Western Diamondback Rattlesnake
The Western Diamondback Rattlesnake (WDR), Crotalus atrox is a member of the family Viperidae (vipers) and the subfamily Crotalinae (pit vipers). The body tends to be bulky and moderate in length in Arizona. Size records of over 210 cm (seven feet) and mass of 6.5 kilograms (14 pounds) have been reported in the eastern part of its range, but in the Tucson area, anything over 140 cm, and/or one kilogram would be a remarkable specimen. The adult females are often in the range of 80 to 90 cm, the males are a bit larger and reach at least 100 cm (1 meter) with mass averaging 350 and 450 grams.
As currently understood the Western Diamondback ranges from western Arkansas, west to Oklahoma, south through central Texas, and westward to Southern California, and southward again to central Mexico. In Arizona, an imaginary line drawn from Lake Mead across the southern edge of the Mogollon Rim would roughly encompass its range. However, I suspect that at some point the WDR may be divided into two or more species. At least one eastern species and one western species.
The WDR inhabits sand dunes near sea level to grasslands and oak juniper woodlands at over 2000 meters (6000 feet). As such, it is a habitat generalist that adapts to many variables within its environment.
Its coloration shows considerable variability from one location to another. The dorsum coloration can range from light silver with dark gray blotches to a light tan with darker brown blotches. Its blotches are outlined by dashed white lines, sometimes forming the shape of a diamond—which has earned this snake its name. At least locally, WDRs can change its coloration to match the soil and rocks it lives on but it can also change color slightly with temperature changes, going from brownish when cold to grayish hues when warm. Its triangular-shaped head has a pale band that arcs from nostril to upper labials, with a similar but narrower band behind the eye. The belly coloration runs from cream to dull yellow. The tail is symmetrically encircled with white and black bands of near-equal width, and, of course, ends with a rattle.
As with all members of Viperidae, WDRs are equipped with needle sharp fangs with which to envenomate its prey—or would-be predators. The fangs fold against the roof of the mouth when not in operation, and both swing and flare outward when the snake delivers a strike. The large size of this species equates to large volumes of venom being injected from glands on either side of the head through the hypodermic needle-like fangs. In the United States, this species is responsible for more venomous snake bites than any other snake. Prey items are mainly, but not limited to, small rodents, rabbits, and lizards. These are actively hunted day or night, (depending on season and weather factors), by utilizing their extraordinary sense of smell to locate prey. It then coils in close proximity to the desired area in ambush posture, and waits for the rodent or lizards to wander within striking range. Like all pit vipers, WDRs have yet another offensive sensory system a — a heat-seeking cavities between nostril and eye on each side of its face. These pits are the most sensitive sensory organs found in nature that detect prey by the infrared energy they give off. The pits are thought to form an infrared image in the snake’s brain.
WDRs will also scavenge road killed animals. Eating carrion they find along or on roads.
In the Tucson are Western Diamondbacks are not only the most common rattlesnake encountered—but they are the most commonly encountered snake. Their abundance is possibly related to their bimodal mating seasons, (late August to early October, March through April); large size; tolerance to both cold and heat; maximum habitat utilization (finding warm hibernacula); ability of females to store sperm for long periods of time; live bearing; maternal accompaniment of young, venomous; long lived (15 years +); and a complex social structure.
In our area, two independent and ongoing studies (Emily Taylor and Dale DeNardo, and Gordon Schuett and Roger Repp) have discovered that young are born early to late August, and tend to be in smaller clutches than might be recorded elsewhere. (2-7, 3-8, 4.4 and 4.6 average respectively). The more commonly quoted figure of 10 to 20 young may transpire in wetter parts of its range, or exceptional years when weather factors are perfect. There is much work to be done along the lines of reproduction, but evidence is mounting that females skip a year or more between birthing events. Fat, healthy females seem to attract mates, and body fat may play a key role in triggering ovarian follicular growth. Females will mate with multiple males and males compete for females using a ritualized combat. The combat behavior is often mistake for mating behavior.
The wide range this species occupies assures its survival well into the next century. But as Tucson begins to expand ever outward, trends reveal that increasing numbers of road kills versus live snakes are being encountered. Once humans settle into any given area in number, C. atrox eventually lose yet another portion of its range.