SPECIES NAMECrotalus oreganus
RISK TO HUMANSHighly venomous
Great Basin Rattlesnake (Crotalus oreganus lutosus)
Northern Pacific Rattlesnake (Crotalus oreganus oreganus)
Southern Pacific Rattlesnake (Crotalus oreganus helleri)


Great Basin Rattlesnake Range Map


Northern Pacific Rattlesnake Range Map


Southern Pacific Rattlesnake Range Map


Western Rattlesnake activity chart

A heavy-bodied pit viper with a large triangular head, elliptical pupils, keeled scales, and a rattle. 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 corner of the mouth across the eye. Adult length ranges from 15-36 inches, less commonly reaching lengths up to 65 inches. Neonates measure approximately 10.5 inches at birth. Males are generally larger than females.

Widespread throughout California inhabiting 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 other 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.

Feeds 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 prey . Adults eat once every 2-3 weeks and juveniles once a week. They swallow their prey whole and are able to digest everything except teeth, hair, and feathers.

Typically unaggressive. Will remain motionless and silent as first line of defense, but may rattle and posture to strike if threatened. May strike if startled or provoked. Most active from late-March through early November depending on local environmental conditions. May remain active year-round in Southern California where temperatures remain relatively consistent. Primarily active during the cooler periods of the day, will bask in open areas during the morning hours and seek shelter during the hottest parts of the day. May stay active into the evenings past midnight on warm nights. Overwinters in subterranean dens, rock crevices, caves, and under houses, outbuildings, wood piles, and debris. 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.

Mating occurs shortly after emergence from overwintering retreats and again in the fall. The female stores sperm until the following year and gives birth to live young between August and October. Males reach sexual maturity between the ages of 3 and 5 and females from 5 to 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 mixture of haemotoxic and myotoxic chemicals. Haemotoxic proteins target the circulatory system by interfering with blood pressure, platelets and clotting factors, causing hemorrhaging through weakening of the blood vessel walls. Myotoxic proteins 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.