San Francisco State University
Department of Geography
Geography 316: Biogeography

The Biogeography of the Mojave Rattlesnake
(Crotalus scutulatus)

by Charles Brigham, student in Geography 316, 2000


MOJAVE RATTLESNAKE

COMPLETE TAXONOMIC CLASSIFICATION

KINGDOM----->ANIMALIA

PHYLUM------->CHORDATA

CLASS----------->REPTILIA

ORDER---------->SQUAMATA

FAMILY--------->SERPENTES

GENUS----------->VIPERDAE

SPECIES---------->CROTALUS SCUTULATUS






Description of Species:
Rattlesnakes have been portrayed throughout history as a sign of fear, pain, and death. They have been symbols of power and authority for kings and rulers. These creatures have become mysterious because of their cunning ability to stun and immobilize their prey. This is a typical strategy for war.

rsnake1.jpg (15144 bytes)
(Shaw 1974)

The Mojave rattlesnake (Crotalus scutulatus) is one of the most lethal rattlers of North America. Adults average 2-4 feet in length from head to tail (Shaw-Cambell 1974). It has a diamond shaped pattern that dissolves into an offset striped pattern towards the tail. The offset stripe pattern helps set it apart from the very similar diamond back. The color of the Mojave rattlesnake varies from a greenish gray to a yellowish brown. These color variations provide camouflage in certain terrain.

Habitat:
Herpetologists have studied the natural history of snakes for centuries and despite current research they continue to pose new questions. Approximately 2700 known snakes form the suborder Serpentes of the order Squamata (Goin 1978). The Mojave rattler shares its home range closely with two other rattlesnake species, the Southwestern speckled rattlesnake and the sidewinder (Stebbins, 1954). All three are native to the Mojave Desert region. The Mojave rattler and the sidewinder share the region more closely. Both snakes prefer the flat desert scrubland.  The Southwestern speckled rattler is adaptive to rocky terrain and usually found in and near mountainous areas. Preferences for brush land and rocky areas between snakes are difficult to determine with accuracy except for continuous local observation (Klauber 1982).

The Mojave rattler, as with other ectothermic species, is dependent on the temperature of the desert.The desert supplies a comfortable habitat for the Mojave rattlesnakes survival. A four-year study concluded that mean monthly temperatures of the Mojave Desert were between 48F and 93F (D.S.C. 2000). The temperature of the Mojave Desert fluctuates often making long stable temperatures uncommon.The body temperature range most suitable for the Mojave rattlesnakes is between 80F and 90F (Klauber 1982). Extreme temperatures above 113F will kill snakes if they are exposed for long periods of time. Snakes maintain their body temperatures by moving to warmer or cooler areas depending on weather. If the temperature is cooler than normal they expose themselves to the sun, raising their body temperature. If hotter, then the snake will find cover often enough to cool off. The Mojave rattlesnakes are most active from April to September. During these months they are most visible due to the warm temperatures of the desert. In spring more snakes are seen because the snakes are hungry after winter hibernation and it is the mating season. Late, in mid-summer the newborns are roaming about. From November to April it is significantly colder, forcing the snake to find cover and hibernate. Most snakes house themselves in old burrows made by rodents and other animals. Rocks and desert brushes also provide shelter for them.

Evolution:
Representative Genera : Elaptidae
Family : Viperidae
Genus : Crotaline

Select Genera
rsnake2.jpg (26541 bytes)

-Crotalus scutulatus (Ernst-Zug 1996).

    Fossil records show that snakes originated over 130 million years ago (Ernst-Zug 1996). Evolutionary evidence concludes that they evolved from lizards through 130 million-year period due to early divergence of lineages and the initial anatomical streamlining associated with limblessness, little distinctive physical evidence exists with which to estimate evolutionary relationships (Ernst-Zug 1996). It is speculated that this occurred when Gondwana was a single continent during the breakup of Pangea. North American snakes are thought to have evolved during late Miocene Era, 23 million years ago. This was the “age of snakes“ an era of greatest diversification, modern faunas, and evolution of the venomous families. The common ancestor of the Viperidae (Family) is the Collubridae (Genus) group. The Collubridae group consists of many unrelated subgroups (Acrochordidae and Elapidae) which are difficult for biologists to classify. They are otherwise known as the “trash can” group (Ernst-Zug 1996). Preceding the Collubridae group is the Viperidae, which represent an early divergence prior to the advent of the Collubrids, the more recent arrival. (See Fig.1)Morphologies in snake skull evolution pose new evolutionary questions. The highly flexible jaws of snakes indicate marine bone structures and composition go against other evolutionary theories (Chicago Herpetological Society 1999). Protein structure and mitochondrial DNA patterns are among the new studies researched in order to determining more accurate evolutionary processes (Ernst-Zug 1996). These more recent techniques are being used in an effort to identify evolutionary paths. The Mojave rattlesnake has a life span of 24 years, indicative of a healthy adaptation to its desert ecosystem. If we consider the close coincidence of environment and weather and habitat, the reported recent evolution of the species and its longevity could be concluded that the Mojave rattlesnake evolved by interaction with recent geographical conditions of today (Shaw-Cambell 1974).

Distribution:
The Mojave rattlesnake’s continuous distribution throughout North America is shared with many other snake species distributed throughout regions adjacent to each other.

Concentrations of snake populations are usually explained by climatic or biotic factors. These snakes are found from 500ft. to 5000ft. elevations. Their home range is from the Mojave Desert through extreme Northern Colorado deserts (Zeiner 1988). This species is endemic to the desert region of Southwestern America.

The favorite plant of the Mojave Rattler is the creosote bush shrub (Stebbins 1954). It is a low-lying bush, occupying roughly 38% of the Mojave Desert, which provides cover for snakes and other animals. The snake’s fondness to this particular plant is unknown but two factors may lead to some explanation. 1. The creosote brush shrub is the most widespread shrub in the desert area. 2.The creosote brush shrub gives off a distinguishable smell which may attract the snake.

Most snakes are nocturnal because of their hunting habits. They seldom roam and hunt during the day. During the daylight they enjoy basking in the hot weather just as we prefer a hammock in the summertime. The desert flats, scrubland, and woodland are prime hunting grounds for the Mojave Rattlesnake. Its ability to camouflage itself draws prey to them unknowingly.Rattlesnakes remain the most prevalent widespread venomous snakes found in the U.S.

Interesting Issues:
Rattlesnake’s evolutionary history from a lizardlike form to a limbless reptile is not clear. The relation between marine animals and reptiles concerning snakes poses many questions. Individual species of snakes vary from one another, making it hard to pinpoint evolutionary transitions.

Differences in determining preferences of brush land and rocky terrain is difficult. It requires further locational observations of snake habitats (Klauber 1982).

The Mojave rattlesnakes’ fondness of the creosote brush shrub is unknown and also requires further investigation.
 

Bibliographic Works
Ernst, H. Carl, George R. Zug. 1996.   Snakes in question. London: Smithsonian Institution.

Goin, J. Coleman. 1978.  Introduction to herpetology. San Francisco: W. H. Freeman and Company.

Hardy, L. David Sr. 1988.  “Male-male copulation in captive Mojave rattlesnakes (Crotalus Scutulatus): its possible significance in understanding the
behavior and physiology of crotaline copulation.” Bulletin of the Chicago Herpetological Society. 33(12), December): 258-262.

Jaeger, C. Edmund. 2000.   The California Deserts. California: Stanford University Press.

Klauber, M. Lawrence. 1982. Rattlesnakes. London: University of California Press.

Selby, A. William. 2000.  Rediscovering the Golden State. New York: John Wiley and Sons, Inc.

Shaw, E. Charles, Sheldon Cambell.  1974. Snakes of the American West. New York: Alfred A. Knopf  Inc.

Stebbins, C. Robert. 1954.  Amphibians and Reptiles of Western North America. New York: McGraw-Hill.

Zeiner, C. David, William F. Laudenslayer, Kennith E. Mayer. 1988. California's Wildlife. 1 (May 2) 244-245.

Websites

National Park Service. 2000. Mojave National Preserve; [Online]. Available: file:///A:/rattlesnake%20papernsite/www.nps.gov/moja/mojaansn.htm

Desert Studies Center. 1997. Desert Climate; [Online]. Available: http://biology.fullerton.edu/facilities/dsc/zz_climate.html

Mattison, Chris and David, Jean Hosking. 1997. Vipers - Viperidae ; [Online]. Available: http://www.thesnake.org/vipers.html