National Geographic

Rattlesnakes Two Hours Apart Pack Totally Different Venoms

If you’re walking through the flat desert of Phelan, California, and you’re bitten by a Southern Pacific rattlesnake, you will start to bleed badly.  The snake’s venom is loaded with proteins that break down the walls of your blood vessels and that prevent the now-leaking blood from clotting.

Let’s say you survive. You bid goodbye to the desert and drive up some twisting mountain roads to the town of Idyllwild, swapping Joshua trees for pine trees. But the Southern Pacific rattlesnake lives here too, and you get bitten again. And this time, the venom doesn’t go for your blood. The toxins of these snakes include proteins that stop nerves from sending signals into muscles. They start to paralyse you.

It takes two hours to drive between these two sites. In one, you’ll find a rattler with purely haemotoxic (blood-destroying) venom. In the other, you’ll find snakes of the same subspecies with purely neurotoxic (nerve-destroying) venom.

Scientists who study snake venom know that it’s an incredibly variable weapon. Its composition can differ dramatically between different species, subspecies, individuals, or even sexes.

Still, the differences between the Phelan and Idyllwild snakes are extreme. “It’s the most complex variation that I’ve ever seen especially within such a geographically short distance,” says Bryan Fry from the University of Queensland, who led the study that team that analysed the different venoms. Even the haemotoxic venoms varied considerably in how potent they are, what toxins they contain, and what targets those toxins attack.

Fry suspects that the rattlesnakes use such diverse cocktails because they live in such different environments. The Idyllwild snakes, in particular, live on high mountain ridges that are 1,600 metres above sea level. They are extremely isolated from the other populations. “It’s like they’re on islands,” says Fry.

The mountains also contain different prey to the deserts, and the snakes there might need to kill their prey more quickly. “Your ability to track prey is very different if you’re in a rocky outcrop than if you’re in grassland. If an animal gets away, it might disappear down to a crack and you’ll never see it again,” says Fry. “We hypothesise that the neurotoxic venoms are needed to drop the prey faster.”

If that’s the case, why don’t all the rattlesnakes have the faster-acting venom? It may be that the desert-dwellers simply haven’t had the pressure to stray from their traditional haemotoxic blends, or that their venoms are adapted to killing their local prey. The short answer is: we don’t know. We barely know what these different populations eat, let alone how their venoms are adapted to killing those prey.

“It’s a perfect example of the importance of basic evolutionary studies,” says Juan Calvete, a venom researcher from the Biomedical Institute of Valencia. In 2012, he found a similar pattern in the Mojave rattlesnake from southern Arizona, whose venom also changes from haemotoxic to neurotoxic as you from east across the state. “Geographic variability in venom composition [within a species] seems to be the rule rather than the exception, particularly for wide-ranging species,” says Calvete. “However, the variability is unpredictable, and must thus be experimentally determined.”

Indeed, people who are bitten by rattlesnakes often experience very different symptoms and complications depending on where they are. For example, Calvete’s team found that if you’re bitten by a Mojave rattlesnake in Cochise County rather than in neighbouring Pima County, you’re 10 times more likely to die.

In California alone, around 800 people are bitten by rattlesnakes every year. Although just a handful die, the venom is painful, debilitating, and can lead to lengthy hospital stays. To make things worse, Fry says that the antivenom that Americans use for rattlesnake bites—CroFab—is ineffective against the Southern Pacific rattler.“It’s notoriously poor,” he says. “People have to be kept in the hospital for up to a week getting continuous infusions just to keep them alive.”

There are two problems. First, CroFab uses antibodies that are less allergenic than those in other antivenoms, but get cleared from the body very quickly. “You end up with very expensive urine,” says Fry. Second, it doesn’t contain antibodies that target the specific proteins used by the Southern Pacific rattlesnake. “They were relying on toxins to be similar to stuff from other rattlesnakes, but even within this one [subspecies], you get completely different venoms. It’s been a debacle.”

Fry thinks that both the effectiveness of antivenoms and our ability to care for patients will be greatly improved if we get a better understanding of the idiosyncracies of venom in local snakes.

The media should take note too. Several news reports have suggested that rattlesnakes in southwest USA are becoming deadlier, and rapidly evolving more toxic venom. Fry says that’s rubbish—the venoms are naturally very varied, and evolved that way a long time ago. It’s not the toxins that have recently changed, but our appreciation of just how diverse they are.

Reference: Sunagar, Undheim, Scheib, Gren, Cochran, Person, Koludarov, Kelln, Hayes, King, Antunes & Fry. 2014. Intraspecific venom variation in the medically significant Southern Pacific Rattlesnake (Crotalus oreganus helleri): Biodiscovery, clinical and evolutionary implications. Journal of Proteomics.

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There are 6 Comments. Add Yours.

  1. Mike Lewinski
    January 27, 2014

    Rattlesnakes might not be getting deadlier, but there’s some evidence that they’re becoming less prone to rattling (and so more likely to bite when stumbled upon).

    Because humans are so aggressive and kill them so readily when encountered, snakes that don’t rattle have an advantage over those that do. This is apparently a behavioral selection, and not a change to their rattles.

    We relocated two rattlesnakes out of our garden in Embudo, NM last summer. Capturing them was probably one of the scariest things I’ve done in some time.

  2. Keith Barker
    January 27, 2014

    Another piece of the story may be predation. At least one mammal in the US is venom resistant, and there may be ithers. See:
    http://onlinelibrary.wiley.com/doi/10.1111/j.1469-185X.2012.00222.x/abstract;jsessionid=5AFB174079A11797EC2CEAADBC9F2D25.f02t02

  3. Adele Edwards
    January 27, 2014

    Many years ago, while camping 90 miles east of Vancouver, my husband and friend were down at the river washing up for dinner. My husband happened to look up just in time to see a Pacific rattlesnake headed right for his friend’s buttocks. He told Al who had the presence of mind to pick up a stick and hold the snake’s head down. Jack who had a knife sliced off the snake’s head.
    Now the old Indian legend is a snake never dies until sundown.
    The two men started back to the campsite with this headless snake. A car from California just happened to drive up to them asking where was the best place to camp. Now Jack and Al didn’t want to scare them away so Jack held the headless snake behind his back. But the snake kept twitching. So the Californians asked them what it was. So Jack told them and they turned around high tailed out of the campsite. Jack did tell the

  4. Shannon Farrell
    January 28, 2014

    iv’e been an ametuer herpetologist my whole life. According to my observations , crotalus helleri who feed primarily on squirrels develop a different , more potent venom . This is due to the fact that squirrels have a higher immunity to snake venoms. Thus requiring a more potent venom to overcome the squirrels natural antibodies. Desert snakes require more nuerotoxic venoms to feed on lizards and other desert dwelling prey. Further studies need to be performed to fully understand the dynamic biodiversity of venomous reptiles and their prey.

  5. Kara Jones
    January 31, 2014

    Adele, rattlesnakes strike so fast that there’s no way your husband could have stopped one mid-strike. People put themselves far more at risk when they try to hold down a venomous snake and kill it than if they scare it away from a distance or move away from the snake. In a retrospective study of venomous snakebites, 40% occurred in people who had been drinking alcohol and 67% of bites occurred when people intentional engaged the snake. Rattlesnakes have been persecuted in this country for 100 years and many eastern populations have been extirpated. For instance, timber rattlesnakes are endangered or threatened in almost every state in which they occur and the massasauga rattlesnake is a candidate for federal protection through the Endangered Species Act. The loss of pit vipers can have knock-on effects, since they prey largely on mammals (especially rodents, which are vectors for Lyme disease and the hanta virus, amongst other things). Mammals are more likely to carry diseases which can spread to humans than rattlesnakes are to kill humans through bites. The loss of predators such as rattlesnakes will ultimately have far more negative effects on people in the U.S. than the snakes could cause themselves.

  6. Juan Cole
    February 2, 2014

    When I was a boy in Salt Lake City, one of the ways we would amuse ourselves was to hunt down rattlesnakes in the benches on the outskirts of town. The fun part was to grab the snake by the rattle, whirl it around your head, and sling that sucker down the hillside. A couple of kids would get bit eat summer, but that was no big deal – just a quick trip to the emergency ward to get patched up and all was well.

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