Description
A heavy-bodied pit viper with keeled scales, elliptical eyes, spade-shaped head, infrared pits on either side of the head between their eye and nostril, and a distinctive rattle at the tip of its tail. Coloration is highly variable ranging from gray, tan, brown, olive-green to yellowish and reddish. Patterns consist of repeating dark blotches with dark edges and light borders along the dorsal surface with similar but smaller blotches along the sides. Blotches transition into distinct light and dark bands of uniform width near the rattle. Juveniles are generally lighter in color but have more contrasting patterns. An angled stripe extends upward from the rear corner of the mouth across the eye. Adult range from 15-36 inches in length and less commonly reach lengths up to 65 inches. Males are generally larger than females.

Habitat
The most widespread rattlesnake species in California. Inhabits a variety of environments including grasslands, woodlands, savannah, montane forests, pinyon-juniper woodlands, sagebrush, and coastal scrub often near areas of rocky outcrops, talus slopes, riprap, streams, brushy areas, wood piles and suburban areas that provide cover. Brumation (the process in reptiles of metabolic slowing to conserve energy similar to hibernation in mammals) occur in dens, burrows, crevices, caves, and under houses, debris piles, and other refugia in suburban and rural areas.

Diet
Preys on small mammals, particularly small rodents including mice, rats, voles, and ground squirrels, which account for two-thirds of their diet. Secondary prey includes birds, snakes, lizards, salamanders, frogs, and insects. Neonates and juveniles feed more frequently on secondary prey than adults. Feeding frequency varies seasonally peaking in June-August. Adult snakes show a preference for holding onto small prey after envenomation but will strike, release, and track larger prey such as ground squirrels. One study reported that western rattlesnakes initiated strikes at 21% of prey that passed within strike range and were 49% successful at envenomating their prey⁵. Adults eat once every 2-3 weeks and juveniles once a week on average. They swallow their prey whole and are able to digest everything except teeth, hair, and feathers.

5. Putman, B.J., M.A. Barbour, and R.W. Clark. 2016. The Foraging Behavior of Free-ranging Rattlesnakes (Crotalus oreganus) in California ground squirrel (Otospermophilus beecheyi) Colonies. Herpetologica 72(1): 55-63.

Behavior
Typically unaggressive. Will remain motionless and silent as first line of defense, but will rattle, posture, and strike if threatened. Active from late-March through early November depending on local environmental conditions. Primarily active during the cooler periods of the day during the summer; will bask in open areas during morning hours and seek shelter during the hottest parts of the day. May stay active into the evenings well past midnight on warm nights. Overwinters in subterranean dens, rock crevices, caves, and under houses, outbuildings, and wood and debris piles. Species migrate seasonally between overwintering sites and summer ranges and have been reported moving up to 1.5 km between sites. Species typically use the same pathways when traveling between overwintering sites and summer ranges. Home ranges vary in size from 9-29 acres.

Reproduction
Mating occurs shortly after emerging from overwintering retreats and again in the fall. Females store sperm until the following year and give birth to live young (ovoviviparous) between August and October. Neonates measure approximately 10.5 inches at birth and are born with a single button at the end of the tail that doesn’t rattle until additional segments are added during successive shedding. Males reach sexual maturity between the ages of 3 and 5 and females between 5 and 7. Females eat sparingly or not at all while gravid (with young) and forgo eating after giving birth until they emerge the following spring emaciated. It generally takes 2-3 years to rebuild fat reserves before repeating the breeding cycle.

Venom Profile
Venom is a complex mixture of proteins and polypeptides comprising hemotoxic and myotoxic chemicals with minor neurotoxic components. Hemotoxins target red blood cells and the circulatory system by interfering with blood pressure, platelets and clotting factors. Myotoxins cause generalized damage and destruction of skeletal muscle tissues. Different subspecies and even different populations within the same subspecies may exhibit variability in the hemorrhagic, fibrinolytic, and thrombin pathologies due to minor variations in venom toxins. Antivenin treatment may require multiple doses over several days to neutralize the venom, which may cause permanent damage and disfigurement even when administered shortly after envenomation.

Range⁶
Great Basin Rattlesnake – Alpine, Lassen, Modoc, Mono, Plumas*, Sierra*, and Siskiyou* counties.

Northern Pacific Rattlesnake – Alameda, Alpine, Amador, Butte, Calaveras, Colusa, Contra Costa, Del Norte, El Dorado, Fresno, Glenn, Humboldt, Inyo*, Kern, Kings, Lake, Lassen, Madera, Marin, Mariposa, Mendocino, Merced, Modoc*, Mono*, Monterey, Napa, Nevada, Placer, Plumas, Sacramento, San Benito, San Bernardino*, San Francisco, San Joaquin, San Luis Obispo, San Mateo, Santa Barbara*, Santa Clara, Santa Cruz, Shasta, Sierra, Siskiyou, Solano, Sonoma, Stanislaus, Sutter, Tehama, Trinity, Tulare, Tuolumne, Yolo, and Yuba counties.

Southern Pacific Rattlesnake – Imperial*, Kern*, Los Angeles, Orange, Riverside, San Bernardino, San Diego, Santa Barbara, and Ventura counties.

6. *Indicates counties where the species’ range extends only into a minor portion of the county.