Why Do Caterpillars Whistle? Acoustic Mimicry of Bird Alarm Calls in the Amorpha Juglandis Caterpillar

Caterpillar species possess a range of anti-predator defenses, from regurgitation to sonar jamming. The North American walnut sphinx caterpillar (Amorpha juglandis) produces a variety of whistling noises when pinched. Limited observations indicate that this causes avian predators to retreat, leaving the caterpillar alone. However, it is unknown why this whistle is such an effective deterrent. Interestingly, the A. juglandis whistle is acoustically similar to the “seet” alarm call that many bird species produce in response to their own predators. We tested the hypothesis that the caterpillar whistles mimic bird alarm calls by playing recordings of A. juglandis whistles, chickadee seet calls, and a control sound to flocks of small songbirds and comparing the birds’ responses to the different stimuli. Birds tended to freeze or dive for cover in response to the seet call and two types of caterpillar whistles. Our results suggest that these caterpillar whistles may protect caterpillars by mimicking the alarm calls of their avian predators.