Crotalus atrox

Common names: western diamondback rattlesnake,^[2] Texas diamond-back,^[3] more.

Crotalus atrox is a venomous pitviper species found in the United States and Mexico. It is likely responsible for the majority of snakebite fatalities in northern Mexico and the second greatest number in the USA after C. adamanteus.^[4] No subspecies are currently recognized.^[2]

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Description

  Adults commonly grow to 120 cm in length. Specimens over 150 cm are infrequently encountered, while those over 180 cm are very rare. The maximum reported length considered to be reliable is 213 cm (Klauber, 1972). Males become much larger than females, although this difference in size does not occur until after they have reached sexual maturity.^[5]

The color pattern generally consists of a dusty looking gray-brown ground color, but it may also be pinkish brown, brick red, yellowish, pinkish or chalky white. This ground color is overlaid dorsally with a series of 24-25 dorsal body blotches that are dark gray-brown to brown in color. The first of these may be a pair of short stripes that extend backwards to eventually merge. Some of the first few blotches may be somewhat rectangular, but then become more hexagonal and eventually take on a distinctive diamond shape. The tail has 2-8 (usually 4-6) black bands separated by interspaces that are ash white or pale gray. There is a postocular stripe that is smoky gray or dark gray-brown and extends diagonally from the lower edge of the eye across the side of the head. This stripe is usually bordered below by a white stripe running from the upper preocular down to the supralabials just below and behind the eye.^[5]

Common names

Western diamondback rattlesnake, western diamond-backed rattlesnake,^[2] Adobe snake, Arizona diamond rattlesnake, coon tail, desert diamond-back, desert diamond rattlesnake, fierce rattlesnake, spitting rattlesnake, Texan rattlesnake, Texas diamond-back (rattlesnake),^[3] western diamond rattlesnake.^[6]

Geographic range

Found in the United States from central Arkansas and southeastern California, south into Mexico as far as northern Sinaloa, Hidalgo and northern Veracruz. Disjunct populations exist in southern Veracruz and southeastern Oaxaca. The type locality given is "Indianola" (Indianola, Calhoun County, Texas, USA).^[1]

In the United States it occurs in the following states: central and western Arkansas, Oklahoma excluding the northeast, north-central region and the panhandle, Texas excluding the northern panhandle and the east, southern and central New Mexico and Arizona, extreme southern Nevada, and in southeastern California on either side of the Chocolate Mountains. Records from extreme southern Kansas (Cowley and Sumner Counties) may be based on a natural occurrence of the species, while multiple records from near Kanopolis Reservoir in Ellsworth County seem to indicate a viable (although isolated) population.^[5]

In Mexico it occurs in the following states: Nuevo León, Coahuila, Chihuahua, Sonora, extreme northeastern Baja California (state), northern Sinaloa, northeastern Durango, Zacatecas, most of San Luis Potosí, northern Veracruz, Hidalgo and Querétaro. Specimens have been collected in the mountains, northwest of Tehuantepec, Oaxaca on numerous occasions, but have not been reported there since the 1940s.^[5]

This species has also been reported on a number of islands in the Gulf of California, including San Pedro Mártir, Santa María (Sinaloa), Tíburon and the Turner Islands.^[5]

Habitat

Found in areas ranging from flat coastal plains to steep rocky canyons and hillsides. It is associated with many different vegetation types, including desert, sandy creosote areas, mesquite grassland, desertscrub, and pine-oak forests. Towards the southern edge of its range, this species may be found in thornforest and tropical deciduous forest.^[5]

Conservation status

This species is classified as Least Concern (LC) on the IUCN Red List of Threatened Species (v3.1, 2001).^[7] Species are listed as such due to their wide distribution, presumed large population, or because it is unlikely to be declining fast enough to qualify for listing in a more threatened category. The population trend is stable. Year assessed: 2007.^[8]

Feeding

A comprehensive study by Beavers (1976) on the prey of C. atrox in Texas showed that by weight 94.8% of prey consisted of small mammals.^[5] According to Pisani and Stephenson (1991), who conducted a a study of the stomach contents of C. atrox in the fall and spring on Oklahoma, mammalian prey included prairie dogs (Cynomys ludovicianus), kangaroo rats (Dipodomys ordii), pocket gophers (Geomys bursarius and Cratogeomys castanops), voles (Microtus ochrogaster), woodrats (Neotoma floridana), pocket mice (Perognathus hispidus and P. flavescens), white-footed mice (Peromyscus leucopus and P. maniculatus), Old World rats and mice (Rattus norvegicus and Mus ssp.), harvest mice (Reithrodontomys megalotis), fox squirrels (Sciurus niger), cotton rats (Sigmodon hispidus), ground squirrels (Spermophilus spilosoma), rabbits (Sylvilagus floridanus), jackrabbits (Lepus californicus), and an unidentified mole species.^[5] Klauber (1972) mentions that large specimens are capable of swallowing adult cottontail rabbits and even adult jackrabbits, although he figured the latter required confirmation.^[9]

Bird and lizards are also preyed upon, with lizards mostly being eaten by young snakes. Avians include: mockingbirds (Mimidae), quail, a nearly full-grown Gambel's quail, a burrowing owl (Athene cunicularia),^[9] a fledgling horned lark (Eremophila alpestris) a black-throated sparrow (Amphispiza bilineata), and an eastern meadowlark (Sturnella magna).^[5] Lizards include: a whiptail lizard (Cnemidophorus), Spiny lizards (Sceloporus), a Texas banded gecko (Coleonyx brevis),^[9] and a side-blotched lizard (Uta palmeri). One case reported by Vorhies (1948) involved a juvenile specimen that had attempted to east a horned lizard (Phrynosoma solare), but died after the lizard's horns had punctured its esophagus, leaving the lizard stuck there.^[5]

Hermann (1950) reported that C. atrox also feeds on lubber grasshoppers (Brachystola magna). Klauber (1972) once found a single specimen in which the stomach contents included grasshoppers, beetles and ants. However, mammal hairs and an iguanid lizards were also found in the same stomach, which made it more likely that the insects had first been eaten by the mammal or the lizard before they had been eaten by the snake.^[9]

Behavior and reproduction

Western diamondbacks can live for more than twenty years, but life expectancy is typically shorter because of hunting and human expansion. Solitary outside of mating season, they are one of the more aggressive species found in North America because they rarely back away from confrontation. When threatened they usually coil and shake their rattle to warn an aggressor that it has stumbled upon something dangerous. There is suspicion that some rattlesnakes (and the diamondback in particular) which generally live around populated areas do not rattle as often because it leads to the snake’s discovery and consequent destruction. However, there is little available evidence of this hypothesis.

  C. atrox, like other desert snakes, can go for up to two years without food in the wild. A 5½ month starvation study showed that the snakes reduced energy expenditures by an average of 80% over the length of the study. The snakes also feed from within on energy-rich lipid stores. The most interesting finding was that the snakes grew during the study, indicating that while the snake's mass was shrinking, it was putting its resources into skeletal muscles and bone.^[10]

In the winter, western diamondbacks hibernate in caves or burrows sometimes with many other species of snakes.

The snake is a poor climber and primarily hunts small mammals, but will also feed on birds, small reptiles and amphibians. They hunt (or ambush prey) at night or early morning using a type of infrared sense prominently found in pit vipers. Although adult specimens have no natural predators, hawks, eagles, and other snakes can prey on young or adolescent individuals.

Rattlesnakes, including C. atrox, are viviparous. Gestation period lasts six or seven months and broods average about a dozen young. However, the young only stay with the mother for a few hours before they set off on their own to hunt and find recluse, thus the mortality rate is very high.The Western Diamondback Rattlesnake is the most commonly encountered rattlesnake in Texas. The Western Diamondback is the longest rattlesnake in the state, and one of the two heaviest (the other is the Timber Rattlesnake). The record length is over 213 cm (84 in); adults found in the wild typically measure between 0.91-1.21 m (3-4 ft). Its common name does not come unearned; a series of diamond-shaped blotches runs down the length of its back, and each blotch is surrounded by a light border. A background coloration of tan or brown surrounds the blotches, and the dorsal coloration varies tremendously over its distributional range. This rattlesnake can easily be distinguished by its black and white tail, which also earns the western diamondback the name "coontail" (also, see below). Its off-white belly is usually unmarked, its anal scale is undivided, and its dorsal scales are extremely keeled, often in rows of 25 to 27 near midbody.

Similar species

Crotalus atrox may be confused with a number of snakes found in Texas, most of which are non-venomous. The majority of these snakes, however, lack the key feature of a rattle found at the end of the tail. Many snakes, including gopher snakes (Pituophis) and hognose snakes (Heterodon) may show an impressive threat display and have similar brown and tan markings, but lack the rattles. Some rattle-less snakes such as rat snakes and copperheads may vibrate their tails. Like all rattlesnakes, Crotalus atrox has a heat-sensing pit between the eye and nostril. The dorsal markings of rattlesnakes range from the distinct diamond-shaped (rhomboid) marks in Crotalus atrox and the Mojave Rattlesnake (Crotalus scutulatus), to dorsal blotches in the Prairie Rattlesnake (Crotalus viridis) and the Western Massasauga (Sistrurus catenatus), to narrower crossbands extending down the sides of the body in the Rock Rattlesnake (Crotalus lepidus). The pattern of the Western Diamondback is most similar to that of the Mojave Rattlesnake and the Prairie Rattlesnake. The color of the tail is a useful key to discriminate C. atrox from most other rattlesnakes. In C. atrox, the tail is completely encircled with white and black bands of equal widths. The Mojave Rattlesnake, Crotalus scutulatus, is the only other Texas rattlesnake with such prominent black and white bands around its tail, but the white bands are twice as wide as the black bands. Crotalus scutulatus does have similar dorsal rhomboid markings running the length of its body, yet these blotches flatten into bands in the posterior third of the body; the diamond markings of C. atrox do not flatten into bands.

Natural History

 All rattlesnakes are venomous, and therefore potentially dangerous if approached or handled. Rattlesnakes are not generally aggressive and will most likely flee if given a chance to retreat. Found from the flatlands and prairies to the rocky hills and low mountains, the Western Diamondback is a key participant in the food web; it is an important predator of many small rodents, rabbits, and birds. The Western Diamondback is in turn preyed upon by a variety of larger mammals and birds, such as coyotes, foxes, and hawks. It is primarily a nocturnal animal, hunting for its prey on warm summer nights. It is, however, seasonally diurnal, moving between hunting sites during the day during the cooler spring and fall months. The Western Diamondback is usually inactive between late October and early March, though an occasional rattlesnake may be seen sunning itself on warm winter days. Mating occurs in the spring and the females give birth (they are viviparous) to as many as 25 young, which may be as long as 30 cm (12 inches) in length. The young are fully capable of delivering a venomous bite from the moment they are born.

Venom

Although the venom of the diamondback isn't particularly toxic, the size of the snake allows a larger capacity of venom which is released from its two prominent fangs. It's not uncommon that only one bite mark from one fang is visible after a strike. Fangs can break or bend, or the bite area may be small, causing a miss. All pit vipers have the ability to control the flow of venom through their fangs, allowing the diamondback to release most of its venom in a single strike (though often a pit viper will not release any of its venom).

Most of the toxin released is proteolytic like all other American pit vipers. Proteolytic venoms are, in fact, advanced and concentrated fluids that destroy tissues and other cells through intramolecular digestion. A few toxic effects include: cytotoxic (destoys cells), hemotoxic (destroys red blood cells), myotoxic (causes paralysis and muscle destruction), hemorrhagic (causes persistent bleeding). Smaller amounts of neurotoxins are also present. Unlike neurotoxins, hemotoxin envenomations becomes quickly apparent; the area around the wound swells at a rapid rate. Discoloration and pain are also experienced shortly after being bitten. Professional medical attention should be sought immediately, especially when the victim is a child. The smaller the victim the less time it takes for the venom to spread. Although it is commonly believed that baby or young rattlesnakes deliver more concentrated venom and are thus more dangerous, this idea is not supported by scientific evidence. The amount of venom delivered is a much more important indicator of the bite's danger than the venom's concentration, and since larger (older) snakes can deliver much more venom, larger rattlesnakes should always be considered more dangerous even though many bites from adult snakes are "dry"

Captivity

C. atrox is frequently bred in captivity, and readily available in the exotic animal trade. Many color variations are bred, including albinos, patternless, and melanistic. They are also heavily collected from the wild, frequently being drawn out of their hiding places with gasoline, and used in rattlesnake roundups where they are killed for entertainment.^[1] Despite this, their population is not considered to be threatened.

Gallery

See also

• List of crotaline species and subspecies
• Crotalus by common name
• Crotalus by taxonomic synonyms
• Crotalinae by common name
• Crotalinae by taxonomic synonyms
• Snakebite

References

Further reading