Crotalus catalinensis

Species of venomous pit viper native to Isla Santa Catalina, Mexico

The Santa Catalina rattlesnake (Crotalus catalinensis) is a species of pit viper endemic to Isla Santa Catalina in the Gulf of California just off the east coast of the state of Baja California Sur, Mexico. Like all other pit vipers, it is venomous. No subspecies are currently recognized. A relatively small and slender species, its most distinctive characteristic is that it lacks a rattle. They are also a generally nocturnal species. Though the species is classified as Critically Endangered on the IUCN Red List, there are few efforts for the snake's conservation. Natural selection has also given this snake a more elongated body than featured in related mainland forms, and measurably longer teeth; These are both characteristics associated with snakes that hunt in trees and shrubs such as this species. This species also lacks the standard bite and release tactic employed by usual rattlesnakes, preferring to instead hold prey in its jaws until the venom can take effect. In this way, it avoids the possibility that prey might fall out or fly away prior to succumbing. Of the two distinct color variations, the predominant variant has a light cream colored base, with reddish brown blotching down the back, and black and white banding around the tail. The other variant has a much lighter, ash gray color, with darker gray blotching. The banding around the tail is still present. The banding tapers off and becomes more obscure. The two different colors were originally thought to be due to natural selection and therefore match different habitat backgrounds, but that has not been supported. There is no difference between the brown and grey within the habitat, as well as no difference in the frequency of color between males and females.

This species is classified as Critically Endangered on the IUCN Red List with the following criteria: B1ab(v) (v3.1, 2001). This means that the best available evidence indicates it is considered to be facing an extremely high risk of extinction in the wild because its geographic range is estimated to be less than 100 km2, it exists at only a single location, and a continuing decline has been observed, inferred, or projected in the number of mature individuals. It was assessed in 2007. It is threatened due to collecting and the introduction of alien predator species, such as domestic cats. There is no guarantee that the cats will not be reintroduced into the area, which could cause problems for the snake, whether that be from being preyed on themselves or through the population decline of their main food sources. The presence of these cats have negatively affected many populations of several other island reptile species. Because the rattlesnake has such limited options for prey on the island, it is vital to maintain the populations of prey for the conservation of C. catalinensis, especially because the main species of prey for the snake, Peromyscus slevini, is endemic to the island. There are currently no movements towards the conservation of this species. Bahía de Loreto National Park considered the rattlesnake as "very difficult to observe" which adds to the lack of conservation efforts. Because this species is endemic to the island and the territory is fairly small, the area being 40.99 km2, the snakes are faced with possible inbreeding, demographic and environmental stochasticity, and more. However, there are no studies, as of 2005, to support these speculations.

The habitat on Santa Catalina Island consists of Gulf Coast desert plants. This snake is often found on the western side of the island within the many arroyos. Further inquiry into the arboreal behavior of this island species has shown individuals actually spend the majority of their time on the ground. Individuals were most likely to be found in low branches of bushes during July, the hottest month on the island. The Santa Catalina rattlesnake is thought to climb off of the hot ground and into bushes to thermoregulate. For example, the main prey is the endemic Santa Catalina Deer Mouse, Peromyscus slevini, which are measured to 21 cm (8.3 in) in total length including the tail, which is 10 centimeters (3.9 in). Like other rattlesnake species from central and northern Mexico, the Santa Catalina rattlesnake has greater foraging activity during the summer and rainy season (July–October).

Laurence Klauber (1972) suggested this species is closely related to C. scutulatus. However, a later study by Murphy and Crabtree (1985) used allozyme data to conclude the Santa Catalina rattlesnake shares its most recent ancestor with C. ruber. Most of the morphological, biogeographic, and biochemical data suggest the same.