Ongoing South Dakota research shows that the venom a prairie rattlesnake uses to disable its prey is different in the higher elevations of the Black Hills than at lower elevation, perhaps because the cooler, more rugged terrain of the Black Hills requires the snake to disable its prey faster.

As scientists learn more, that could lead to different strategies for treating humans bitten by rattlesnakes, depending on where they were when they were bitten.

Black Hills State University biologist Brian Smith, his student Mallory Ageton, Eduardo Callegari from the University of South Dakota and Steve Mackessy from the University of Northern Colorado participated in the study.

Smith explained that the scientists collected venom from snakes from known den sites in the Black Hills area. The venom came from highlands snakes (collected at den sites at high elevations in a closed canopy ponderosa pine forest), and lowland snakes (collected at den sites at lower elevations around the Black Hills, in prairie habitat). Mackessey provided venom from den sites on the prairies of eastern Colorado.

The scientists then “fractionated” these venoms to identify the proteins found in all three populations and to compare the three populations. Callegari, who runs a proteomics facility at the University of South Dakota in Vermillion, worked with Ageton on that part of the study.

They found that venoms of the three rattlesnake populations had 75 proteins in common, and South Dakota’s highland snakes from closed canopy forest were more similar in venom composition to the snakes of eastern Colorado. They shared 15 proteins in common, none of which are found in South Dakota’s lowland snakes, even though those lowland snakes are only 50 miles from the Black Hills.

“We became particularly interested in three specific proteins that are found in our South Dakota highland snakes that are not found in South Dakota lowland snakes. These proteins affect muscular coordination. So, the question is: Why do snakes found in closed canopy forests of the Black Hills have these three proteins, which are not found in the grasslands snakes surrounding the Black Hills? We suspect these three proteins kill prey faster and affect prey such that they can’t travel as far before they die,” Smith said.

The scientists have three hypotheses about why venom composition differs, Smith said. 1) Diet. It probably differs between the areas. “We know, for instance, that prairie dogs aren’t available as food items for the highlands snakes. There may be other differences in prey base as well, but we haven’t done mammal trapping so we can’t comment on that.” 2) Habitat. The forests have a complex habitat compared to the grasslands. “It might be important to immobilize a prey item faster in such complex habitat, so it is easier for the snake to locate the prey item that was bitten,” Smith said.

3) Temperature regulation. “The ability to immobilize a prey item quickly may make it easier - less energy intensive - for snakes to reach the prey,” Smith said. That could be an important factor at higher elevations because reptiles are “cold-blooded” animals and depend on the warmth of the sun. The warmer they get, the faster and more coordinated they are in their movement. “Closed canopy forest doesn’t provide too many opportunities to regulate body temperature, since the sunlight reaching the forest floor is, at best, dappled sunlight, but more often is completely shaded, such that the snakes don’t have an opportunity to warm up to temperatures that are above air temperatures,” he added. Smith noted that snakes that are capable of soaking up the sun can raise their body temperatures up to 98 degrees F, even if the air temperature is in the 60s or lower.

Smith’s studies with venom are continuing.

“People are interested in our work because of the possible ramifications to snakebite treatment,” Smith said. “It could be that snakebites might have to be managed somewhat differently, depending on where the snakes that bit the patient came from. It would not be necessary to use a different antivenom, but we considered the fact that the symptoms might have to be managed differently, depending on the geographical location where the victim was bitten. For example, a bite might have to be managed more aggressively depending on where the patient was when they were bitten.”

Smith said his research on the rattlesnake’s venom underscores for him how finely adapted the prairie rattlesnake is to the terrain in which it hunts. But he adds that mice or other very small animals are its typical prey of choice. That famous rattle is the snake’s way of signaling that it would just as soon avoid confrontations with humans or their pets, Smith said.

Research reported in this article was supported by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under grant number P20GM12345.

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