Common names: black-tailed rattlesnake, green rattler, Northern black-tailed rattlesnake, more.

Crotalus molossus is a venomous

Venomous snake

"Poisonous snake" redirects here. For true poisonous snakes, see Rhabdophis.Venomous snakes are snakes which have venom glands and specialized teeth for the injection of venom...

 pitviper

Crotalinae

The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...

 species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 found in the southwestern United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

. Four subspecies

Subspecies

Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...

 are currently recognized, including the nominate subspecies described here.

Description

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

They range in color from olive greens, to yellows, to browns, all the way to black. As their name implies, one of their most distinguishing features is that despite variations capable in body color, the tail scales are entirely black. Often these rattlesnakes have a black band that goes across their eyes and diagonally down to the corners of their mouth forming a sort of facial 'mask'.

Like other rattlesnakes, C. molossus has a rattle composed of keratin

Keratin

Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

 on the end of the tail. Each time the snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

 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

Rattlesnake

Rattlesnakes are a group of venomous snakes of the genera Crotalus and Sistrurus of the subfamily Crotalinae . There are 32 known species of rattlesnake, with between 65-70 subspecies, all native to the Americas, ranging from southern Alberta and southern British Columbia in Canada to Central...

's rattle is not an accurate measure of its age, unless the terminal button is intact.

Common names

Black-tailed rattlesnake, green rattler, northern black-tailed rattlesnake, dog-faced rattlesnake, dog-headed rattlesnake, mountain diamondback.

Geographic range

Found in the southwestern United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 in Arizona

Arizona

Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

, New Mexico

New Mexico

New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

 and west and central Texas

Texas

Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

, and Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

 as far south as Oaxaca

Oaxaca

Oaxaca , , officially Estado Libre y Soberano de Oaxaca is one of the 31 states which, along with the Federal District, comprise the 32 federative entities of Mexico. It is divided into 571 municipalities; of which 418 are governed by the system of customs and traditions...

. Also found in the Gulf of California

Gulf of California

The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...

 on San Estéban Island and Tiburón Island

Tiburón Island

Tiburón Island is both the largest island in the Gulf of California and the largest island in Mexico, with an area of . It was made a nature reserve in 1963 by President Adolfo López Mateos. Tiburón is Spanish for shark. The etymology of the Seri name is unknown...

. The distribution reaches a maximum elevation of 2930 m. 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 (LC) on the IUCN

World Conservation Union

The International Union for Conservation of Nature and Natural Resources is an international organization dedicated to finding "pragmatic solutions to our most pressing environment and development challenges." The organization publishes the IUCN Red List, compiling information from a network of...

 Red List of Threatened Species

IUCN Red List

The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...

 (v3.1, 2001). Species are listed as such due to their wide distribution, presumed large population, or because it is unlikely to be declining fast enough to qualify for listing in a more threatened category. The population trend is stable. Year assessed: 2007.

Behavior

All rattlesnakes are carnivorous

Carnivore

A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

, 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

Diurnal animal

Diurnality is a plant or animal behavior characterized by activity during the day and sleeping at night.-In animals:Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects, reptiles and birds...

. 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 snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

. They are variable in their form of locomotion depending on what substrate the need to traverse and will actively change between sidewinding or rectilinear movement. Although they are able climbers and expert swimmers, C. molossus is primarily a terrestrial species and inhabits grasslands, desert areas, rocky and mountainous areas, as well as higher altitude forested habitats.

Reproduction

Breeding occurs in the spring when males follow the pheromone

Pheromone

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

 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 months, and the babies only 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 averages 4–6. Their lifespan averages 15–20 years.

Subspecies

Subspecies	Taxon author	Common name	Geographic range
C. m. estebanensis Crotalus molossus estebanensis Crotalus molossus estebanensis is a venomous pitviper subspecies endemic to San Estéban Island, Mexico.-Geographic range:Known only from the type locality, which is "San Estéban Island, Gulf of California, Mexico.-Conservation status:...	Klauber Laurence Monroe Klauber Laurence M. Klauber , was an American herpetologist, and was considered to be the foremost authority on rattlesnakes... , 1949	San Esteban Island black-tailed rattlesnake	Mexico Mexico The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of... : Isla San Esteban (Gulf of California Gulf of California The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland... ).
C. m. molossus	Baird Spencer Fullerton Baird Spencer Fullerton Baird was an American ornithologist, ichthyologist and herpetologist. Starting in 1850 he was assistant-secretary and later secretary of the Smithsonian Institution in Washington, D.C...  & Girard Charles Frédéric Girard Charles Frédéric Girard was a French biologist specializing in ichthyology and herpetology.Born in Mulhouse, France, he studied at the College of Neuchâtel, Switzerland as a student of Louis Agassiz. In 1847, he accompanied Agassiz as his assistant to Harvard... , 1853	Northern black-tailed rattlesnake	United States United States The United States of America is a federal constitutional republic comprising fifty states and a federal district...  (Arizona Arizona Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix... , New Mexico New Mexico New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S... , southwest Texas Texas Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in... ), Mexico
C. m. nigrescens	Gloyd Howard K. Gloyd Howard Kay Gloyd was an American herpetologist who is credited with describing several new species of reptile, such as the Florida Cottonmouth, Agkistrodon piscivorus conanti... , 1936	Mexican black-tailed rattlesnake	Mexico (South Sonora Sonora Sonora officially Estado Libre y Soberano de Sonora is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided into 72 municipalities; the capital city is Hermosillo.... , southwest Chihuahua, southern Coahuila Coahuila Coahuila, formally Coahuila de Zaragoza , officially Estado Libre y Soberano de Coahuila de Zaragoza is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico... , south to Oaxaca Oaxaca Oaxaca , , officially Estado Libre y Soberano de Oaxaca is one of the 31 states which, along with the Federal District, comprise the 32 federative entities of Mexico. It is divided into 571 municipalities; of which 418 are governed by the system of customs and traditions...  and Veracruz Veracruz Veracruz, formally Veracruz de Ignacio de la Llave officially Estado Libre y Soberano de Veracruz de Ignacio de la Llave , is one of the 31 states that, along with the Federal District, comprise the 32 federative entities of Mexico. It is divided in 212 municipalities and its capital city is... , Tlaxcala Tlaxcala Tlaxcala officially Estado Libre y Soberano de Tlaxcala is one of the 31 states which along with the Federal District comprise the 32 federative entities of Mexico. It is divided into 60 municipalities and its capital city is Tlaxcala.... )
C. m. oaxacus	Gloyd, 1948	Oaxacan black-tailed rattlesnake	Mexico (Oaxaca Oaxaca Oaxaca , , officially Estado Libre y Soberano de Oaxaca is one of the 31 states which, along with the Federal District, comprise the 32 federative entities of Mexico. It is divided into 571 municipalities; of which 418 are governed by the system of customs and traditions... )

External links

• Crotalus molossus at Herps of Texas. Accessed 24 February 2007.
• Black-tailed Rattlesnake at Centennial Museum & Chihuahuan Desert Gardens Natural and Cultural History. Accessed 24 February 2007.
• Crotalus molossus, Black-tailed Rattlesnake at California Reptiles and Amphibians. Accessed 24 February 2007.
• Crotalus molossus molossus at Munich AntiVenom INdex. Accessed 24 February 2007.