The Western rattlesnake is a venomous pit viper species that is native to North America. The size of this species varies greatly, with some populations being stunted and others growing very large. Juveniles usually have more or less distinct patterns, but these fade as the animals mature. The color of the iris often matches the ground color, which may be bronze, gold, or different shades of tan, pink, or gray. For example subspecies, Northern Pacific rattlesnakes can have a dark-brown, dark-gray, olive-brown, or sometimes black or pale yellowish ground color overlaid dorsally with a series of large, dark blotches with uneven white edges. Their belly is pale yellow, usually with brown spots. A large, dark-brown blotch on the snout has a pale border behind it that forms transverse bars on the supraoculars. There is a dark brown postocular stripe with a white border that extends from the eye to around the angle of the jaw.
Western rattlesnakes are found in North America from southwestern Canada, through much of the western half of the United States, and south into northern Mexico. In Canada, they are found in southern British Columbia. In the US, they occur in Washington, Oregon, western and southern Idaho, California, Nevada, Utah, Arizona, and likely west-central New Mexico. In northern Mexico, they are found in western Baja California and the extreme north of Baja California Sur. Western rattlesnakes inhabit grasslands, chaparral-covered foothills, canyons, mountain forests, and rocky areas.
Western rattlesnakes are live on the ground but sometimes may climb into shrubs or trees. They are generally not aggressive and lead a solitary life. Western rattlesnakes are usually active at dawn and dusk but during hot summer may become nocturnal. They may come out sometimes during the day to bask in the sun, but usually spend most of the day hidden in their shelters. During cold months they hibernate in mammal burrows, crevices or caves.
Western rattlesnakes are carnivores that feed on birds, bird eggs, and small mammals, from mice to rabbits. They also eat small reptiles and amphibians. The juveniles prey mainly on insects.
Western rattlesnakes mate in spring when they emerge from hibernation. Females give birth to as many as 25 live young. Baby rattlesnakes are born precocial (fully developed).
There are no major threats to Western rattlesnakes at present. However, locally they do suffer from habitat loss due to residential and commercial development. In Baja California, these snakes suffer from urbanization, agro-industry, and the construction of new roads. Western rattlesnakes are also often persecuted because of fear.
According to IUCN, the Western rattlesnake is locally common and widespread throughout its range but no overall population estimate is available. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.