- University of Southern Denmark
- University of Toronto
- Cornell University
Echolocating bats and eared moths are a model system of predator–prey interaction within an almost exclusively auditory world. Through selective pressures from aerial-hawking bats, noctuoid moths have evolved simple ears that contain one to two auditory neurons and function to detect bat echolocation calls and initiate defensive flight behaviours. Among these moths, some chemically defended and mimetic tiger moths also produce ultrasonic clicks in response to bat echolocation calls; these defensive signals are effective warning signals and may interfere with bats' ability to process echoic information. Here, we demonstrate that the activity of a single auditory neuron (the A1 cell) provides sufficient information for the toxic dogbane tiger moth, Cycnia tenera, to decide when to initiate defensive sound production in the face of bats. Thus, despite previous suggestions to the contrary, these moths' only other auditory neuron, the less sensitive A2 cell, is not necessary for initiating sound production. However, we found a positive linear relationship between combined A1 and A2 activity and the number of clicks the dogbane tiger moth produces.
Animals and acoustic presentation
Experiments were conducted at Queen's University Biological Station (QUBS) in southeastern Ontario, Canada. Cycnia tenera eggs were taken from wild-caught adults and raised to pupae on dogbane, Apocynum androsaemifolium, and Indian hemp, Apocynum cannabinum. Pupae were stored at 4°C (12 L : 12 D cycle) for several months, and then transferred to 25°C (16 L : 8 D) rooms. Adults emerged two to three weeks later and matured for 12–24 hours. Moths were exposed to pulsed synthetic sounds generated by Matlab (v. R2006b, MathWorks, USA), broadcast via a high-speed data acquisition card (National Instruments, Austin, TX, USA), ultrasonic amplifier (70101, Avisoft Bioacoustics, Germany) and ultrasonic speaker (ScanSpeak 60102, Avisoft). The speaker was 20 cm behind and ventral to the moth in the chamber (behaviour) and foam-lined Faraday cage (electrophysiology). This system was calibrated and intensities were measured as described in Fullard et al. (2003).
Moths were tethered from their dorsal thorax using wax and a rigid wire, suspended in a foam-lined chamber and left in darkness for 20 min before playbacks began. After acclimatization, moths remained relatively motionless throughout trials. Acoustic stimuli and tymbal MCs produced in response to these stimuli were detected using an Avisoft CM16 microphone and recorded using an ultrasound acquisition board (Avisoft USG 416) connected to a laptop running Avisoft Recorder at a sampling rate of 250 kHz. The .wav files were subsequently analysed using BatSound Pro v. 3.2 (Pettersson Elektronik, Sweden). The microphone was equidistant from the moth and the speaker. We noted the total number of MCs produced to each pulse. Moths were exposed to 32 short-rise pulse, 32 long-rise pulses and then 32 short-rise pulses once more (each series 10 s apart; total duration 145 s; figure 1). We then ablated one ear and re-exposed each moth to the stimulus paradigm to later determine how many afferent spikes would have been necessary to elicit the observed number of click MCs in monaural versus binaural subjects prior to electrophysiological preparation.