Batesian mimics––harmless species that converge on the warning signals of a dangerous species––are spectacular examples of adaptation, but few documented cases involve acoustic signals. Even fewer studies have documented microevolutionary change in mimicry of any kind. Here, we describe potential evolutionary change in acoustic mimicry. Many nonvenomous snakes vibrate their tail tip when threatened, making a sound resembling a venomous rattlesnake. When we compared this behaviour between gopher snakes from mainland California where rattlesnakes are present versus nearby derived island populations where rattlesnakes are absent, we found that island snakes vibrated their tail for a shorter duration. Thus, defensive tail vibration may be acoustic mimicry of rattlesnakes that is undergoing erosion in an area lacking rattlesnakes, providing evidence of possible microevolutionary change in mimicry.