Crotalus molossus
black-tailed rattlesnake

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)

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)

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; eNature.com, 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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)