Why do caterpillars whistle at birds? Insect defence sounds startle avian predators.

Behav Processes. 2017 May;138:58-66. doi: 10.1016/j.beproc.2017.02.002. Epub 2017 Feb 20. Dookie AL1, Young CA2, Lamothe G3, Schoenle LA4, Yack JE5. Author information 1 University of Ottawa, School of Psychology, Ottawa, ON K1N 6N5, Canada; Carleton University, Department of Biology, Ottawa, ON K1S 5B6, Canada. 2 Carleton University, Department of Biology, Ottawa, ON K1S 5B6, Canada. 3 University of Ottawa, Department of Mathematics and Statistics, Ottawa, ON K1N 6N5, Canada. 4 Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061 USA. 5 Carleton University, Department of Biology, Ottawa, ON K1S 5B6, Canada. Electronic address: jayneyack@cunet.carleton.ca. Abstract Many insects produce sounds when attacked by a predator, yet the functions of these signals are poorly understood. It is debated whether such sounds function as startle, warning or alarm signals, or merely serve to augment other defences. Direct evidence is limited owing to difficulties in disentangling the effects of sounds from other defences that often occur simultaneously in live insects. We conducted an experiment to test whether an insect sound can function as a deimatic (i.e. startle) display. Variations of a whistle of the walnut sphinx caterpillar (Amorpha juglandis) were presented to a predator, red-winged blackbirds (Agelaius phoeniceus), when birds activated a sensor while feeding on mealworms (Tenebrio molitor). Birds exposed to whistles played back at natural sound levels exhibited significantly higher startle scores (by flying away, flinching, and hopping) and took longer to return to the feeding dish than during control conditions where no sounds were played. Birds habituated to sounds during a one-hour session, but after two days the startling effects were restored. Our results provide empirical evidence that an insect sound alone can function as a deimatic display against an avian predator. We discuss how whistles might be particularly effective 'acoustic eye spots' on avian predators. KEYWORDS: Acoustic; Avian; Caterpillar; Defence; Deimatic display; Insect; Startle