New Mexican Ridgenose Rattlesnake

Crotalus willardi obscurus

Description

The relatively short and slender-bodied New Mexican ridgenose rattlesnake, Crotalus willardi obscurus, attains a maximum length of about 24 in (60 cm). It is grayish brown above with indistinct and irregular white cross bars edged with brown; it is buff and spotted beneath. This venomous snake's most distinctive characteristic is a prominent ridge on the snout. The New Mexican ridgenose rattlesnake is one of five subspecies of C. willardi.

Behavior

The New Mexican ridgenose rattlesnake is most active in daylight hours from July through September. An active forager, it feeds on small mammals, birds, lizards, other snakes, and arthropods. The snake prefers to hide in leaf litter among cobbles and rocks along canyon floors. It frequently climbs into trees or shrubs and uses its perch to spot prey. It probably relies to a considerable extent on concealment in obtaining prey, both from an immobile position and while cruising. Concealment is also a key factor in escaping predators. It hibernates to talus areas and formations, such as rock outcrops, cliffs and ledges through the coolest part of the winter. These areas allow the snake to move below the frost line.

The female rattlesnake is ovoviviparous, a livebearer. She retains fertilized eggs within her body during hibernation and gives birth to live young when warm weather returns in April or May. The gestation period is about 13 months. Hatchlings disperse within a few days after birth.

Habitat

This snake is found in belts of pine woodland in narrow mountain canyons at elevations between 5,500 and 9,000 ft (1,675 and 2,745 m). These woodland areas typically consist of scattered, open stands of pine and oak or pine and fir, with associated shrubs and grasses. Annual precipitation is about 20 in (51 cm).

Distribution

Never common, this species is found very locally in the Animas Mountains of southwestern New Mexico and adjacent portions of the Sierra de San Luis in northern Mexico. Other subspecies of ridgenose rattlesnake range through southern Arizona and New Mexico and as far south as Durango and Zacatecas in Mexico.

The New Mexican ridgenose rattlesnake occurs in isolated enclaves in Hidalgo County in the extreme southwestern corner of the state. Habitat area is estimated to total no more than 2 sq mi (5.2 sq km). Populations have been found in the main stem and west fork of Indian Creek Canyon, in Bear Canyon, and in Spring Canyon. The U. S. population in the wild has not been surveyed but was estimated to consist of about 500 individuals in the 1960s. Collection may have reduced the population by one-fourth. The status of the Mexican population is unknown.

Threats

Because of its small size and distinctive characteristics, this snake was intensely exploited by collectors for the commercial market. After the species was documented in the Animas Mountains in 1957, collectors came from all parts of the country to obtain specimens. Collectors also destroyed or altered habitat. Turning rocks and logs as well as using gasoline and crowbars on rock outcroppings were common techniques for collection. Excessive grazing may have reduced suitable habitat, further restricting the distribution of this species. In captive populations diseases have caused the death of ridge-nosed rattlesnakes but it is not known if any diseases have infested wild population.

Conservation and Recovery

In 1974, an agreement was reached with the landowner to restrict unauthorized access to the canyons. Although the state of New Mexico prohibits collecting without a permit, the snake's low reproductive rate has slowed its recovery. Portions of the habitat, particularly along the canyon floors, were heavily grazed by cattle in the past, which denuded ground cover and disrupted leaf litter. The former landowner cooperated enthusiastically with the U.S. Fish and Wildlife Service to reduce the level of grazing in the canyons. In 1982, however, most of the property was sold to American Breco, a California-based corporation, and the conservation agreements were not renewed. A resumption of heavy grazing would pose a serious threat to the snake's survival. While mining is also a potential threat, mineral exploration has so far been minimal.

Because of its limited range and numbers, the New Mexican ridgenose rattlesnake will probably always remain threatened. The goal of the recovery plan is to minimize habitat disturbance and discourage collection through negotiations with the current landowner. The highest elevations of the Animas Mountains are managed as a wildlife preserve.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
P. O. Box 1306
Albuquerque, New Mexico 87103-1306
Telephone: (505) 248-6911
Fax: (505) 248-6915
http://southwest.fws.gov/

Reference

Dobrott, S. J. 1980. "A Management Plan for the Animas Mountains, Submitted to the Victorio Company." Hidalgo County, New Mexico.

Harris, H. S., and R. S. Simmons. 1975. "An Endangered Species, the New Mexican Ridgenose Rattlesnake." Bulletin of the Maryland Herpetological Society 11: 1-7.

U. S. Fish and Wildlife Service. 1985. "New Mexico Ridgenose Rattlesnake Recovery Plan." U. S. Fish and Wildlife Service, Albuquerque. 59 pp.