Black-tailed Rattlesnake

Black-tailed Rattlesnake, Crotalus molossus

Black-tailed Rattlesnake, Crotalus molossus. Snake photographed within the confines of the Arizona-Sonora Desert Museum, Tucson, Arizona, February, 2022.

The Black-tailed Rattlesnake, Crotalus molossus, is a member of the Viperidae family of Pit Vipers. The Viperidae family has three hundred fifty members placed in three genera. Members of the Viperidae family are venomous and are commonly known as vipers. They are characterized by a head that is distinct from the body and a single pair of long, hollow, venom-injecting fangs that can be folded against the top of the mouth (tipped inward) when their mouth is closed. There are twenty-nine species in the Crotalus genus. The Black-tailed Rattlesnake is known in Mexico as Serpiente De Cascabel de Cola Negra. 

Black-tailed Rattlesnakes range in color from olive-gray, greenish-yellow, and yellow to reddish-brown and black. Their tail is entirely black in color and their scales are sharply keeled. They have a body pattern of dark irregular bands, some of which widen into diamond-shaped blotches with yellow centers. Single scales bordering the pattern are often a lighter shade than the body color. Their pattern resembles beadwork because each scale is a single color. They possess a characteristic dark band across their eyes that extends diagonally down to the corners of their mouth, forming a sort of facial ‘mask’. Like other rattlesnakes, they have a rattle composed of keratin on the end of their tail. They display sexual dimorphism with females being larger in size and have larger and thicker tails compared to the males. Adult Black-tailed Rattlesnakes measure 71.1 (2 feet 4 inches) to 1.25 m (4 feet 1 inch) in length and weigh between 6.8 kg (15 lbs 0 oz) to 10.9 kg (24 lbs 0  oz). 

Black-tailed Rattlesnakes are considered to be monogamous and breeding occurs from February through May. Males follow the chemical trails of receptive females and then attempt to mate. Mating typically takes place on rocks or in low vegetation. The male rattlesnake taps its chin down the female’s spine while flicking its tongue on her skin. After mating occurs, the male remains with the female to guard her from other prospective mates. Black-tailed Rattlesnakes are ovoviviparous and bear live offspring. While most females breed once per year, some Black-tailed Rattlesnakes have been known to have a biennial reproductive system. Females give birth to 3 to 16 young during July and August. Once neonates are born, they remain with their mother for protection for only for a short period that does not exceed 24 hours, and left to find for themselves. Females reach sexual maturity at four years of age; the age males reach sexual maturity in unknown. In addition, there is little information regarding the specific lifespan and longevity of the Black-tailed Rattlesnake. Rattlesnakes generally have a lifespan of seventeen and a half years and it is expected that Black-tailed Rattlesnakes possess a similar lifespan.

Black-tailed Rattlesnakes are active in warm temperatures, and during the cold winter months they hibernate underground below the frost line in animal burrows or rock crevices. In the spring and autumn they are diurnal, but they shift to a nocturnal pattern in the summer months to avoid exceedingly hot daytime temperatures. They prefer to sleep above the ground in trees or shrubs. Depending on the environment they need to traverse, they move by slithering via horizontal waves, rectilinear movement, or side-winding. They can also climb trees to heights of 2.5 m (8 feet) to 2.7 m (9 feet) and are capable of swimming quickly in water. Black-tailed Rattlesnakes are carnivorous and primarily feed on rodents, birds, small lizards, and various other small mammals. When hunting for prey, they use their pit organs, located between their eyes and nostrils, to sense infrared radiation and detect warm-blooded prey. In addition, they flick their forked tongues to pick up scents in their surroundings. To capture prey, they strike with their two hollow fangs. Once the fangs penetrate the skin, glands on each side of their head release lethal venom into their prey. In turn the Black-tailed Rattlesnakes are preyed upon by bobcats, coylotes, eagles, hawks, and other snakes such as the western diamondback rattlesnake. They use their cryptic coloration to avoid predation and will also hiss, rattle their tail, and as a last resort, strike with their fangs as defense mechanisms. 

Black-tailed Rattlesnakes are found from central and western Texas, the southern half of New Mexico, northern and western Arizona, south to the Mexican Plateau, Mesa Del Sur, and Oaxaca. They are also found on San Esteban and Tiburon Island in the Gulf of California. They primarily occupy grasslands, deserts, and rocky, mountainous areas. At lower elevations, they live in mesquite grasslands and deserts prefering warm, rocky sites such as the sides of canyons or small ledges in caves. Black-tailed Rattlesnakes are found at altitudes between 300 m (985 feet) to 3,750 m (12,300 feet) within high-altitude pine-oak and boreal forests.

The Black-tailed Rattlesnake may be confused with the Western Diamondback Rattlesnake, Crotalus atrox  (dark diamond-shaped patterns along its back and black and white bands just above their rattle).

From a conservation perspective, the Black-tailed Rattlesnake is currently considered to be of Least Concern with a wide distribution and large population.

No current threats have been identified for this species. Even though Black-tailed Rattlesnake venom is about two-thirds as toxic as Western Diamondback venom, EXTREME CAUTION should be used if encountered. While their venom is generally not lethal to humans, a bite from a fully-grown snake may result in severe envenomation that can cause pain, swelling, nausea, vomiting, an even shock and multiple organ damage.  The Black-tailed Rattlesnake will normally announce its intention to strike by coiling, hissing, and rattling their tail to indicate that they want to be left alone.