Tuesday, December 3, 2013

The Proof is in the Field Notes: Cottonmouths Rarely Bask in Trees --Guest Post--

    Some observations come before you realize how important they are. Only later do you slap your forehead and realize that you should have taken more detailed notes, because the likelihood of you seeing such a thing again is slim. This is why I encourage everybody interested in wildlife to take down field notes. 

    When I was in high school, every afternoon when I got home I would take a canoe out behind my parent’s house, which backed up to a diverse swamp forest. This was the floodplain of the upper headwaters of the Flint River—one of Georgia’s least polluted and least dammed rivers, despite its proximity to Atlanta. Part of the reason why it’s not terribly polluted is because of these swamps near its headwaters. The beavers have had their way there, constructing natural dams to block nearly every tributary, and the swamps and marshes created by the dams naturally filter the filthy runoff and occasional bursts of raw sewage that overflow Atlanta’s primitive and downright criminal water treatment system. What comes out below the headwaters in Clayton and Fayette Counties is as clean a stream as you’re likely to find in this region of Georgia. 

Photo by Kerry Nelson.
    These swamps are also loaded with Cottonmouths. There are also four species of watersnakes, and I will admit that before I became familiar with how to tell the species apart from a distance, I thought all the watersnakes were Cottonmouths. 

On the right, a Brown Watersnake (Nerodia taxispilota) basking in a branch overhanging a stream in Georgia. These are heavy-bodied, non-venomous watersnakes frequently mistaken for Cottonmouths.

    I kept up my visits to these swamps while I was an undergraduate living with my parents, something that was not considered very cool during the booming economy of the 90s, but which has since mysteriously come back into fashion. Anyhow, I made a key observation on Cottonmouths back in 1996. 

Photo by Sean Graham.
    During a flood I observed a group of cottonmouths basking in shrubs about 1 m above the water’s surface. Now, this forest floods a lot, so it wasn’t an unusually massive flood that triggered this behavior. However, it was pretty early in the season, and the snakes were hanging out near one of their usual hibernation sites—a small hill and rock outcrop out in the floodplain. The hill is usually just high enough to prevent flooding, but this time it was submerged. I assumed the snakes were caught off guard and took refuge about a meter up into a shrub rather than meet the cold floodwaters their usual way. 

Here, some Cottonmouths (Agkistrodon piscivorus) hanging in a shrub during an early spring flood. There are two in this picture. Can you see them both?

    I have accumulated a lot of experience studying Cottonmouths, but since this early observation and I have rarely seen Cottonmouths perched in shrubs or trees near the water’s edge. I began to realize these observations contradict the legend of the Cottonmouth that hangs in shrubs at the water’s edge and falls into boats. I usually hear this story any time I tell people what I’m doing out in the swamp, when I pass people while paddling my canoe, or when I encounter people at bridge crossings over creeks during salamander surveys. It’s usually a warning. I have heard this myth nearly as often as the myth that Cottonmouths are aggressive

    When I first read a paper written by Whit Gibbons and Mike Dorcas about Cottonmouth aggressive behavior (citation at the end of the post), I slapped my forehead and thought to myself, “How stupid that nobody has done a study like that before!” These guys studied the behavioral responses of Cottonmouths to humans, and provided evidence that they are nowhere as sinister as most people think, and most of the time they try to escape when confronted by humans (in this case, the researchers themselves). Even when they picked up the snakes with an ingenious phony arm, the snakes rarely tried to bite. So much for Cottonmouths being aggressive. Most people familiar with the snake knew this already, but the genius here was that they actually went through the trouble to design an experiment to prove it. 

Photo by Kerry Nelson.
    At some point years after reading their paper, and having studied the Cottonmouth for nearly a decade, I had the bright idea of similarly tackling the “Cottonmouths in trees” myth. Most people familiar with the Cottonmouth will tell you that a good way to tell them apart from nonvenomous watersnakes is that Cottonmouths rarely, if ever, bask in trees near the water’s edge, whereas watersnakes do this frequently. Since most people can’t tell the difference between a nonvenomous water snake and a Cottonmouth (neither could I at first!), folks just assume all those watersnakes plunking into the river are Cottonmouths, just like I did when I was a teenager. Observations of watersnakes doing this surely spawned the legend.

A Queen Snake (Regina septemvitatta) basking in a branch. Most members of the live-bearing natricine snake radiation are prone to spring basking in vegetation at the water’s edge.

    Most people familiar with Cottonmouths do occasionally see them perched in trees, but of course you’re more likely to remember the odd observation than the usual situation, which may give you a false impression of how common it is. But how frequently does it happen? Fortunately, I took detailed notes of the position of each snake that I encountered during both my Master’s and PhD research, which I got in the habit of doing early on. For almost every snake I saw, I estimated where it was vertically, horizontally (in relation to the water’s edge), and noted its posture (coiled, in an ambush pose, etc.). This allowed me to not only estimate how frequently I saw them perched in a bush near the water, but also to provide the critical information on how frequently they don’t do that, to put it in perspective. 

Photo by Kerry Nelson.
    The dataset revealed some interesting patterns, the most obvious of which was that Cottonmouths very rarely climb onto slender branches and hang there like watersnakes. Only two of 804 Cottonmouths (< 1%) exhibited this behavior. Instead, about 24% of the snakes were in aquatic habitats (open water, aquatic vegetation, etc.), 43% were on elevated substrates (on hummocks, logs, or piles of driftwood—lying there high and dry but not hanging in vegetation like a watersnake), and 33% were on terrestrial substrates (mud, banks, etc.). 

A Cottonmouth in a more typical situation: in an ambush posture in shallow water at the water’s edge, in this case, facing the terrestrial environment. They are also frequently seen terrestrially facing the other way.

    Whether in water or on land, most of these snakes were within a meter of the water’s edge, the valuable real estate Cottonmouths use for hunting. Here they can zap any creature that comes to the water’s edge for any reason—fish, frogs, birds, mammals, you name it—and Cottonmouths feed on nearly anything they can get their heads around, alive or dead. 

    So, case closed, right? Well, not really. I had to address potential biases in my observations. For the most part I waded through the swamp looking for Cottonmouths, so if I became accustomed to seeing them in certain places, wouldn’t I stop looking in unlikely places, which would potentially bias my observations toward certain sites? The best way to eliminate a bias like this is to spend an equal amount of time searching in all habitat categories, something I did not do. I was able to suggest that I did at least try to look in trees and shrubs along the water’s edge, because during one of my studies watersnakes were also captured for part of the research. I also argued that, with my experience, I would have a better chance of seeing a Cottonmouth in a tree than an average member of the public. So if I only saw 2 out of 804 in trees, an average Joe would probably see even fewer. 

Photo by Kerry Nelson.
    I figured my paper might cause the same reaction on people that Gibbons and Dorcas’ Cottonmouth aggression paper had on me. I assumed a lot of people would slap their heads and think to themselves, “how stupid of me not to think of this!” I kind of also expected dozens of people to come out of the wood work to tell me about their observations about a Cottonmouth in a tree. The paper has certainly generated a lot of interest, probably more than any other paper I’ve written. I’ve gotten emails from total strangers applauding the paper, and of course some people have told me their data show more frequent aerial basking than I showed. A researcher in Louisiana wrote that aerial basking in Cottonnmouths occurs at his study area in as high a frequency as 3.8%. Watch out!

Above, a Cottonmouth in a tree. The photographer was sure to point out on his Facebook post about this observation that it was unusual.

Photo by Mark Herr.
Sean Graham is currently a professor of Biology at the University of Findlay in Ohio. He received his PhD from Auburn University in the same lab as David Steen. His Master’s and PhD research focused on the behavior, reproductive physiology, and immunology of the Cottonmouth. He welcomes you to share your Cottonmouth stories in the Comments below. 

Further Reading:

S. P. Graham (2013). How frequently do Cottonmouths (Agkistrodon piscivorus) bask in trees? Journal of Herpetology, 47, 428-431 DOI: 10.1670/12-082

Gibbons, J. W., and M. E. Dorcas. 2002. Defensive behavior of cottonmouths (Agkistrodon piscivorus) toward humans. Copeia 200(2002): 195-198.

Related Posts:

View the entire Cottonmouth Myth series.

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