Black-tailed rattlesnake (Crotalus molossus)

Crotalus molossus is a venomous pit viper species found in the southwestern United States and Mexico. Four subspecies are currently recognized, including the nominate subspecies described here.


This medium-sized species averages from 76 to 107 cm (30 to 42 in) in length. Large specimens are usually not much more than 100 cm (39 in) long, although lengths of 125.0 cm (49.2 in) (Gloyd, 1940), 125.7 cm (49.5 in) (Klauber, 1972), and 129.5 cm (51.0 in) (Shaw & Campbell, 1974) have been reported. The females tend to be larger than the males.

They range in color from yellows and olive greens to browns and black. As their name implies, one of their most distinguishing features is, despite variations in body color, the tail scales are entirely black. Often, this rattlesnake has a black band across its eyes and diagonally down to the corners of its mouth, forming a sort of facial ‘mask’.

Like other rattlesnakes, C. molossus has a rattle composed of keratin on the end of its tail. Each time the snake sheds its skin, a new segment is added to the rattle. They can shed their skin several times a year, and the rattle is fairly fragile and can be broken, so the length of a rattlesnake’s rattle is not an accurate measure of its age, unless the terminal button is intact.

Geographic range

It is found in the southwestern United States in Arizona, New Mexico and west and central Texas, and Mexico as far south as Oaxaca. Also it is found in the Gulf of California on San Estéban and Tiburón Islands. The distribution reaches a maximum elevation of 2930 m. Although it has been recorded being seen at as high of an altitude as 6900 feet at the McDonald Observatory in the Davis Mountains of West Texas. The type locality given is “Fort Webster, St. Rita del Cobre, N. Mex.” (Fort Webster, Santa Rita del Cobre, Grant County, New Mexico, USA).

Conservation status

This species is classified as Least Concern IUCN Red List of Threatened Species (v3.1, 2001). Species are listed as such due to their wide distribution, presumed large population, or because they are unlikely to be declining fast enough to qualify for listing in a more threatened category. The population trend was stable when assessed in 2007.


All rattlesnakes are carnivorous, their primary food sources being rodents, other small mammals, birds, and small reptiles. The behavior of northern black-tailed rattlesnakes varies over the course of a year. In the spring and fall, they are primarily diurnal. In the summer, they shift to a nocturnal behavior, to avoid the heat of summer. In the winter, they hibernate in dens created and abandoned by other animals, often with other species of snakes. They are variable in their form of locomotion depending on what substrate they need to traverse and will actively change between sidewinding or rectilinear movement. Although it is an able climber and expert swimmer, C. molossus is primarily a terrestrial species and inhabits grasslands, desert areas, and rocky and mountainous areas, as well as high-altitude forested habitats.

Docility and Defense

C. molossus is often considered the most docile rattlesnake ever, because of their placid and curious attitude. Bites rarely occur because of this. They rely mostly on camouflage to avoid trouble. C. molossus normally trys to back away when confronted, but will rattle when cornered and threatened.


The venom of the C. molossus has a hemotoxic venom, like most Crotalids. The venom is not considered fatal to humans, but victims still need to seek medical attention if bitten. Fatalities are extremely rare. CroFab antivenom is often used to treat bites.


Breeding occurs in the spring when males follow the pheromone trails of the females. Copulation can sometimes last for hours and happen multiple times over a period of days. After mating, the male often stays near the female for several days to prevent any other males from mating with her. The female gives birth to live young in the summer, and the babies stay with the mother only until they wander off on their own, usually less than a day or two. Females are believed to breed every year, and can have litters as large as 10-12 young, but usually average four to six. Their lifespans average 15–20 years.


Text is available under the Creative Commons Attribution-ShareAlike License. Photograph is available under a Creative Commons License from Patrick Alexander.

About the author

Jim Mattern

Jim is a scapegoat for the NPS, an author, adventurer, photographer, radio personality, guide, and location scout. His interests lie in Native American and cultural sites, ghost towns, mines, and natural wonders in the American Deserts.

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