After traveling 600 miles southwest of Missouri, we had finally made it to the rolling red plains of Sandy Sanders in Oklahoma. Although it was bone dry, I was itching to find snakes after the winter months.. yet reptile activity on the whole seemed scarce. We meandered up and down the terrain until the sun had set, and while perusing the crumbly cliffsides, an unmistakeable rattle kicked up a few meters from me. Glancing behind my left shoulder, I saw an outstretched figure of a western diamondback (Crotalus atrox) slip down into the rock crevices, ungraceful but fortunately no loud plop from the fall. Nothing is quite so exciting as stumbling upon wildlife when you least expect it, especially when being alerted by such an astounding evolutionary innovation that is the rattlesnake rattle.
The rattle is not a hollow husk with little particles inside, but rather a series of interlocking segments that rub against one another to produce sound. Extending from the tip of the snake’s last fused vertebrae is a bony club-like projection that forms a mold for keratinous outgrowths added during shed cycles. The irregular surface of the modified tail provides attachment sites for muscle bundles capable of extremely high rates of contractions. Each developing rattle segment is shaped by epithelial tissues, fitting neatly against its adjacent segment with grooves that prevent them from becoming detached. However, the entire rattle structure does not retain blood vessels and nerves throughout. When the snake is in shed, the epithelial tissues are resorbed from the caudal end, concurrent with the emergence of a new rattle segment, which frees up any inflexibility at the rattle tip. Being completely dissociated from living tissues allows the rattle to vibrate effortlessly, buzzing like the stridulatory organs of many insects.
The sound of a rattlesnake rattling is an instantly recognizable deterrent. So much so that the acoustic displays are mimicked by burrowing owls to deceive mammalian predators. But not all animals are as easily confused. When confronted by a rattlesnake, ground squirrels use their keen hearing to assess the snake’s temperature and body size through the rattle frequency range and click rate. Warmer and larger snakes pose more formidable threats, and ground squirrels will pick up on the subtleties in sound to change their behaviors accordingly. It may be obvious that the rattle is used as an antipredator mechanism, but it’s evolutionary origin remains less clear.
During the incipient stages of a proto-rattle, two hypotheses have been put forth that may explain how selection has led to its current form. The first, and most intuitive, is that tail vibration and thrashing against the substrate served as a warning to predators. A more durable tail tip would then protect the tail from damage and also enhance the volume of the produced sound. A second explanation involves a shift in the rattle’s ecological role from prey capture to predator defense. Many vipers will use conspicuously colored tail tips as a lure for prey, wriggling it up in the air as they lie in ambush. A proto-rattle may have functioned in this way, bearing resemblance to an insect’s exoskeleton. This is similarly the case for an extraordinary genus of Iranian viper (Pseudocerastes) which attracts bird prey with a fringed tail that mimics a sun spider.. though keratinous structures have solely evolved in rattlesnakes and not the Viperinae. To evaluate the two evolutionary stories, behaviors of extant species with reduced rattles are often implicated, yet it’s important to understand that selection does not necessarily act the same way when a structure is elaborated versus atrophied over time. Support for either evolutionary story remains equivocal, and it’s plausible that during the rattle’s early formation, both antipredator and caudal luring behaviors shaped its evolution.
At dawn we began exploring the magnum gypsum hills, hopping from rocky bluffs to open grasslands and stands of redberry juniper (Juniperus pinchotii) and honey mesquite (Prosopis glandulosa). My eyes were peeled for exposed patches of rubble reminiscent of the rattlesnake den site we had encountered the night before. I called to Josh after spotting a promising area, but he was already there, standing over the skeleton of another diamondback. The head was completely removed, a strategy common in mammalian predators to handle these dangerous prey items. Throughout our time in Sandy Sanders, it was clear that coyotes were extremely abundant. We could hear them calling throughout the evening and night, and there was an inordinate amount of poop along the roads and animal trails. It’s hard to say whether a coyote had an altercation with this rattlesnake. However, the since the body of the snake remained intact (and uneaten), it could be that a person came along and killed the rattlesnake out of fear or malice.
Only a dozen meters from the dead rattlesnake, we were greeted by a stunning young male diamondback sunning itself at the highest plateau of the flat terrain. It didn’t seem to be too perturbed until we came too close, and it immediately arched its front half of the body, rattling away, as it retreated to a nearby crevice. After disappearing, I turned around to see a much larger individual moving slowly at the edge of the brush. My longing for the rattlesnake that escaped was soon replaced by excitement to observe this golden female. Having noticed our shift in attention, it similarly reared up in defense, but stood its ground firmly instead of fleeing. Pairs of male and female rattlesnakes are most often seen during March and April, when they court and mate prior to female ovulation in early summer and parturition before the fall. So it’s no coincidence that at both den sites we happened upon, one of each sex were in close proximity.
Moving from the upper strata of gray shale and gypsum to the lower red mudstones, I was curious whether these more erosion-prone gulleys still provided suitable denning habitat for rattlesnakes. Quickly we came up upon a diamondback entirely enveloped by vegetation, an exciting but also alarming find given the low visibility of the grassy area. The snake retreated as I veered close to get a better look, and I soon realized there was not one, but two snakes receding into the red crevices. I sat on the opposite side of the gulley for about 15 minutes and watched as one of the two rattlesnakes emerged cautiously, first poking its head out from under the shadows with great hesitation. Meanwhile, Josh seemed to be finding more rattlesnakes left and right, including an absolutely gorgeous female coiled in a perfect circle. As we ate a meal looking down on these slopes, we felt honored to visit rattlesnake dens in various kinds of geological terrain, and couldn’t help but think of the astonishing numbers of diamondbacks that must inhabit the rolling red plains.