Crotalus molossus

Geographic Range

Crotalus molossus (the black-tailed rattlesnake) is found in the United States from central and western Texas west through the southern half of New Mexico, northern and western Arizona, and south toward the Mexican Plateau, Mesa Del Sur, and Oaxaca, Mexico. They are also found on the islands of Tiburon and San Esteban in the Gulf of California.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Black-tailed rattlesnakes are terrestrial and occupy grasslands, deserts, and rocky, mountainous areas. Crotalus molossus are also found at high-altitude pine-oak and boreal forests. This species prefers warm, rocky sites such as the sides of canyons or small ledges in caves. At lower elevations, this species lives in mesquite grassland and deserts. Individuals living on dark lava flows often have darker coloration that matches the dark earth. Crotalus molossus are found at altitudes of 300-3,750 m.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune
• savanna or grassland savanna or grassland
• forest forest
• mountains mountains

Physical Description

Typical of all rattlesnakes, Crotalus molossus has a series of rattles on the end of its tail. This species ranges in color from olive-gray, greenish-yellow, and yellow to reddish-brown and black. The tail of Crotalus molossus is entirely black. In addition, this species is distinguished by a dark band between the eyes and a diagonal dark stripe from the eye to the corner of the mouth. Running down the length of the body are a series of darker vertical crossbands. The females tend to be larger than the males and have larger & thicker tails. Scales are sharply keeled. While the exact metabolic rate of Black-tailed rattlesnakes is unavailable, factors that affect their metabolic rate are mass, temperature, sex, time of day, and place of origin.

There are 3 recognized sub-species of black-tailed rattlesnakes: C. molossus nigrescens (Mexican black-tailed rattlesnake), C. molossus estebanensis (San Esteban Island rattlesnake), and the U.S. subspecies, C. molossus molossus . The San Esteban Island morph of Crotalus molossus is the smallest of the three.

• Other Physical Features
• heterothermic heterothermic
• venomous venomous

• Sexual Dimorphism
• female larger
• sexes shaped differently

Development

During adolescent growth, black-tailed rattlesnakes shed their skin 2-4 times in the first growing season and 1-4 times in the second season. Each time the skin is shed, a new rattle segment is added to the tail. The shedding is essential for growth and wear on the skin. Once the species reaches maturity, the skin is still periodically shed, but the rattle stops developing and old segments begin breaking off.

Reproduction

Males follow the chemical trails of receptive females and then attempt to mate. Mating takes place on rocks or in low vegetation. A male black-tailed rattlesnake taps its chin down the female's spine while flicking its tongue on her skin. The male and female of the species mate sexually. After mating occurs, the male remains with the female to guard her from other prospective mates. Based on observations, Crotalus molossus has a monogamous mating system.

• Mating System
• monogamous monogamous

Very little is known about this species' reproductive behavior. Black-tailed rattlesnakes are ovoviviparous and bear live young. They normally breed once a year in a span of four months during spring. The young are born in the months of July and August. Some female black-tailed rattlesnakes are said to have a biennial reproductive cycle. One pair was observed to copulate in the wild for 105 minutes. Females reach sexual maturity in an average of four years. It is still unknown when the males are able to successfully breed.

• Key Reproductive Features
• seasonal breeding seasonal breeding
• sexual sexual
• fertilization fertilization
• ovoviviparous ovoviviparous

Females bear live young. Once the neonates are born, they remain with the mother for protection for only a few hours to a day maximum. After this, the young of C. molossus are on their own and lack maternal care.

• Parental Investment
• no parental involvement
• pre-fertilization
• pre-hatching/birth
  • protecting
    • female

Lifespan/Longevity

There is not much available information about the lifespan and longevity of Crotalus molossus . Rattlesnakes generally have an average lifespan of 17.5 years. The expected lifespan of black-tailed rattlesnakes is unknown.

Behavior

During the cold winter months, this species hibernates underground below the frost line in animal burrows or rock crevices. Black-tailed rattlesnakes are active when the temperature is warm. In the spring and autumn, Crotalus molossus are diurnal, but they shift to a nocturnal pattern in the summer months because of exceedingly hot temperatures. The species moves by slithering via horizontal waves, rectilinear movement, or side-winding, depending on the environment they need to traverse. They can climb trees to heights of 2.5-2.7 m. and are also able swim quickly in water. Crotalus molossus prefer sleeping above ground in trees or shrubs. After a cool rain, it is not uncommon to find these rattlesnakes warming themselves on paved highways.

• Key Behaviors
• arboreal arboreal
• terricolous
• diurnal diurnal
• nocturnal nocturnal
• hibernation hibernation
• aestivation
• daily torpor
• solitary solitary

Home Range

The average range size has been reported at 3.49 ha.

Communication and Perception

Crotalus molossus uses its tongue, aided by the Jacobson's organs in its mouth as a system to detect chemicals, scents, and tastes. Crotalus molossus also possess two pits to the anterior of the nostrils in the upper labial region of the head which serve to detect heat emitted from living prey items. The ability to detect heat doesn't limit these rattlesnakes to reliance on diurnal activity. Thus, they are able to function perfectly well at night time or in pitch-black caves and tunnels. There is no evidence of communication among individuals other than the emission of pheromones by the female to attract a mate. However, black-tailed rattlesnakes communicate threats to their enemies by using three devices. First, a Crotalus molossus will rattle its tail to startle its aggressor. If this does not work, it will then hiss loudly and rapidly flick its tongue in addition to the rattling. Another warning threat typical of vipers is to puff up and coil its body, making it look much larger. Since snakes are deaf to air sounds, the hiss is a means to communicate to a threat, not for intraspecific communication. Sensitive nerves connecting to ventral scales help Crotalus molossus detect minute ground vibrations such as an approaching predator or prey item.

• Communication Channels
• visual visual
• acoustic acoustic
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• visual visual
• infrared/heat infrared/heat
• tactile tactile
• vibrations vibrations
• chemical chemical

Food Habits

Crotalus molossus are carnivores. Generally, black-tailed rattlesnakes feed on rodents , birds , small lizards , and various other small mammals . When hunting for prey, this species uses its heat-sensitive organs on the sides of its head to detect infrared heat, and flicks its tongue to detect scents in its surroundings. Prey is caught by means of two hollow fangs tucked away in the front of the upper jaw. Upon striking, the fangs are extended. Once the fangs penetrate the skin of the prey item, glands at each side of the head release lethal venom into the prey.

• Primary Diet
• carnivore carnivore
  • eats terrestrial vertebrates

• Animal Foods
• birds
• mammals
• amphibians
• reptiles

Predation

Black-tailed rattlesnakes have dark blotches along its body to serve as cryptic coloration and hide away from its predators. The species is prey to eagles and hawks , coyotes , bobcats and other snakes such as the western diamondback rattlesnake Crotalus atrox . Humans are an unnatural enemy, and predation by humans is not for food but for control and protection of livestock. To avoid being preyed upon, a Crotalus molossus will hiss, rattle its tail, and as a last resort tactic, strike with its fangs.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Black-tailed rattlesnakes plays the ecosystem role of rodent control. By hunting and feeding on rodents, the species helps to control rodent populations that may destroy crops and vegetation.

Economic Importance for Humans: Positive

Rattlesnakes have been used as an attraction in zoos and carnivals. In fact, there is a business in snake dealing. The venom of rattlesnakes are used in scientific research to produce antivenin. There was a small market for rattlesnake oil and fat in the past, mainly to reduce swelling, relief of aches, bruises and sprains. Rattlesnake skins are used to make leather products such as belts, wallets, purses, and jackets. In general, rattlesnakes are an important factor in the control of crop-harming rodents.

• Positive Impacts
• body parts are source of valuable material
• source of medicine or drug source of medicine or drug
• research and education
• controls pest population

Economic Importance for Humans: Negative

This species as well as other rattlesnakes kill livestock and domestic animals, such as horses and cattle. These snakes have also bitten humans as well, and although the venom is only mildly toxic by rattlesnake standards, it can still cause sickness, and possibly death in young children or the elderly. The venom causes hemorrhaging in many cases, and some symptoms of the bite include swelling, ecchymosis of the bitten area, and thrombocytopenia. The typical treatment for a bite is antivenin. According to a rough estimate, there are approximately 1,500 bites by venomous snakes each year in America. 1,000 or 66% of these bites are caused by rattlesnakes. Only 3% of those bites are fatal, and the mortality rate is 0.02 per 100,000 of population per year.

• Negative Impacts
• injures humans
  • bites or stings
  • venomous venomous

Conservation Status

There is no evidence to the status of this species. As a whole, this species appears to have steady population sizes. However, unnecessary killings from fear or hatred of the snakes run rampant and education must be undertaken to ensure a stable future for Crotalus molossus .

Additional Links

Encyclopedia of Life

Contributors

David Armitage (research), Animal Diversity Web.

Matthew Wund (editor), University of Michigan-Ann Arbor.

Megha Desai (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

References

Beaupre, S. 1993. An ecological study of oxygen -consumption in the mottled rock rattlesnake, Crotalus-lepidus-lepidus, and the black-tailed rattlesnake, Crotalus-molossus-molossus. Physiological Zoology , 66: 437-454.

Beck, D. 1995. Ecology and energetics of three sympatric rattlesnake species in the Sonoran Desert. Journal of Herpetology , 29: 211-223.

Ernst, C. 1992. Venomous Reptiles of North America . Washington and London: Smithsonian Institution.

Goldberg, S. 1999. Reproduction in the blacktail rattlesnake, Crotalus molossus. Texas Journal of Science , 51: 323-328.

Greene, H. 1997. Snakes, The Evolution of Mystery in Nature . Berkeley, CA: University of California.

Hickman, C., L. Roberts, A. Larson. 2003. Animal Diversity . New York, NY: McGraw-Hill.

Klauber, L., K. McClung. 1982. Rattlesnake . London, England: University of California Press.

Klauber, L. 2000. Rattlesnake. Pp. "32,187" in The Columbia Encyclopedia , 6th Edition. Columbia University Press. Accessed March 15, 2003 at http://web3.infotrac.galegroup.com/itw/infomark/365/836/59332311w3/purl=rc1_GRGM_0_A68486159&dyn=3!xrn_1_0_A68486159?sw_aep=lom_umichanna .

NatureServe Explorer, 2004. "Crotalus molossus" (On-line). Accessed July 06, 2004 at http://www.natureserve.org/explorer .

eNature.com. 2004. "Black-tailed rattlesnake" (On-line). eNature.com. Accessed July 05, 2004 at http://www.enature.com/fieldguide/showSpeciesRECNUM.asp?recnum=AR0201 .