Mexican west coast rattlesnake

Identification

The Mexican west coast rattlesnake is one of the largest rattlesnakes: adults longer than 1.5 m are not rare, and the largest reported are just over 2 m, including a captive-born female of 2.05 m^2,5. It is heavy-bodied, with a clearly triangular head and strongly keeled scales⁵. Compared with the western diamondback and other large rattlesnakes, it has a prominent ridge along the spine and especially rough scales towards the tail².

Its colour changes a great deal with age. Young snakes are bright red-brown with a row of 26–41 dark diamonds edged in cream, while fully grown adults become olive green with faded diamonds and a tail that is almost black above². Newborns are about 33 cm long, and adults can grow very large rattles².

Distribution

This rattlesnake is found only in Mexico^5,7. It lives along the Pacific side of the country, from extreme southern Sonora through Sinaloa, Nayarit, Jalisco and Colima to western Michoacán^3,6. The Reptile Database also lists records from Chihuahua, Zacatecas and Aguascalientes¹, and in Zacatecas it is known only from the west and south-west of the state⁷.

It is mainly a lowland snake, and most records come from below 1,000 m³. A few have been found much higher, including one at 2,225 m in the mountains of Michoacán³.

Habitat

The Mexican west coast rattlesnake mainly lives in tropical thorn forest, tropical deciduous forest and arid tropical scrub³. It also enters the transition zone between tropical deciduous forest and pine-oak forest, and occasionally humid pine-oak forest higher up^3,7. It shelters in the burrows of other animals, mainly mammals⁵.

Behaviour

Field observers have often found this rattlesnake active by day³. At Chamela, Jalisco, most were seen by day or at dusk, mainly in the dry season⁵. Elsewhere it seems most active during the summer rainy season, when many are found crossing roads, and in Sinaloa young rattlesnakes were often found at night on the highway^3,4.

Herpetologists who studied it in the field described it as one of the gentlest of the Mexican rattlesnakes³.

Diet

The Mexican west coast rattlesnake feeds on small mammals and reptiles⁵. Its venom changes as it grows: young snakes have more lethal venom, with more crotamine and crotoxin, while adults have more of the enzymes that damage tissue⁹.

Reproduction

The Mexican west coast rattlesnake gives birth to live young^1,5. Its reproductive activity runs from late spring to early autumn, both sexes can store sperm until the following season, and females usually breed every year⁶.

In Chamela, mating seems to happen in June of the year before the birth, and litters of 24–35 young are born at the start of the rainy season⁵. In Sinaloa, a group of 20 recently born young was found sunning near a burrow in late July⁴.

Venom and medical significance

This rattlesnake is dangerous to people. The World Health Organization lists it in Category 2 in Mexico, meaning a highly venomous snake that can cause illness, disability or death but is less often involved in bites or has fewer clinical data¹⁰. About two-thirds of its venom is made up of metalloproteinases, which cause bleeding and tissue damage; in mice, most of the venom’s lethal effect comes from phospholipases that also damage nerves and muscle⁸.

Its venom also varies from place to place: most snakes tested from Zacatecas, Sinaloa and Michoacán lacked the nerve toxin crotoxin, while those from other areas had it⁹. Its venom is used, together with terciopelo venom, to make one of Mexico’s antivenoms, which neutralised its lethal effects in laboratory tests^8,9. Anyone bitten should go to hospital immediately.

Conservation

The IUCN Red List classifies the Mexican west coast rattlesnake as Least Concern, based on an assessment published in 2007¹¹. In Mexico it is listed as subject to special protection (Pr) under the official list of species at risk, NOM-059-SEMARNAT-2010⁵.

Its main threats are the clearing of the Pacific slope’s tropical dry forests and killing by people who fear venomous snakes⁵.

Similar species

It is easily mistaken for black-tailed rattlesnakes (Crotalus molossus and its relatives), which live mostly further north and east but meet it in a few areas, where the two may interbreed^6,7. The western diamondback (Crotalus atrox) also occurs nearby in southern Sonora³; the Mexican west coast rattlesnake has a more prominent spinal ridge and rougher scales². In the Sierra de Coalcomán it lives close to the Neotropical rattlesnake Crotalus culminatus, which has clear dark stripes on the neck, whereas the Mexican west coast rattlesnake’s neck pattern is faint^2,3.

These differences are not a safe basis for handling any snake. Keep your distance from every snake you cannot identify.

In captivity

The Mexican west coast rattlesnake is a dangerous venomous snake and is not suitable as a pet. In Mexico it is a legally protected species⁵. Rattlesnakes of this species were once shipped from Colima to dealers in the United States and sold as “Mexican green rattlesnakes”².