Sound strategy: acoustic aposematism in the bat–tiger moth arms race
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The night sky is the venue for an ancient arms race. Insectivorous bats with their ultrasonic sonar exert an enormous selective pressure on nocturnal insects. In response insects have evolved the ability to hear bat cries, to evade their hunting maneuvers, and some, the tiger moths (Arctiidae), to utter an ultrasonic reply. We here determine what it is that tiger moths “say” to bats. We chose four species of arctiid moths, Cycnia tenera, Euchaetes egle, Utetheisa ornatrix, and Apantesis nais, that naturally differ in their levels of unpalatability and their ability to produce sound. Moths were tethered and offered to free-flying naïve big brown bats, Eptesicus fuscus. The ability of the bats to capture each species was compared to their ability to capture noctuid, geometrid, and wax moth controls over a learning period of 7 days. We repeated the experiment using the single arctiid species E. egle that through diet manipulation and simple surgery could be rendered palatable or unpalatable and sound producing or mute. We again compared the capture rates of these categories of E. egle to control moths. Using both novel learning approaches we have found that the bats only respond to the sounds of arctiids when they are paired with defensive chemistry. The sounds are in essence a warning to the bats that the moth is unpalatable—an aposematic signal.
ESM1 (video) Interaction of big brown bats with palatable, silent control moths. This video is a collage of different big brown bats capturing control palatable and silent (C-S-) wax moths. The approach angles and handling of the moths is different (wing vs. tail captures) however the outcome is the same: the experienced bats capture and consume the unprotected moths (avi; 10.4MB). Visual interactions were recorded at 250fps on a pair of digital, infrared, high-speed Photron® FastCam PCI 500 cameras. Acoustic interactions were recorded with an UltraSound Advice QMC100 bat detector in BatSound Pro v3.2. Audio and video tracks were synchronized digitally by non-linear editing in Adobe Premier™-v6.5. Playback at 10fps (slowed down approximately 25x)
ESM2 (video) Interaction of a big brown bat with an unpalatable, sound-producing tiger moth. This video shows the approach of an unpalatable and sound-producing (C+S+) Cycnia tenera tiger moth by an experienced big brown bat. As the bat emits its ultrasonic cries to locate the moth, the moth responds with a series of clicks that warn the bat of its chemical defense and the bat aborts the attack (avi; 9MB). Methods are the same as in supplemental video 1
ESM3 (Table 1) Comparison of the signs of the slopes of the a-priori predictions and the empirical results