Western Rattlesnake (Northern Pacific Rattlesnake)

Most western rattlesnakes are small, and only measure about three feet long.

The western rattlesnake is native to the western United States and Southwestern Canada. It’s a medium-sized rattler that’s the only species in many parts of native range. It’s not a western diamondback, even though their names are similar.

Rattlesnakes are the notorious rattling sound you hear in many old western movies. It usually precedes the bad guy’s appearance and, since rattlesnakes scare most people, it fits. But, however frightened that rattling sound makes you, rattlesnakes aren’t out to get you.

Western rattlesnakes are common in the Pacific Northwest of the United States and southwestern Canada. Read all about these amazing animals!

3 Amazing Western Rattlesnake Facts

• Also called northern Pacific rattlesnakes, they are native to California, Oregon, Washington, Colorado, Idaho, and possibly Montana in the U.S. and British Columbia in southwestern Canada.
• Once considered a subspecies of prairie rattlesnake, the western rattlesnake and several others are now treated as full species.
• It’s not the same species as the western diamondback rattlesnake, even though the names are similar.

Western Rattlesnake Scientific Name

Rattlesnakes are members of the Crotalinae subfamily of snakes. All the pit vipers are in this subfamily and are native to the Americas and Eurasia. 

If you thought you knew this snake scientifically, Crotalus viridis oreganus, you’d have been right until a few years ago. Then, phylogenetic studies on hundreds of snake species caused scientists to reorganize many species. As a result, several rattlesnakes that were considered subspecies (like the western rattlesnake) became species in their own right.

Like many snakes, there are many common names, which confuses people! They’re mostly called western rattlesnakes and northern Pacific rattlesnakes. Unfortunately, prairie rattlesnakes (Crotalus viridis) are also sometimes called western rattlesnakes. The confusion that common names can cause is why most herpetologists prefer scientific names alongside common names.

Western rattlesnakes go by the scientific name Crotalus oreganus. Crotalus is from the Greek word krotalon, which means rattle or castanet. Their specific name, oreganus, refers to Oregan, where they recorded the western rattlesnake species’ holotype.

Evolution & Origins

According to scientists, rattlesnakes evolved more recently than other snakes. They’re part of the larger Viperidae family, which scientists believe developed in the early Eocene Era, between 56 and 48 million years ago. 

Crotalinae subfamily snakes, known as pit vipers, evolved sometime during the Miocene Era, 23-5.3 million years ago. They took evolution a step further and developed a heat-sensing pit between each nostril and eye, and some of those in the Americas developed rattles — rattlesnakes. 

Why they developed rattles, no one knows for sure. However, some scientists have proposed that the rattling behavior could have been triggered by the bison that roamed the same areas where rattlesnakes live.

How rattles developed is in ongoing discussion and a couple of theories exist.

The first theory says that the snakes that rattled their tails most vigorously were more successful at deterring predators and other threats. So, over time they evolved a more sophisticated warning system. 

The second theory proposes that snakes with the most tail-rattling skills developed keratin callouses on their tails, eventually becoming rattles. 

Regardless of how rattlesnakes got their rattles, the fact is that these snakes have a unique warning system.

Venom, on the other hand, has evolved separately in dozens of species. Some seem to have had venom, lost it, then gotten it back again! 

Venom evolution is complicated and not well understood. What type of venom a snake employs has a genetic component, but also environmental. For example, the prey it eats and what its prey eats can affect venom composition.

In the case of Crotalinae snakes, most venom is hemotoxic. This type of venom acts on the blood cells themselves and causes massive swelling, internal bleeding, and sometimes death. The exception seems to be the Mojave rattlesnake, which has a crazy cocktail of hemotoxic and neurotoxic venom — the percentages of each type depend on where it lives and what it eats.

Researchers agree that animals in nature are in an ongoing arms race, each trying to out-evolve the other.

Western Rattlesnake Appearance and Description

Measuring almost five feet long at times, the western rattlesnake usually averages between two and three feet long. Like other rattlesnakes, it has a big triangular head, a thin neck, with a thick body ending in a rattle. 

Northern Pacific rattlesnakes vary in color depending on their environment. These snakes have strongly keeled scales that look rough to the touch — however, we don’t recommend touching one! Their base color can be gray, yellowish, greenish, tan, or reddish brown, and they have darker blotches down the length of their back. In the last one-third of their body, the markings become relatively evenly-sized dark bands alternating with narrower light bands the end of the tail; the last couple of rings are often black.

As juveniles, western rattlesnakes have strong, clearly defined patterns. However, their patterns become less defined and harder to see as they age. Their tail ends in a bright yellow button until they develop their first rattle segments.

These snakes often have a darker band that starts behind their eyes and a lighter chin color. Their eyes sit forward on their heads and have vertical pupils, like a cat’s; like other rattlesnakes, they have an extra ridge of scales over their eyes. It makes their eyes hard to see when you look down at the top of their head.

Western Rattlesnake Behavior

They’re generally shy and avoid confrontations with people whenever possible. When a predator or other threat approaches, they tighten into a coil, leaving their head and rattle exposed. Although sometimes they strike without rattling a warning, most of the time, snakebite victims have plenty of notice to back away from the terrified reptile! 

These snakes aren’t interested in biting you. Generally, if you take a couple of slow steps away from the snake, it will use that opportunity to beat a hasty retreat — sometimes even as it hisses and rattles.

Although mostly terrestrial, this species can climb! They’ve been found in trees and shrubs, presumably hunting baby birds or eggs.

Rattlesnakes are more sedentary than racers or kingsnakes and don’t need to eat as often. Western rattlesnakes are ambush predators. So, while they wait for a meal, they may sit for days without moving. 

Western rattlesnakes are primarily solitary. However, they often brumate through the winter in communal dens, sometimes with other snake species. In the spring, they’ll sometimes stay near one another as they bask and warm up after the long winter.

Habitat

This species prefers a drier habitat, yet they don’t live in deserts. Western rattlesnakes are versatile and live in shrubby basins, canyons and chaparral, dry forests and flatlands. They’re also found in open fields and at the edges of housing developments. These snakes occur at altitudes of up to 8,000 feet above sea level. 

They seek shelter from extreme temperatures and to brumate through the winter in abandoned burrows, caves, crevices, and other hidden areas. 

Diet

Western rattlesnakes use their camouflage to their advantage. They wait until something gets close enough that they can reach with a lightning-fast strike. 

Their diet includes small rodents like rats and mice, they’ll also eat rabbits, birds, and bird eggs. Young snakes feed on lizards and insects until they get big enough to take on rodents. 

This species only needs to eat about once a month. However, in the early spring they eat as much as possible to rebuild glucose and fat stores lost during winter brumation.  

Venom: How Dangerous Is It?

As many as 30-40% of all rattlesnake bites are dry, where there’s no venom injected. Rattlesnakes prefer to save their venom for their prey!

Although they prefer to avoid people, they do bite when threatened. Most bites happen because someone either stepped on the snake or picked it up. 

Western rattlesnake venom carries hemotoxins that directly attack the blood. It can cause massive swelling, blood clotting problems, and internal bleeding. However, they also have some neurotoxins that attack the nervous system and can cause respiratory and heart distress. Although it’s rarely fatal in people, snakebite victims must get prompt medical care to minimize the tissue damage. 

Reproduction, Babies, and Lifespan

Females typically mature by the time they reach their third year, with a slower maturity rate observed in northern populations. Most females mate in the spring every two to three years, and give birth to between one and 21 babies in the summer. In cooler climates, they may wait up to six or seven years between breeding cycles; additionally, they often mate in the fall before brumation and give birth in the spring. 

While they’re gravid, females stay close to the dens, so the babies are born near shelter. Often, the mother snakes stay near their newborns for a week or two after birth.  

Threats, Population, Conservation Status

Rattlesnakes of all kinds are prey for other larger predators. This includes king snakes and coachwhips, hawks, eagles, and coyotes. Their vulnerability to so many predators explains their highly defensive behavior. If everything in nature were out to get you, you’d be afraid too!

IUCN Redlist of Threatened Species classifies western rattlesnakes as “least concern,” because of its wide distribution and tolerance of different habitats. However, the last assessment was in 2007, so things may have changed since then. 

Conservation threats include being hit by cars and killed by people out of fear. However, the bigger challenge is habitat destruction. As more real estate developments pop up in the west, their natural habitat is reduced. Habitat loss also causes more interactions between people and rattlesnakes that can end with the person bitten or the snake injured.

Similar Animals

• Western diamondback rattlesnake
• Prairie rattlesnake
• Southern Pacific rattlesnake