There are few things more chilling than the sound of a nearby rattlesnake. That distinctive sound serves as a warning that trouble could be on the way. The only thing worse than hearing a rattlesnake within striking distance — is not hearing it at all. A herpetologist in South Dakota's Black Hills has discovered a growing number of Prairie Rattlesnakes with atrophied tail muscles; he believes it's a genetic issue that multiplies because those snakes that can rattle usually end up being killed. But others think the situation could be an evolutionary development to avoid detection.