Threat Display - Deimatic or Aposematic?

Deimatic or Aposematic?

In a study of the rattling made by rattlesnakes of different species, Fenton and Licht found that the sounds are always similar: they have rapid onset (starting suddenly, and reaching full volume in a few milliseconds); they consist of a 'broadband' mixture of frequencies between 2 kHz and 20 kHz, with little energy either in the ultrasonic (above 20 kHz) or in the rattlesnakes' hearing range (below 700 Hz); and the frequencies do not change much with time (the rattling after 2 minutes having a similar spectrum to that at onset). There was no clear difference in the sounds made by the different species measured: Crotalus horridus, Crotalus adamanteus, Crotalus atrox, Crotalus cerastes, Crotalus viridis and Sistrurus catenatus. This pattern implies that the rattling "could serve as a general attention-getting device", which "is designed as a deimatic or startle display". Its similarity to the "broadband, harsh sounds" used as warning calls by birds and mammals may enhance its effectiveness. Since rattlesnakes can barely hear the sound, it is unlikely to serve as any form of communication to other snakes of the same species. Finally, the sounds are not in themselves loud enough to cause pain and hence keep predators away.

Fenton and Licht note that the effect of a rattlesnake's rattling could be deimatic (startle) in inexperienced animals, whether predators or large animals that might injure the snake by stepping on it, but aposematic (a warning signal) in animals that are aware of the rattle's meaning. They refer to the work of Bates and Fenton on the responses of the big brown bat, Eptesicus fuscus, to the defensive clicks made by moths in the family Arctiidae, which includes the garden tiger moth, Arctia caja. This family includes large, furry, bitter tasting or poisonous moths. They found that while sounds can startle inexperienced bats, after a few trials the bats ignored the sounds if the prey was edible; but the same sounds can warn experienced bats of bitter tasting prey (an honest signal).