A SERPENT'S TAIL
Those of us from certain parts of North America tend to take rattlesnakes for granted, rarely bothering to appreciate how fantastic they really are. They comprise about 50 species, in two unique American pitviper genera, all with tails that are tipped with a series of complex, interlocking, cornified scales, completely unlike anything else known to have been evolved by snakes—until very recently, anyway. These reptiles are not only specialized at their very tips; the musculature of the tail itself is dominated by three pairs of “shaker” muscles, two of which produce lateral, back-and-forth movements, while the third pair applies torsion, drawing the ventral edge of the rattle outward to either side. The fibers of these muscles are rich in mitochondria, sarcoplasmic reticula, capillaries and glycogen, and capable of sustaining the high respiratory levels necessary to vibrate the tail as rapidly as 100 Hz. for as long as an hour at a time. These speeds are comparable to the oscillations of sphingid moth wings. Among vertebrates, only the hummingbirds (family Trochilidae) can vie with the rattlesnakes in this respect.The rattling system's main function is to warn away dangerous animals like predators and large grazing animals, although in some of the small Sistrurus species, it is only audible at close range, and appears to be of little use in this area. I've never witnessed a wild ungulate or carnivore interacting with a rattlesnake, but have many times seen how effective rattling is in deterring domestic analogs like dogs and horses. Whatever the first proto-rattlers used their tails for, they probably enhanced an already existent behavior. Similar tail movements are exhibited in snakes of many species, in many unrelated taxa. Tail-thrashing of various forms can be a prelude to battle or mating, or a means of evading predators. Some fossorial boids like Calabar Pythons (Charina=Calabaria reinhardtii) and Rubber Boas (C. bottae) wave their blunt tails about while hiding their heads (see photograph here). Some elapids, like the Langsdorff's Coralsnake (Micrurus langsdorffii) pictured above, confuse the enemy by moving both ends simultaneously. Many snakes, including some vipers, vibrate the tail defensively. When doing so against dry vegetation, the resulting sound is not unlike a rattler's. Defensive tail-shaking colubrids, like the Common Racer (Coluber constrictor), lack the specialized tail musculature, and cannot sustain the motion more than several seconds, but the tail muscles of the Copperhead (Agkistrodon contortrix), a close cousin of the rattlesnake, have a significantly elevated respiratory capacity. The traditional view of rattler evolution posits that rattles evolved to enhance this behavior, and, since the earliest-known rattlesnake fossils were found in the American Great Plains, it's tempting to visualize the first rattler warding off vast bison herds like in the painting up top. Genetic mapping, though, strongly suggests that rattlesnakes first evolved in America's southeast, severely shaking this attractive theory.
A third form of tail movement is caudal luring (see video here), a not uncommon behavior in vipers and a number of another snake taxa. It is possible that the earliest rattlers drew potential prey within striking distance by writhing and twitching a simple rattling tail appendage. Caudal luring is practiced by some of the earlier mentioned Sistrurus rattlesnakes, particularly the young ones; in fact, in many of the Crotalus spp. as well, the rattle could function more as a caudal lure in neonates, which can't produce sounds until their first shed. Concurrent with the young snakes' diet shift from ectotherms to mammals is the rattle's increased effectiveness as a sounding device, and the fading of bold colors and patterns on the tail. Both caudal luring and defensive tail-shaking are behaviors seen in the Copperhead.
A new species of viper sheds a bit of new light on the subject. Pseudocerastes urarachnoides was described just over a year ago, from two specimens collected in Iran (a pdf is available here). The holotype, an adult male (above, top), was collected in 1968 and deposited in Chicago's Field Museum. At the time, it was identified as a Persian Horned Viper (P. persicus), but its tail bore a strange appendage resembling a small solifugid (below). This was assumed to be a tumor or other aberration, until a second specimen, a young male (above, lower), was collected in 2003.
The tails of both specimens were carefully examined, and confirmed to be normal and uninjured. The assumption is that these structures are caudal luring devices, although so far, nothing is known of the species' behavior, and caudal luring has not to my knowledge been observed in the other two Pseudocerastes spp (--update--a video of the tail appendage can be seen here. The stomach of the paratype contains a partially digested, unidentified passerine bird. Latifi's Snakes of Iran lists P. persicus' diet as consisting of mice and lizards. Since the female P. urarachnoides is unknown, it's anybody's guess whether or not this tail appendage is a sexual characteristic. Both specimens were preserved in formalin, and their tissues were deemed to be unsuitable for molecular analysis. We can only hope that live specimens will be found and observed. If these hopes are realized, surely they will give us insight into the evolution of their distant cousins on the other side of the northern hemisphere, as well as into the nature of all life.
upper: PRAIRIE SENTINEL--PRAIRIE RATTLESNAKE (2002) acrylic 40" x 15"
middle: LANGSDORFF'S CORALSNAKE AND BLUE-CROWNED MOTMOT (2008) acrylic 15" x 20"
lower two: Photos of P. urarachnoides taken from BOSTANCHI et al : NEW SPECIES OF PSEUDOCERASTES FROM IRAN. Proceedings of the California Academy of Sciences, ser 4, 57(14): 443-450 figs 1-4, 8-9