- University of Calgary
- University of Toronto
Summary
1. Most studies examining interactions between insectivorous bats and tympanate prey use the echolocation calls of aerially-feeding bats in their analyses. We examined the auditory responses of noctuid (Eurois astricta) and notodontid (Pheosia rimosa) moth to the echolocation call characteristics of a gleaning insectivorous bat, Myotis evotis.
2. While gleaning, M. Evotis used short duration (mean ± SD = 0.66 ± 0.28 ms, Table 2), high frequency, FM calls (FM sweep = 80 − 37 kHz) of relatively low intensity (77.3 + 2.9, −4.2 dB SPL). Call peak frequency was 52.2 kHz with most of the energy above 50 kHz (Fig. 1).
3. Echolocation was not required for prey detection or capture as calls were emitted during only 50% of hovers and 59% of attacks. When echolocation was used, bats ceased calling 324.7 (±200.4) ms before attacking (Fig. 2), probably using prey-generated sounds to locate fluttering moths. Mean call repetition rate during gleaning attacks was 21.7 (±15.5) calls/s and feeding buzzes were never recorded.
4. Eurois astricta and P. rimosa are typical of most tympanate moths having ears with BFs between 20 and 40 kHz (Fig. 3); apparently tuned to the echolocation calls of aerially-feeding bats. The ears of both species respond poorly to the high frequency, short duration, faint stimuli representing the echolocation calls of gleaning M. evotis (Figs. 4–6).
5. Our results demonstrate that tympanate moths, and potentially other nocturnal insects, are unable to detect the echolocation calls typical of gleaning bats and thus are particularly susceptible to predation.
Methodology
Foraging experiments were conducted at the University of Calgary's Kananaskis Centre for Environmental Research (KCER), Kananaskis Valley, Alberta, Canada (51° 02' N, 115° 03' W, elevation 1390 m) between May and August 1987 and 1988. Bats were caught in mistnets or Tuttle traps (Tuttle 1974) and housed in a 2.3 x 2.3 x 1.8 m indoor flight cage. To determine if M. evotis could glean, we presented 35 flying bats (some used more than once) with one or more of 3 foraging situations: (1) moths fluttering on the ground, (2) moths fluttering on a barkcovered vertical trellis, and (3) flying moths. We recorded foraging behaviour and capture success during a 5 min test period for each bat. All experiments were conducted the evening following capture to minimize the bats' habituation to the laboratory environment.
To characterize the echolocation calls of gleaning M. evotis, we recorded the bats' vocalizations during prey presentations. The flight cage was dimly lit (3–4 lux) with a red bulb to permit observation. We lined the cage walls with plastic, except for one corner where the bats roosted, so that each bat started from the same position for each experiment. Moths, collected at an ultraviolet light at KCER, were pinned through the abdomen to a bark-covered vertical trellis 1.85 m from the roosting corner. At the start of an experiment each bat was induced to fly, and once flying, moths were prodded to flutter. An ultrasonic broadband microphone (flat ±5 dB 15–80 kHz; Simmons et al. 1979a) was placed 5 cm behind the moth, and by coupling it to a Tektronix Type 502A Storage Oscilloscope and/or a Racal Store 4DS instrumentation tape recorder operating at 76 cm/s, we monitored/recorded echolocation calls.
Calls were analyzed using the Interactive Laboratory System (ILS) software package (Signal Technology Inc., 5951 Encina Road, Goleta, CA 93117, USA). Oscillograms and power spectra using Fast Fourier Transform (FFT) and a Hamming analysis window were obtained for each call (Fig. 1). Call durations and repetition rates were calculated from oscillograms while highest, lowest and peak (maximal spectral) frequencies were obtained from power spectra.
In 1989, echolocation call intensities during gleaning attacks were measured with a Brüel and Kjaer (B & K) Type 2606 measuring amplifier fitted with a B & K Type 4135 1/4 inch condenser microphone (flat ±5 dB 0.02–120 kHz). Intensities from known distances were recorded as oscilloscope voltages. Simulated bat calls, produced with a Wavetek Model 22 11 MHz Stabilized Sweep Generator (90–35 kHz, sweep speed 0.1 s) and broadcast to the condensor microphone with a Technics EAS–10TH400B Leaf Tweeter (flat ±5 dB 5–70 kHz), were used to determine dB SPLs. The peak-to-peak intensity of the generated signal was adjusted until the output peak-to-peak voltage equalled that produced by the bats. The dB reading from the recorded distance and from 10 cm (i.e. emitted intensity) was then read from the measuring amplifier.
Auditory studies of moths collected at KCER were conducted at KCER and at the Queen's University Biological Station (QUBS), Chaffey's Lock, Ontario, Canada using techniques modified from Fullard (1984). We used the moths Eurois astricta (Noctuidae) and Phoesia rimosa (Notodontidae) because both species are abundant in the Kananaskis Valley. Wild-captured moths were decapitated and fastened dorsum up to a block of modelling clay and extracellular recordings were made of the moths' tympanic nerve (IIIN1b, Roeder 1967) with stainless steel hook electrodes. Responses of the auditory receptors were observed on-line with thresholds defined as the first discernible change in baseline activity to acoustic pulses of 10 ms, 1 ms rise/fall, delivered at 1 pulse/s. Auditory threshold curves (audiograms) were derived from these responses for stimulus pulses from 5 to 120 kHz at 5 kHz increments. Duration/response relationships of the moths' A1 auditory receptor were derived from pulses ranging from 1 to 25 ms at intensities of 60, 70 and 85 dB SPL and frequencies of 30, 40 and 60 kHz.