Monday, July 22, 2013

Readers Write In: Are Water Moccasins and Cottonmouths Different Species?


I have recently heard from two independent sources that a cottonmouth and a water moccasin are "in fact" two different snakes. Both stated that coloration, body size/shape, habitat, and swimming behavior were the differentiating traits. I was taken aback by both accounts, never before hearing this in any of my herpetology classes or seeing this for myself in the field. Afterwards, my first thoughts were "Are Agkistrodon piscivorous (Cottonmouths) sexually dimorphic?"...and "Are they confusing a particular non-venomous water-snake for the other?     

Kevin P.
Mississippi 

      It is generally well understood that all species that have been officially discovered, recognized, and described are given a scientific name. These scientific names are made up of two words representing their genus and species. For example, the scientific name of the Cottonmouth is Agkistrodon piscivorus. This name tells us that Cottonmouths are in the genus Agkistrodon (along with some other closely related snakes) and they are the species piscivorus (a name which they don’t share with anything else in that genus).

    The scientific name of the Copperhead is Agkistrodon contortrix. This tells us that Copperheads and Cottonmouths are in the same genus (i.e., they are closely related) but they are different species.

    Common names, on the other hand, are a different story because there are not really any hard and fast rules about them. When I say common names, I mean the words we use to refer to a species in casual conversation. Both Copperhead and Cottonmouth are common names. You can imagine that depending on where you are and because there are no rules about common names, different people might refer to the same species with different common names. Or two different species with the same common name. For example, what comes to mind when I say black snake? Depending on where you are from, you may be imagining a Black Racer (Coluber constrictor), a Black Ratsnake (Pantherophis alleghaniensis) or even an Indigo Snake (Drymarchon couperi). 

    You can see that these things can get tricky. We even recently discussed in the comments that a blog post just on local and colloquial names might be worthwhile, so I won’t go into much detail about them now. But many of them are quite colorful, like Swamp Lion (apparently this is sometimes used to refer to Cottonmouths).

    In any case, it makes it quite difficult to answer technical questions about common names. That said, species do have standard common names and no species has water moccasin as its standard name. I wrote to Kevin that if people say water moccasin when referring to a venomous snake that lives in the water they are thinking of the Cottonmouth and they are the same species. If people say water moccasin and are referring to a non-venomous snake that lives in the water, they are actually thinking of a watersnake within the genus Nerodia (not a Cottonmouth). I touch on this a little bit on a previous post and include the relevant information here:

    Before we examine how to accurately differentiate between venomous Cottonmouths and non-venomous water snakes, let's first address their names. A Cottonmouth's scientific name (Agkistrodon piscivorus) cannot be mistaken with anything else, but there aren't rules regarding what other names it can be called. For example, Cottonmouths are often referred to as water moccasins. It's not entirely accurate to call other snakes moccasins, but many do. Much confusion can be averted by using the word moccasin only when speaking of the venomous Cottonmouth.

    I thought this would answer Kevin's question, but he responded:

These people seemed very adamant that they were describing two different, but co-occurring venomous snakes.


    Oh.

    Well, that is a different question, one that I can't attribute to people simply being a little loose with terminology. 

    There are not two different aquatic and venomous pit viper species inhabiting the same wetlands in the southeastern United States. There is only the Cottonmouth and various non-venomous watersnakes. The name water moccasin is most often used to refer to the Cottonmouth and less often (and mistakenly, I would argue) used to refer to these other non-venomous snakes. It is not a separate species (here is where I need to add the disclaimer that I base this statement on the fact that there has never been any scientific evidence ever presented that would suggest there is a species in the southeastern United States similar to the Cottonmouth and that shares the same habitats but is actually a different species. Based on this e-mail, there is apparently some local knowledge that would suggest that there are two aquatic pit vipers).*

    I tried to imagine how a Cottonmouth could be considered two different species. It is true that the species exhibits sexual dimorphism (males are different than females) but this difference is just in body size, not enough to mistake them for two species.

   When they are young, Cottonmouths have a bright yellow or green tail that they use to attract prey. Adults do not. Young snakes are also often more brightly colored than adults. In fact, I once saw a young Cottonmouth that was so bright and distinctly patterned that it took a long time to convince myself that I wasn't actually looking at a Copperhead. Perhaps the differences between young and adult Cottonmouths are significant enough to convince some people that they are actually different species.

    Has anyone heard this particular tale before, that water moccasins and Cottonmouths are two separate (but similar) venomous snakes?







* I have to clarify this statement somewhat. There is genetic evidence that there are two different "types" of Cottonmouths. Specifically, Cottonmouths in peninsular Florida are genetically distinct from Cottonmouths elsewhere (the citation for the paper on the subject is below). Whether or not this means they are separate species has not been decided. If they were, they'd probably be called the Northern Cottonmouth and the Southern Cottonmouth, not the Cottonmouth and the Water Moccasin. Because this topic is not directly relevant to the question of whether there are two kinds of Cottonmouths in the same area, I did not discuss it above.

Guiher TJ, & Burbrink FT (2008). Demographic and phylogeographic histories of two venomous North American snakes of the genus Agkistrodon. Molecular phylogenetics and evolution, 48 (2), 543-53 PMID: 18539486

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