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Crotalus molossusBlack Tailed Rattlesnake

By Megha Desai

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. (Ernst, 1992)

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. (Ernst, 1992)

  • Range elevation
    300 to 3,750 m
    984.25 to ft

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. (Beaupre, 1993; "Black-tailed rattlesnake", 2004; Klauber and McClung, 1982; Klauber, 2000)

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. (Klauber and McClung, 1982)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range mass
    6.8 to 10.9 kg
    14.98 to 24.01 lb
  • Range length
    71.1 to 125 cm
    27.99 to 49.21 in

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. (Klauber and McClung, 1982; Klauber, 2000)

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. (Greene, 1997; Klauber and McClung, 1982)

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. (Ernst, 1992; Goldberg, 1999; Klauber and McClung, 1982)

  • Breeding interval
    Normally, black-tailed rattlesnakes breed once a year in the spring months. However, some females have been known to have a biennial reproductive system.
  • Breeding season
    February-May (4 months)
  • Range number of offspring
    3 to 16
  • Average number of offspring
    6.7
  • Range time to independence
    3 to 24 hours
  • Range age at sexual or reproductive maturity (female)
    3 to 5 years

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. (Ernst, 1992; Klauber and McClung, 1982; Klauber, 2000)

  • 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. (Ernst, 1992; Klauber and McClung, 1982)

  • Range lifespan
    Status: wild
    15 to 20 years
  • Range lifespan
    Status: captivity
    18 (high) years
  • Average lifespan
    Status: captivity
    20.7 years
    AnAge

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. (Ernst, 1992; Klauber and McClung, 1982)

Home Range

The average range size has been reported at 3.49 ha. (NatureServe Explorer, 2004)

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. (Klauber and McClung, 1982)

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. (Ernst, 1992; Klauber and McClung, 1982)

  • Primary Diet
  • 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. (Ernst, 1992; Klauber and McClung, 1982)

  • Anti-predator Adaptations
  • 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. (Klauber and McClung, 1982)

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. (Klauber and McClung, 1982)

  • Positive Impacts
  • body parts are source of valuable material
  • 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. (Ernst, 1992; Klauber and McClung, 1982)

  • Negative Impacts
  • injures humans

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. (Klauber and McClung, 1982)

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.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

arboreal

Referring to an animal that lives in trees; tree-climbing.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

infrared/heat

(as keyword in perception channel section) This animal has a special ability to detect heat from other organisms in its environment.

monogamous

Having one mate at a time.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

References

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

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. 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.

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

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

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