19. The Rattlesnake That Wasn’t: Mimicry in Nature

It was along a back road in rural Oklahoma that I received the greatest fright ever bestowed upon me by a snake. Strange to say having dealt with such lethal serpents as rattlesnakes, water moccasins, cobras, and kraits, but this fellow (or so I call it) was just an ordinary “ole” colubrid.

You may first rightly ask, what is a colubrid? A colubrid is simply a member of a certain taxonomic family (Colubridae) of snakes. About eighty percent of all the snake species in the world are colubrids. These are typically the common, non-venomous (with certain potentially deadly exceptions) snakes of the world. And yes, I know that the question of which snakes are venomous is a whole other topic for discussion. In our area of southwestern Indiana, typical colubrids would include the prairie kingsnake, garter snake, black rat snake, and northern water snake. These are snakes that simply don’t deserve the fear many people experience should they come upon one of them.

My tale begins along a remote, gravel backroad in Oklahoma. A summer road trip embarked upon with my graduate school advisor John Whitaker was the motive. Our destination was Texas A&M University in College Station. This was the site of the annual convention of the American Society of Mammalogists. It was a journey I had anticipated with much excitement as the gathering was sure to draw the top mammal scientists in the country. At that stage of my life, I couldn’t get enough of the research regarding such esoteric topics as the food habits of the short-tailed shrew, the territorial behavior of chipmunks, or the ecological relationships of woodland mice.

The trip down there was a jaunt of around nine hundred miles. We traveled in one of the venerable VW minibuses belonging to Indiana State University’s life science department.  Dr. Whitaker was not about to let all of the intervening territory between Indiana and Texas pass by without sampling its small mammal fauna. No barrier was too great to stop his quest for specimens for our collections. I once saw him dig a pocket gopher from its burrow with his bare hands. I’m not kidding. It was like watching the speed and power of the bionic man, Steve Austin, in the old TV series The Six Million Dollar Man. I also have a distinctly imprinted memory of him sprinting through the Texas scrub like an Olympian in pursuit of a fleeing nine-banded armadillo. The armadillo lost.

John was also the most intensely hard-working, diligent scientist with whom I’ve had the pleasure of associating. As a student, and as an adult, I’ve found myself to be alarmingly prone to bouts of daydreaming and flights of fantasy. Thus if one was to surreptitiously observe me, I might be found  idly sitting at my desk staring, with discouraging regularity,  off into space. In contrast, throughout the years I worked with Dr. Whitaker, I was amazed to never find him in a similar state of reverie. I know he must have done so. The devotion of time to contemplating, hypothesizing, or just plain thinking about things are necessary commitments for a scientist. Yet, I never walked into his office to find him idle. On every occasion, throughout the years, he would be busily examining mites beneath his microscope, engaged in typing the text of one of his hundreds of research papers, or pouring over the rough draft of a research project submitted by one of his graduate students. He simply had the most strenuous work ethic I’ve ever seen. And so, there were small mammals to be collected on the way to Texas.

Our routine was to drive an allotted number of hours and check into a motel. We would then cruise into the surrounding countryside to look for a place to set traps. John is a specialist in the arcane world of small mammals, shrews and mice. He is also one of the world’s leading experts on the food habits and parasites of bats. However the latter mammals require more specialized collecting equipment so, on this trip, it was mice and shrews. The animals we sought would be used to enrich the storehouse of mammal specimens in the vertebrate collections at Indiana State. The device we used for capture was the simple mouse trap baited with peanut butter. I suppose the biggest difference in our collecting with mouse traps and that which you might pursue in your house was a matter of numbers. Whereas you might be satisfied with two or three traps strategically placed along a baseboard or in a cabinet, John was a bit more industrious. He had brought along several hundred mouse traps and these we dutifully set each evening after finding lodging. Once the traps had been set, we would venture back to town for dinner. Having eaten, we would go back to the trap line, collect any specimens taken, and pick up our traps.

Back at the motel we would then check the mice and shrews for parasites, record standard measurements of the specimens, and then prepare study skins of the various specimens. This was done by removing the skin and fixing it over a cotton form we constructed which approximated the size and shape of the original animal. Left to dry, these museum specimens would then be ready for classroom or research use. Such specimens, properly protected from insect pests, will last for many decades. I suppose it must sound quite strange, to a non-scientist, to hear of two grown men engaged in such an abstruse pursuit.  However, I recall a pleasant blend of the camaraderie associated with field work and a sense of creative accomplishment in producing an artistically done museum specimen. I will freely admit, finding the world of shrews and mice fascinating to the point of infatuation is not everyone’s cup of tea. However, I have no doubt that it is through the varied interests and pursuits by scientists, of even the most obscure aspects of the natural world, that we eventually come to better understand the intricate workings of the natural world.

And so it was that, on this particular evening, I found myself walking along a road side ditch, in central Oklahoma, engaged in the setting of the three hundred or so mouse traps we had previously baited with peanut butter. John was walking ahead of me, removing mouse traps from a bag, and dropping one of them every foot or so. Walking as I was in the ditch, it was very handy for me to reach down and pick up a trap lying on the berm above the ditch. I had managed to set a dozen or so traps when I reached down into the grass to retrieve the next one. Suddenly, I was greeted by the one sound in nature that is guaranteed to instantly send one’s heart into overdrive. From the grass directly at my face came the very distinct sound of an exceedingly unhappy rattlesnake.

My heart leaped into my throat and began to gallop at a pace which was palpable. I couldn’t see the snake lurking in the grass before me, but it couldn’t have been more than an arm’s length away. The sound was too close, too loud, and too distinct to be otherwise. My mind raced through the possible reactions I should take. Running didn’t seem like a good option; I thought a sudden movement might provoke a strike. Another possibility was mistaking the exact location of the snake and blundering into during a precipitous flight. It seemed the best course of action was to remain still, try to visually locate the snake, and hope that it wasn’t feeling overly aggressive today. All the while, I mentally ran through a catalog of the possible rattlesnake species that I might be confronting. Western diamondback and prairie rattler seemed the two prospects.

While all these thoughts were rapidly presenting themselves, the serpent began to reveal itself. The tuft of grass in which it had hidden began to slowly rise, as though being inflated by some invisible pumping mechanism. Now I could begin to make out the coils of a very large snake (too big for a prairie rattlesnake I thought) emerging from the grass. A background color of brownish-yellow was revealed and upon this was imprinted a series of very dark brown dorsal blotches. They were not really diamond-shaped, more rectangular, and my fear began to subside somewhat.  But the rattling sound continued unabated; I was still puzzled. At last the serpent’s head came into view, perched on the end of a wicked looking, rattlesnake-like “ S-loop” at the front of the body. Finally the mystery, at least to the snake’s identity, was revealed. Running from the eye to the angle of the jaw was a quite distinct dark band bordered by a yellowish band above it. The snake’s head was also familiar to me. I experienced an instance of what biologists often refer to as “gestalt”. This is the presence of a mental image, often difficult to describe to a neophyte, which allows one to recognize a given type of organism.  There was a certain conformity of head shape and scale pattern which clearly said bullsnake. I wasn’t in a life-threatening situation after all – whew!

The bullsnake lying before was nevertheless an impressive animal. Not many snakes in the United States are larger than a mature bullsnake. The record length for this species is nearly eight and a half feet. Coupled with their size is a propensity to act aggressively when disturbed.  I once narrowly avoided being bitten in the face by a grumpy bullsnake that launched a strike from what seemed to be an impossibly distant reach. But the rattling sound being produced by the snake here in my presence continued to intrigue me. It was so nearly identical to the sound of a large

rattlesnake that I could hardly believe it was issuing forth from this individual. It is extremely common for snakes of many species to vibrate their tail when agitated. If this is done against dry leaves or grasses, the sound can be remarkably similar to that of a rattler. However, my bullsnake was not doing that. As I watched it more intently, I noticed that this rattler-like sound seemed to coincide with a very noticeable exhalation of air. Of course many snakes hiss as part of their aposematic display. But this was no hiss; it was a rattlesnake rattle. I continued, with now normal heart rate, down the trap line still pondering how the snake was making this sound.

Later, back in Terre Haute, I delved into the bullsnake’s secret. When snakes hiss they forcefully expel air from their glottis. The glottis is basically the opening into their trachea or windpipe if you will. We have one too of course. Ours lies in the back of our throat. In snakes, the glottis is extended forward to open as a tube on the floor of the mouth just behind the lower, front teeth. This adaptation allows them to breathe while their mouth is stuffed full of a large prey item during the swallowing process (which may take many minutes). I found that, in bullsnakes, there is a flap of soft tissue at the end of the glottis which vibrates back and forth as they expel air from the lung. This flapping imitates the sound of a rattlesnake rattle to an amazing degree. It is easy to see how the evolution of such structure and behavior in the bullsnake would be adaptive. Adaptive is a fancy way biologists have of saying that an evolved characteristic is beneficial to an organism.  Predators or grazers which have learned by experience to avoid the dangerous rattlesnake species would certainly be put off by the excellent mimicry of the bullsnake.

Thus it was, once again, that a chance encounter with a denizen of the wild led me down an unexpected yet rewarding learning path. The bullsnake who would be a rattler became not just another coincidental animal meeting, but a guide who opened the door to yet another secret niche lying within the wondrously biodiverse world in which we all live.

Additional Notes Regarding Mimicry:

Mimicry takes many forms within the animal kingdom. The harmless scarlet kingsnake has a color pattern which looks much like that of the dangerously venomous coral snake. Hover flies are easily mistaken for bees by novice observers. Mimicry in which a harmless or edible species resembles a harmful or distasteful one is called Batesian mimicry. The name honors  the nineteenth century naturalist Walter Henry Bates. In other cases, such as heliconid butterflies, several bad tasting (to birds) species resemble one another so all achieve a collective protection. This is an example of Muellerian mimicry. Sometimes mimicry can be achieved through structure or color. Technically, this might be better categorized as having cryptic structure or color. For example, certain lepidopteran caterpillars resemble the droppings of birds. Others, should the branch upon which they are climbing be jostled, will suddenly stiffen and rear up at an angle. Thus posed, they look remarkably like the petiole from which a leaf has broken. There are butterflies in the tropics which resemble dead leaves, complete with what appear to be holes formed by the action of fungi. Katydids often bear extraordinary resemblance to living leaves. Walking stick insects look, of course, like the twigs upon which they clamber about. The natural world is replete with examples of mimicry, protective resemblance, camouflaging coloration, and behavioral copycats.

Photos by the author.