It may be terrifying to hear a rattlesnake's signature sound—but, hey, at least it's giving you a heads-up. Now, hikers in South Dakota's Black Hills have to keep their eyes peeled for rattlesnakes that can't rattle, NPR reports. These snakes have curled tails like pigs thanks to what seem to be atrophied muscles, says an expert. It may be a genetic defect.
"Defect" or not, it's probably helping the snakes: Those that do rattle are likely to get spotted and killed, so the non-rattlers have built-in protection that they may be passing on to their descendants. On the other hand, it could just be that certain snakes have had their tails broken "during failed predation attempts," says a researcher. Either way, it seems hikers may need more than just their ears to avoid them.