Western Diamondback Rattlesnake (Crotalus atrox)

     Possibly one of the most enigmatic reptiles to reside in the lone star state. The Western diamondback rattlesnake (Crotalus atrox) is also one of the most misunderstood of Texas' reptiles. "Cold-blooded," "calculating," and "honery" are some of the attributes credited to this reptile. Yet, despite this negative portrayal, it has become a hallmark symbol of the rugged west.  Also, recent research into the natural history and ecology of rattlesnakes has yielded interesting glimpses into the complex lives led by these fascinating reptiles.

Habitat
     Crotalus atrox can be found in a variety of habitats including grasslands, scrub oak-juniper forest, rolling plains, thorn scrub, rocky canyons and outcroppings, and deserts. Trash piles and junk yards can often provide refuge for many vermin such as rats and mice. Sites such as these in rural settings may attract rattlesnakes and other snakes as well.

Distribution
     Western diamondback rattlesnakes occupy a geographic range that includes western Arkansas, eastern and south-central Oklahoma, Texas (except the extreme east and northern panhandle), central and southern New Mexico and Arizona, extreme southern Nevada, southern California, and southward into Mexico to extreme northeastern Baja California and northern Sinaloa in the west, and northern Veracruz, Hidalgo and Querétaro in the east. Most of its range lies below 1,500 m. (Ernst, 1992). However, in San Luis Potosí, Mexico this species has been found at elevations of 2,438 m (Werler and Dixon, 2000). The southernmost distribution for the western diamondback rattlesnake is northwest of Tehuantepec, Oaxaca, Mexico (Campbell and Lamar, 2004).

Activity
     The activity patterns of the western diamondback rattlesnake is similar to many other snakes. C. atrox has been noted to be active as early as March 9 (Minton, 1958). In 1998 I found specimens at night while crossing the road in Cameron, Hidalgo, and Willacy counties in south Texas as late as December 23. Like other reptiles, the activity of C. atrox is largely influenced by temperature. During the spring months of April to May when mild temperatures prevail, these snakes are active during the day. Their diurnal activities come to a halt in June through August during the oppressive heat of the summer. During these months, activity is largely limited to early mornings and late evenings when there is some relief from the heat. The cooler temperatures of September seem to also entice some specimens into diurnal activity. However, this activity is not as prevalent as that during the spring.

     A communal den site is used for shelter during the winter. Den sites are typically found within southerly facing rocky outcroppings. The winter dormancy may be periodically interrupted during periods of warm weather. This was evident during a night drive in Llano county in mid November of 1998 as a small male specimen was found crossing the road at night. The ambient air temperature at the time of the sighting was 48-50° Fahrenheit. Denning is also advantageous as there is a good chance of males mating with females as they leave the hibernacula.

Size
    The largest reported measurement for a western diamondback rattlesnake is 92.5 inches (Jones, 1997). Interestingly the second largest specimen to be measured was found in Cedar Hill, Texas and measured 92 inches. (Curtis, 1949).   While gigantically proportioned specimens such as these are unlikely to be found in the wild again near Dallas, specimens measuring more than five feet in overall length are still found in southwestern Dallas County. The average size for adult specimens is between three to four feet.

Description
     The base coloration of the western diamondback can vary from location to location. This is due to genetic expression from different populations. The basal color can range from brick red, buff pink, straw-yellow, beige, brownish and light gray. Boldly marked chevrons form an interlacing rhombic diamond-like pattern down the middle of the snake's back, thus affording this species its' common name. Often these markings are outlined in black and white. The head is broad and two white diagonal lines are present on either side of the eye extending to the supra labial scales.

     Another common name used in reference to this species is "coon-tail rattler." This moniker is derived from the white tail with encircling black bands. Although some specimens bear different colors no subspecies are recognized. While crossing the road, rattlesnakes hold their head and tail up at roughly a 45° angle. Although this may not always be the case, it is a useful means to help identify a snake as a rattlesnake while even at a considerable distance.

Similar Non-Venomous Species
      Possibly the most crucial component for positively identifying a snake as a rattlesnake is the presence of a rattle. This may seem as quite an elementary component, yet I am compelled to include this feature as many inexperienced people confuse rat snakes (Elaphe sp.) and bull snakes (Pituophis sp.) for rattlesnakes. When alarmed, either species will rapidly vibrate their tails. Should the vibrating tail contact loose leaves or dried vegetation the sound more closely resembles that of a rattlesnake.

     The best imitation of a rattlesnake by a non-venomous species is the performance provided by the bull snake (Pituophis melanoleucus sayi). These large and heavy bodied serpents share their range and habitat with rattlesnakes. Whenever frightened, bull snakes will vibrate their tails, but their talent for creating sound while forcefully expelling air is the most convincing. In many specimens, this noise resembles the sound of a rattlesnake so closely that the two species can be difficult to differentiate based on sound alone.

Reproduction
     Males achieve a larger size than females. During the spring males leave their hibernation sites and begin searching for food and mates.  Occasionally this leads to an encounter with another male which often results in a bout of ritualistic combat. Often time, the two contestants engage in this behavior under the attention of a nearby female. The males intertwine the anterior portions of their bodies while spiraling upward. The victor is determined as the male who forces his opponent to the ground. This spiraling demonstration often results in both snakes toppling over and continuing to wrestle on the ground.

     Alas, to the victor goes the spoils. The winner of these 'wrestling matches' usually receives mating privileges with the nearby female. Courtship involves the male actively tongue flicking the dorsal surface of the female's body. As the courtship ensues, the male's head begins to move to and fro in a jerky yet rhythmic motion. As the courtship continues, the male aligns his cloaca with the female's. Should his intentions be undesired, she will shove the male aside by using an arch of her coils. Females give birth in late September to early October to 9 to 14 young measuring 9 to 13.5 inches (Tennant, 1998). In some populations, females only reproduce every other year.

Diet
     Western diamondback rattlesnakes prey chiefly upon mammals such as mice, rats, cottontail rabbits, chipmunks, and squirrels. C. atrox will actively forage for prey as well as resort to ambush tactics. When preparing for an ambush, C. atrox lies quietly beside a frequently used rodent trail. At such times it may lay a coil over its rattle, perhaps to muffle it (Thayer, 1988).  Obviously, this species can be of great benefit to man by consuming destructive rodents. Rodents that are known to carry the plague and hantavirus occur in portions of the western United States including areas near Dallas and Fort Worth. These life threatening illnesses are additional reasons why this snake should not be killed when encountered in the wild.

Conclusion
      The western diamondback rattlesnake is the most widespread venomous snake in the state of Texas. It is a highly popular species in zoological displays across North America and Europe. Although its' venom is quite potent few deaths are reported and medical attention can result in a positive outcome for the bite victim.  Because of their value as rodent destroyers, rattlesnakes should not be killed.  The fact, that this species occurs close to suburban areas in Dallas County and the extremely low occurrence of snake bite suggest that this reptile poses a very low risk to the health of the citizens in the Dallas Fort Worth area.

          Literature cited

• Campbell, J. A. and W. W. Lamar. 2004. Venomous Reptiles of the Western Hemisphere. Volume 2. Cornell University Press pp. 477-870
• Curtis, L. 1949. The Snakes of Dallas County, Texas. Field and Laboratory. 18:1:13
• Ernst, C. H. 1992. Venomous Reptiles of North America. Smithsonian Institution Press. Washington and London. 236pp.
• Minton, S. A., Jr. 1958. Observations on amphibians and reptiles of the Big Bend Region of Texas. Southwestern Naturalist 3:28-54.
• Tennant, A, 1998. A Field Guide to Texas Snakes. Gulf Publishing. 291pp.
• Thayer, F. D, 1988. Crotalus atrox (Western Diamondback Rattlesnake). Hunting Behavior. Herpetological Review 19:35
• Werler, J. E. and J.R. Dixon, 2000. Texas Snakes. University of Texas Press. Austin, Texas. 437pp.

Young adult specimen eating a road killed rodent. Texas: Menard County       An active adult male on a rocky ledge. Texas: Val Verde County

Photograph by Carl J. Franklin                                                                                        Photograph by Carl J. Franklin

A female specimen avoiding the hot Texas sun in the shade of a Mesquite. Texas: Jim Hogg County

Photograph by Carl J. Franklin

Texas: Dallas County: Cedar Hill. 

Photograph by K. C. Rudy

Western diamondback rattlesnakes are abundant at Cedar Hill State Park. The second largest measured western diamond back rattlesnake is a 92 inch specimen captured in Cedar Hill, Texas in 1949 another specimen beat the Cedar Hill record by only half an inch.