shows about searching for Bigfoot. Instead, take a step outside to discover the very real creatures that are all around you, because they are just as mystifying as anything you can find on this planet, let alone on television. He would know.

Northern Cottonmouth, southern Illinois. Todd Pierson.

    Cottonmouths are a common species in the southeastern United States. Everybody knows about them, even if the details might get a little hazy, and few would consider them a mysterious creature. Yet Frank Burbrink and Tim Guiher just revealed to us something extraordinary about the Cottonmouth. Or, I should say Cottonmouths. These two researchers just published the results of a study arguing that the animal that we have been calling the Cottonmouth for all these years is actually two different species. That’s right, there are two different species of Cottonmouths. But they weren’t content to stop there. They also suggest that there are two different species of Copperheads as well. Mind blown?

    There is a common misconception that we have already learned all there is to know about the animals roaming through North America. Nothing could be further from the truth. In many cases, we can be looking at an animal in our right hand and another animal in our left hand and not even realize that we are holding two different species. This recent research proves it.

Eastern Copperhead, southern Georgia. Todd Pierson.
    We talk about Cottonmouths and Copperheads a lot here on this blog. After reading this recent scientific paper and realizing that there may be four of these animals instead of two, I knew I had to learn more about this research and write about it here, so I called up Dr. Burbrink and we chatted on the phone.

    He told me that in the past, scientists typically used morphology (physical characteristics) to determine how many different species there were. Color patterns, the number of scales, and geography could all help differentiate between similar snakes. But, these things can be confusing, especially for snakes, which tend to be particularly variable in their color and patterns. Many snakes look alike, even species that are on different continents, because they evolved to live in similar habits and eat similar things. As a result, it can often be hard to tell them apart.

Broad-banded Copperhead,
Clinton and Charles Robertson.
    Instead of focusing on what their morphology is today, many biologists seeking to differentiate species now try to reveal their past evolutionary paths. If a group of organisms has had a different evolutionary path than another group of organisms, they might be considered different species. And, an effective way to explore evolutionary relationships is to identify the genetic make-up of animals over wide areas. If a group of organisms in one region has different genes than similar organisms in a different region, this is evidence that they can be considered different species.


    Dr. Burbrink gave an analogy: if aliens landed on our planet and could not tell apart all the different kinds of apes, they could take a blood sample from chimpanzees, gorillas, and us, characterize our genes, and see that we are quite different, even if that alien didn’t think so from looking at us. That’s basically what he did for the animal we have been calling the Cottonmouth. He found that there was very little gene flow between the two types of Cottonmouths and, separately, between the two types of Copperheads.

    Some scientists think this gene-focused approach is too quick to split one species into two but Dr. Burbrink told me that this was not actually the case. In fact, if there are two groups of animals and just one animal from each generation switches over and breeds with the other group, that’s enough to affect the gene pool such that the genetics of the two groups will be indistinguishable, in other words, they would be considered one species.

Florida Cottonmouth, Dirk Stevenson.
    But that’s not what Drs. Burbrink and Guiher found. Their analysis indicated that there was one species of Cottonmouth in the Florida peninsula, an animal they now call the Florida Cottonmouth (Agkistrodon conanti) and another species of Cottonmouth everywhere else, an animal they dubbed the Northern Cottonmouth (Agkistrodon piscivorus). Regarding the Copperhead, most of them are now called the Eastern Copperhead (Agkistrodon contortrix) and in Texas and Oklahoma (more or less) they are called the Broad-banded Copperhead (Agkistrodon laticinctus).  There are some differences in the patterns and coloration that distinguish these new species, but they look pretty similar, especially when young. There are also significant hybridization zones where the two different species of Cottonmouths and the two species of Copperheads interbreed.

    Now, you may be thinking to yourself, if the two species of Cottonmouths (or Copperheads) interbreed, how can they be different species?

Eastern Copperhead, Northern Georgia. Kevin Stohlgren.
    Thanks to the biology that we learned in high school, I would guess that most of us would instinctively say that if two animals can breed, they are the same species. And, if they can’t, they must be different species. In reality though, nature is a lot messier than that. Evolution is a process that is still going on right now and what we see today is just a snapshot in time. If two species started drifting away from each other a very long time ago, it is unlikely they can breed with each other and produce fertile offspring today. But what if they started branching off from each other relatively recently? In that case, maybe they still look alike, maybe they can still interbreed, but they can still be different enough to consider them different species. Besides, the ability to interbreed is not the end-all-be-all when describing different species. Dr. Burbrink brought up the following example: we have no idea if the different species of dinosaurs could interbreed, but we still feel confident saying that they are indeed different species. So, why use the ability to interbreed as the ultimate criterion when distinguishing between species that are still around?

    Dr. Burbrink was very generous with his time but I couldn’t let him off the phone without getting his answers to a few common questions I often hear about Cottonmouths.

Northern Cottonmouth, South Carolina. Kevin Stohlgren.
Have you ever been chased by a Cottonmouth?*

I’ve caught about 700 Cottonmouths but have never been chased by one. I’ve heard other people say they have though, a lot. I’ve had some swim by me and towards me and I could understand how others might interpret that. Human psychology is fascinating. Cottonmouths want two things in life, to eat and mate. People don’t fit into that equation at all. They have no interest in attacking people.

Some people believe that there are two kinds of co-occurring aquatic pitvipers in the southeastern United States. Is your research, which argues that there are two species of Cottonmouths, consistent with this idea?

No – the two kinds are separated geographically and that’s not the same thing as finding two kinds of viperids living in the same area.

What questions were raised during your research that you think would make for an interesting follow-up study?

There are lots of interesting questions to explore. For example, we don’t know how the behavior of the animals differ. Cottonmouths outside of Florida have to cope with relatively cold temperatures and that may result in some important differences regarding how the two species make it through the winter months. We also don’t know whether one species of Cottonmouth can tell whether another Cottonmouth is the same species or not, this might help us figure out if they preferentially breed with their own species. There has also been some exciting recent research about venom composition and how it may differ among animals within the same species.

Eastern Copperhead, North Carolina. Kevin Stohlgren.

Finally, is there anything else you think is important for the general public to know about your research?

The rate of habitat destruction in this country is very high. We have so much to learn about the biodiversity around us and amazing puzzles to figure out and we need to do it soon. Otherwise, we risk losing our heritage. Heritage is not just things like jazz and rock and roll, it includes things like our native animals. These species are a lot older and in much more danger than our other kinds of heritage and we don’t want to squander them.

*This interview has been condensed and edited.

Burbrink, F. T., & Guiher, T. J. (2015). Considering gene flow when using coalescent methods to delimit lineages of North American pitvipers of the genus Agkistrodon Zoological Journal of the Linnean Society, 173 (2), 505-526


This post is part of the #CrawliesConverge Reptile and Amphibian Blogging Network (RAmBlN) online event about reptile and amphibian evolution. Follow on Twitter or Facebook.

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