The Southern Pacific Rattlesnake

(Crotalus oreganus helleri)

by Jay Sharp

Southern Pacific Rattlesnake Coiled against a Log
Southern Pacific Rattlesnake Coiled against a Log Photo by kevdog818 (Getty Images)

In recent years, the southern Pacific rattler has become an increasingly dangerous snake in the southwestern region of California and in the northern part of Mexico's Baja Peninsula. According to Michael Tennesen, Scientific American, April 2009, the species appears to be expanding its range and inflicting more bites. Moreover, said Tennesen, "Anecdotal reports suggest that the snake's venom contains more neurotoxin that it did a few years ago." It has become known as the "people-biting snake in California," said Loma Linda University Medical Center's Dr. Sean Bush, quoted by Tennesen.

Characteristics of the Southern Pacific Rattlesnake

The southern Pacific rattlesnake is one of the nine subspecies of western rattlesnakes, which range across much of the western United States, Mexico and Canada. Like its relatives, the southern Pacific rattlesnake has a relatively hefty body and stubby tail, a triangular-shaped head, hooded eyes and elliptical pupils. Like not only rattlesnakes but also like the cottonmouth water moccasins and the copperheads, the southern Pacific rattler has a small opening, or heat-sensing organ called a loreal pit, on each side of its face, between its nostrils and eyes, giving it membership in the subfamily called "pit vipers."



The southern Pacific rattlesnake's distinguishing features include:

  • Adult Size and Weight: Typically, its chunky body, covered with ridged, or "keeled," scales, measures two and a half to three feet in length, although it can exceed four feet in length. Its weight ranges from some ounces to perhaps a few pounds.
  • Color and Pattern: Highly variable, with base color ranging from dark brown to greenish brown to gray. Back marked with dark-rimmed blotches separated by light bands. Tail circled by dark bands.
  • Head and Face: Triangular head with thin neck; eyes have vertically elliptical "cat-like" pupils but no eyelids; black stripe from eyes to mouth.
  • Fangs: Like all of its close kin, the southern Pacific rattler has paired fangs that unfold from the roof of its mouth, becoming fully extended as it strikes. Its fangs function much like hypodermic syringes, conducting venom from glands within the skull, behind the eyes, into a victim's tissues. If the snake loses fangs, it has developing fangs, in the same sheath, waiting in line to replace the originals.

    Southern Pacific Rattlesnake rattles
    Southern Pacific Rattlesnake.  Photo by johnaudrey (Getty Images)
  • Rattles: Like all except one rare species within its close kin, the southern Pacific rattler has a blunt tail with jointed rattles—actually a series of nested modified scales. It adds a new rattle with each molt. It may add several rattles within a year, and it may occasionally lose a rattle to breakage. It has, according to A. H. Savitzky and B. R. Moon, Old Dominion University, specialized tail muscles and a uniquely modified tail skeletal structure that it uses to shake its rattles at a very high frequency.
  • Senses: The snake, says University of California's Professor Walter E. Howard, "Control of Rattlesnakes," has a good sense of smell and vision. Although deaf, it has the ability to detect vibrations in the ground. With its forked tongue and Jacobsen's organ (on the roof of its mouth), it can detect microscopic particles it uses to assess its surrounding environment and track prey. Using the heat-sensing organs on the sides of its face, even in total darkness, the snake can, according to the San Diego Zoo, detect nearby "prey that are as little 1/10 of a degree warmer than their background!"
  • Communication: Like all its relatives, the southern Pacific rattler uses its rattles to warn away intruders. It may also hiss a warning. The female, said the San Diego Zoo, leaves scent marks that serve as sign posts for her progeny to follow to a wintering den.
  • Strike and Bite: The southern Pacific rattler, like its kin, usually corners or ambushes its prey. It can deliver a strike across a distance equivalent to a third to a half the length of its body. Oftentimes, its bite does not cause immediate death. The snake typically follows and overtakes its wounded and struggling victim by using its sight, its forked tongue and Jacobsen's organ, and its heat-sensing organs.
  • The Venom and Its Effects: The venom of the southern Pacific rattlesnake "is unique," says Brandon Cornett, "Southern Pacific Rattlesnake – One of San Diego's Rattlers." Most rattlesnakes "have a venom that primarily acts as a hemotoxin. This means it attacks the victim's blood cells," inducing rapid bruising, swelling and pain at the site of the bite. By contrast, the southern Pacific rattler "has a venom that can cause various neurological systems…" This means that it rapidly attacks the victim's nervous system, possibly triggering problems in breathing, loss of muscle control, and collapse into coma. We are, said Dr. Sean Bush, "only now learning how potent and varied rattlesnake venom can be."

Distribution and Habitat

The southern Pacific rattler's range encompasses westernmost counties in southern California, the islands of Santa Cruz and Santa Catalina, and the northwestern quadrant of Mexico's Baja California. It lives in a diverse range of habitats, including, according to the Aquarium of the Pacific, "grasslands, mountain forests [western slopes], coastal dunes, rocky deserts and hillsides, and agricultural fields." In the northern part of its range, it may hibernate in rocky crevices.


Its diet includes, according to the San Diego Natural History Museum, "small mammals, birds, reptiles and amphibians." The adult snake eats perhaps every couple of weeks, typically consuming prey equivalent to about 40 percent of its body weight over the course of a year.

Behavior and Life Cycle

Like other rattlers, the southern Pacific rattler follows a torpid existence, remaining idle some 90 percent of its life. When hungry, it takes to the hunt under the darkness of night, relying heavily on its heat-sensing organs and forked tongue and Jacobsen's organ to locate and take prey. When the weather turns cool in the fall, the snake enters hibernation.

Emerging from hibernation in the spring, the southern Pacific rattlesnake begins its breeding season, which may last for a couple of months. Two males may perform "combat dances" to win the right to mate with a female, who releases chemicals called "pheromones" to signal her receptivity. The triumphant male rubs her with his chin and touches her with his tongue in an elaborate courtship prior to mating.

Unlike some other species, which may lay eggs, the female southern Pacific rattler delivers eight or ten live young about three months after mating. The young, each six to ten inches in length, are born with fangs, with each newborn ready to defend itself and hunt prey. Young snakes leave the mother in short order, becoming independent, though they may follow the mother's scent to a wintering den that may be used for many generations. In the wild, the snake may live for one or two decades.

Southern Pacific Rattlesnake in motion.

Southern Pacific Rattlesnake in motion.  Photo by kevdog818 (Getty Images)


The southern Pacific rattler has not been listed as threatened or endangered, but it may face long-term risks in the wake of human expansion and development. It has always fallen to predators such as coyotes, hawks and roadrunners.

Interesting Facts

Typically, while a rattlesnake consumes prey equivalent to about 40 percent of its weight over the course of a year, in at least one instance, a rattlesnake was found to have swallowed a single prey that weighed 23 percent more than the snake itself.
Given sufficiently large winter den space, rattlesnakes may hibernate by the hundreds, forming large balls through the season.

Research done on pit vipers, says the Aquarium of the Pacific, has contributed advances in our military's night vision and heat seeking technology.
A rattlesnake's head, just severed from its body, "can still bare its fangs and bite," said Howard. "The heat sensory pits will still be functioning, and the warmth of a hand will activate the striking reflex."

Rattlesnake venom, dried and stored, has been found to be "toxic to mice for at least 27 years," according to the USGS Southwest Biological Science Center.

For tips for avoiding rattlesnake bites, see: /reptiles/how-to-keep-rattlesnakes-away.html#yard.

For tips for dealing with a rattlesnake bite, see: /reptiles/rattlesnake-bites.html.


Read more about rattlesnakes:
- Rattlesnakes
- The Sidewinder
- Western Diamondback

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