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The Newsletter of the Colorado Herpetological Society

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Natural History

A Brief History of the Arizona Black Rattlesnake

by Cody Meachum
CHS Vice President

The Arizona Black Rattlesnake (Crotalus viridis cerberus) is one of nine subspecies of the Western Rattlesnake, native only to Arizona and a very small part of western New Mexico. Most of the snake's range in Arizona lies north of Phoenix and south of a line northwest through Show Low, Flagstaff, and to just north of Peach Springs. There are isolated populations occurring in the Santa Catalina, Galiuro, Pinal, and Pinaleno Mountains southeast of Phoenix.

In Arizona, this snake occurs over a wide range of habitats including forest, woodland, and upland desert chaparral, preferring areas with conspicuous and often dense woody cover. It ranges from a elevation of 3500' in desert chaparral to 9150' atop Mount Lemmon in the Santa Catalina Mountains, its highest verified elevation. Seemingly, this snake is most frequently encountered at lower elevations, especially in the transition zone between upland Sonoran Desert vegetation and desert chaparral. According to the records of the Arizona Game and Fish Department, it is a relatively common snake at lower elevations, but is rarely seen because of its secretive nature.

Yearly activity for this subspecies generally begins in April or May, depending on daily temperatures, and extends into the late fall. Arizona Blacks have been seen on the surface as late as November at lower elevations, but generally start looking for hibernacula in October. Nearly all of the animals found in the open after late September are juveniles, with only a few adults being observed, usually around den sites. It is interesting to note that most individuals prefer to den alone or in small numbers, very unlike their cousin the Prairie Rattlesnake (C. v. viridis), where dens containing hundreds of snakes have been found.

Shortly after hibernation ends, breeding begins. Copulating pairs have been found as early as April. Females then gestate their young through the summer and give birth in August or September, with an average litter of seven young. Curiously, the young are born with an entirely different coloration than the adults. Born an overall light brown, within seven to ten sheds they have assumed the light gray to jet black appearance of their parents. It is not known why this ontogenetic color change occurs, but it is believed to be to provide a more cryptic coloration for the young. Even more intriguing is the fact that the adults themselves are capable of changing color, often being light gray with dark blotches at night, and nearly jet black during the daylight hours. It is the only rattlesnake capable of changing colors in this manner, making it unique among crotalids.

Crotalus v. cerberus is a relatively small snake, rarely reaching in excess of 3½ feet in length. Average adults are somewhere between 2½ and 3 feet long, a length reached after their third or fourth year. Being a small snake, they prefer small prey. Rodents make up the bulk of their diet, along with an occasional ground-nesting bird. The stomach contents of juveniles have shown that lizards and amphibians are also eaten when available. One adult was observed in the wild swallowing a young desert quail. After nearly an hour, the snake managed to finish its meal, and then slowly crawled toward a small rock crevice to digest it.

Despite all the information I have provided here, not enough is known about this spectacular example of North American herpetofauna. Sadly, this snake is often overlooked and rarely given the attention it deserves, even among rattlesnake experts. Fortunately, studies of local populations have turned up varying results, giving rise to further research, and providing fuel for a growing interest. Recent studies have turned up noticeable differences in venom, appearances, and even habitat preferences. These studies make it clear that, although it may seem that much is known, in truth we have only just started to understand the secrets of this animal that nature holds so dear.

As it is with the Arizona Black, so it is with so many of the world's herps, that much more research needs to be done, and more questions have to be addressed. How is this snake able to change colors, for instance? And why? Perhaps the future will bring more answers. I for one will keep asking, keep learning -- and keep looking! Who knows, maybe you too can locate one of these great animals yourself. All it takes is a little extra diligence, and maybe a little luck. So the next time you find yourself in Arizona on a lonely road in the middle of the night, and a piece of old tire catches your eye, check it out. It just might be an Arizona Black.