Microevolutionary change in mimicry? Erosion of rattling behaviour among nonvenomous snakes on islands lacking rattlesnakes

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.