- Common names: ridge-nosed rattlesnake, Willard's rattlesnake, Willard's rattler
Crotalus willardi is a venomous pit viper species found in the southwestern United States and Mexico. It is the most recent rattlesnake species to be discovered in the United States. Its specific name is in honor of its discoverer, Professor Frank C. Willard. Five subspecies are currently recognized, including the nominate subspecies described here. The Arizona ridge-nosed rattlesnake is the state reptile of Arizona.
C. willardi is a rather small rattlesnake with all subspecies measuring one to two feet (30-60 cm) in length. Color patterns are generally a dark brown base with pale or white horizontal striping, but vary slightly between subspecies. The distinctive ridges along each side of its nose, which are a series of upturned scales, are unique to its species and are the origin of its name.
C. willardi is rarely found outside habitats at high elevation. Wooded mountain ranges, primarily in the southwest, are where this reclusive species is found. Each subspecies’ range is limited to select mountain ranges, making human encounters rare events.
Conservation status 
This species is classified as Least Concern on the 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.
Although four of the five subspecies are secure, the New Mexico ridge-nosed rattlesnake (C. w. obscurus) is an endangered species and listed as threatened by the US Fish and Wildlife Service. Remaining populations are scattered throughout New Mexico, Arizona and the northern part of Mexico. Habitat destruction is the cause of declining numbers, but critical habitat designations (recovery measures) have been proposed.
Rattlesnakes are primarily ambush hunters; they coil and lie waiting for prey to approach within striking distance. The diet of C. willardi includes small mammals, lizards, birds, and large centipedes. The young feed primarily on large centipedes (Scolopendra spp.) and lizards, whereas adults feed primarily on mammals and birds. 
Like other rattlesnakes, C. willardi is ovoviviparous, meaning it gives birth and does not lay eggs. Contrasting with viviparous animals, the young still develop within an egg inside the female snake until their time of birth. Copulation occurs from late summer to early fall, and gestation lasts about four to five months. Females give birth to two to 9 (average five) young in late July or August. Both sexes appear to reach reproductive maturity around 400 mm (16 in) in body (snout to vent) length. Although captive snakes have reproduced annually, wild females probably reproduce every second or third year.
Due to the generally small size of the snake, venom discharge yields are low; thus, the largely hemotoxic venom is not as life-threatening as that of other rattlesnakes. No documented deaths have been caused by ridge-nosed rattlesnakes, but pain and discomfort can still result from a rare bite.
|Subspecies||Taxon author||Common name||Geographic range|
|C. w. amabilis||Anderson, 1962||Del Nido ridge-nosed rattlesnake||Mexico in north-central Chihuahua|
|C. w. meridionalis||Klauber, 1949||Southern ridge-nosed rattlesnake||Mexico in southern Durango and southwestern Zacatecas|
|C. w. obscurus||Harris & Simmons, 1974||Animas ridge-nosed rattlesnake||The US in extreme southeastern Arizona and extreme southwestern New Mexico, Mexico in extreme northwestern Chihuahua and extreme northeastern Sonora|
|C. w. silus||Klauber, 1949||Chihuahuan ridge-nosed rattlesnake||Western Chihuahua and eastern Sonora|
|C. w. willardi||Meek, 1905||Arizona ridge-nosed rattlesnake||Southeastern Arizona, and northern Sonora|
See also 
- List of crotaline species and subspecies
- Crotalus by common name
- [[:Category:|Crotalus by taxonomic synonyms]]
- Crotalinae by common name
- Crotalinae by taxonomic synonyms
- McDiarmid RW, Campbell JA, Touré T. 1999. Snake Species of the World: A Taxonomic and Geographic Reference, vol. 1. Herpetologists' League. 511 pp. ISBN 1-893777-00-6 (series). ISBN 1-893777-01-4 (volume).
- Wright AH, Wright AA. 1957. Handbook of Snakes. Comstock Publishing Associates. (7th printing, 1985). 1105 pp. ISBN 0-8014-0463-0.
- Biographies of People Honored in the Names of the Reptiles and Amphibians of North America
- "Crotalus willardi". Integrated Taxonomic Information System. Retrieved 16 May 2007.
- Crotalus willardi at the IUCN Red List. Accessed 13 September 2007.
- 2001 Categories & Criteria (version 3.1) at the IUCN Red List. Accessed 13 September 2007.
- Holycross et al. 2002
- Holycross and Goldberg 2001
- Mehrtens JM. 1987. Living Snakes of the World in Color. New York: Sterling Publishers. 480 pp. ISBN 0-8069-6460-X.
- Campbell JA, Lamar WW. 2004. The Venomous Reptiles of the Western Hemisphere. 2 volumes. Comstock Publishing Associates, Ithaca and London. 870 pp. 1500 plates. ISBN 0-8014-4141-2.
- Klauber LM. 1997. Rattlesnakes: Their Habitats, Life Histories, and Influence on Mankind. Second Edition. 2 volumes. Reprint, University of California Press, Berkeley. ISBN 0-520-21056-5.
Further reading 
- Holycross, A.T., C.W. Painter, D.G. Barker and M.E. Douglas. 2002. Foraging ecology of the threatened New Mexico Ridge-nosed Rattlesnake (Crotalus willardi obscurus). In Schuett, G.W., M. Höggren, M.E. Douglas and H.W. Greene (eds.), Biology of the Vipers, pp. 243-251. Eagle Mountain Publishing, Eagle Mountain, Utah. 596 pp.
- Holycross, A.T. and S.R. Goldberg. 2001. Reproduction in northern populations of the ridgenose rattlesnake, Crotalus willardi (Serpentes: Viperidae). Copeia 2001:473-481.
- Meek, S.E. 1905. An annotated list of a collection of reptiles from southern California and northern Lower California. Field Columbian Museum Zoölogical Series 7(1):1-19, Plates I.-III.
|Wikimedia Commons has media related to: Crotalus willardi|
- Crotalus willardi at the Reptarium.cz Reptile Database. Accessed 12 December 2007.
- Taxonomy of the New Mexican Ridge-nosed Rattlesnake at Virginia Tech. Accessed 12 December 2007.
- International Biopark: Ridge-nosed Rattelsnake at Biopark. Accessed 12 December 2007.
- US Fish & Wildlife Service Species Profile: Crotalus willardi at U.S. Fish & Wildlife Service. Accessed 12 December 2007.