MELISSA BLOCK, HOST:
Here's a sound you'd rather not hear out on a hike.
(SOUNDBITE OF A RATTLE)
BLOCK: That's a Prairie Rattlesnake from western South Dakota. Well, there's only one thing worse than a rattlesnake giving you that famous warning, one that gives you no warning at all. That's what's been happening with some rattlesnakes in South Dakota's Black Hills. They have apparently lost their ability to rattle.
As South Dakota Public Broadcasting's Gary Ellenbolt reports, that may be good for those snakes but bad for people.
GARY ELLENBOLT, BYLINE: On a nice day during her senior year of high school, Bonny Fleming decided to take a walk in South Dakota's Black Hills.
BONNY FLEMING: There was a rock outcropping and I went to sit on that rock, but I just hopped kind of over it. And there was a rattlesnake right under my legs.
ELLENBOLT: It was Fleming's first encounter with a rattlesnake, which left her alone once it made its famous noise.
The sound made by Crotalus Viridis, or the Prairie Rattlesnake, is one of the most chilling sounds heard in nature. There's a specific purpose to the rattle: To keep larger animals and humans away. But Terry Phillip, a naturalist at Reptile Gardens in Rapid City, says the rattle often leads to the snake's demise.
TERRY PHILLIP: And the snakes that are discovered are the ones that have a really strong muscle next to their rattles, so that they actually function the way they're supposed to. So people will be out walking or gardening or whatever, and the rattlesnake starts rattling, well, it gets the end of the shovel or the .22 pistol or whatever.
ELLENBOLT: Over the past couple of years, Phillip has noticed an increasing number of rattlesnakes with what he calls curly-Q tails. Imagine the tail on a pig and you'll get the idea. Phillip says the tail muscles on these snakes have apparently atrophied, so they can't shake their rattle. And rattlesnakes that can camouflage themselves well are less likely to be killed.
PHILLIP: And so the snakes that have that genetic defect, those are the ones that are surviving to then reproduce, and they're passing on that genetic defect to their offspring.
ELLENBOLT: A genetic defect is just one of a few theories that might explain this change. Phillip says rattlesnakes that can't rattle tend to be more aggressive, since they're missing a key protective element. Bonny Fleming, the hiker who encountered a prairie rattlesnake in high school, says that's not good.
FLEMING: I think that's terrifying. Every encounter I've had, I've been able to avoid them because of that warning. So it's really helpful.
ELLENBOLT: Black Hills State University professor Brian Smith has made a career out of researching rattlesnakes. He has about 20 Prairie Rattlers near his office on the Spearfish, South Dakota campus. Smith says it may not always be a genetic defect that keeps rattlers from rattling.
BRIAN SMITH: Snakes do get abnormalities. Snakes do get their - tails can be broken during failed predation attempts.
ELLENBOLT: You shouldn't think a rattlesnake has to rattle before biting. It doesn't always do that. So you should always keep your eyes and ears open for rattlesnakes. And if you think you'll be safe from a snakebite during of the cold South Dakota winter, Smith says that's not necessarily true.
SMITH: We've recorded them in temperatures close to 40 degrees. I've collected them every month of the year except February and March. So we have collected them in January, basking at a den entrance.
ELLENBOLT: So, to review, if you're hiking in South Dakota and hear this sound...
(SOUNDBITE OF RATTLING)
ELLENBOLT: ...take it as a polite invitation to stay away. But it's a good idea to keep your eyes on the trail, just in case there's a rattlesnake that won't or can't rattle if you startle it.
For NPR News, I'm Gary Ellenbolt in Vermillion, South Dakota. Transcript provided by NPR, Copyright NPR.