by Greg Mayer

Last month I took a trip, and posted a “spot the _____” post for an animal I’d seen and photographed, but without giving any geographic location clues, so as to make identification more challenging. Despite this, many readers were not only able to spot it (not very hard), but also identify it: a cottonmouth. It is, more specifically a western cottonmouth (Agkistrodon piscivorus leucostoma), and I photographed it in the Cache River State Natural Area in southern Illinois. Here she is again, in a slightly different view. You can see she’s right in the middle of the footpath, and blends in fairly well with her background. By the time this photo was taken, she was pretty agitated and had reared up, so she stood out more.

Southern Illinois is very interesting from a natural history point of view, both geologically and biologically. The southern part of the state narrows to a point where the Mississippi and Ohio rivers join. A bit north of their confluence you find a set of hills crossing the state from west to east, called the Shawnee Hills or “Illinois Ozarks“. Illinois is otherwise exceedingly flat (in Wisconsin, Illinoians are know as “flatlanders”, and considering how flat Wisconsin is, that’s saying something), so the Hills are a distinctive landscape feature. Composed of Carboniferous sandstones and limestones from the mid-Paleozoic epicontinental sea that covered what is now the middle of the U.S., differential erosion has created many interesting land forms, including Camel Rock and Giant City.

Biologically, many southern species reach the northern limit of their range in southern Illinois. (Culturally, too– having driven several times from Chicago to the Gulf coast in Mississippi, I can attest that you enter the Bible Belt somewhere in southern Illinois, and leave it about ten miles north of the Gulf of Mexico.) In the Hills you get a high diversity of oaks (my favorite trees), but just south of the Hills you enter the flat Cache River country full of cypress and tupelo swamps, two species at the northernmost edge of their ranges. The Cache River itself is a remnant of an old bed of the Ohio River.

The giant old cypress above is said to be 1000 years old, with a basal circumference of 43 feet (13 m). A couple of buttresses from a second very large cypress can be seen at the right; that tree is known as the “Winnie the Pooh Tree“. Cypresses are known for having upward projections from their roots that stick above the water, known as “knees”; their function is unclear.

My wife and I spent one day hiking in these swampy areas in a few different places, and were well rewarded by our hike along the edge of Little Black Slough, a tupelo/cypress swamp.

We saw and photographed a bald eagle– it looked much better through binoculars!

After 3 days of hiking, our herpetological findings had been limited, just a few true frogs and painted turtles. Southern Illinois is the northern edge of the range for cottonmouths and copperheads, but I had begun to think the season was not far enough advanced to have a chance to see them. At 3:30 in the afternoon, when basking was likely to be done for the day even if the snakes were active, I thought our snake chances were over.

Just moments after the photograph above was taken, the photograph below was taken. My wife had stepped over or next to the snake. A pace behind her, I saw it—a thick bodied, banded snake, right next to the water—and I immediately thought “water snake or cottonmouth?” I changed my step in mid-stride, awkwardly hopping over the snake and turning to face it– no doubt, it was a  cottonmouth!

It too had turned to face me, and raised and pulled back its head, opening its mouth and exposing its “cotton” mouth. In the picture below, you can see several neat feature of the snake’s biology. First, is the aforementioned open-mouthed threat display, enhanced by the mouth color. Second, note the eye, divided into dark and light halves, that break up its identifiable shape, helping to camouflage the snake. Between the eye and the lip, note the white pit, which is an infrared sensitive structure which enables the snake to get a “thermal picture” of its environment, and the thing from which “pit vipers” get their name. Pit vipers can strike a warm object accurately using only their pits to locate the object. And finally, the fangs are in the long folds of the buccal mucosa alongside the lips on each side of the mouth. The fangs can pivot, and would swing forward out of the surrounding tissue when the snake actually tried to bite, either for predation or defense. (Click on these photos to enlarge them and see more details.)

I took these pictures with a Nikon camera I had gotten for Christmas, and was pleased to find you could actually see the blood vessels in the snake’s mouth, but I was too unfamiliar with it yet to figure out in the moment how to take video. So, my wife used her cell phone.

Note the vigorous “rattling” of the tail– many snakes do this, not just rattlesnakes. Against the substrate and dry leaves, this can be quite noisy, and in a cottonmouth acts as another threat beyond the white, open mouth display. Rattlesnakes have taken this further, with the loose fitting, keratinous, rattle scales at the tip of the tail capable of making a loud buzzing sound. In the video I mention the “quite long” tail to note the smooth tapering of the tail from the cloaca, indicating this is a female. Having said that, I immediately realized that to most people it would seem that the tail is shorter than they expected, so I then say it is a “fairly short” tail. The two statements are not contradictory– just pointing out different aspects of the snake’s tail: the tail shape showing its sex, but also that, as in most snakes, the tail is relatively short compared to the body length.

A natural history tour of this area of Illinois is well worthwhile, with many state and federal protected lands. The one criticism I would make is that in most areas the interpretive materials (i.e. signage) are sparse to lacking. There is, however, a fine, small museum run by the state of Illinois, the Barkhausen Cache River Wetlands Center, and I would recommend beginning a hiking tour with a couple or more hours there first.

Greene, H.W. 1997. Snakes: The Evolution of Mystery in Nature. University of California Press, Berkeley. (excellent general natural history of snakes that we’ve had occasion to recommend here before)

Smith, P.H. 1961. The amphibians and reptiles of Illinois. Illinois Natural History Survey Bulletin 28:1-298. (details on Illinois herpetofauna)