The ultrasonic clicks produced by some tiger moths — all of which possess bat-detecting ears — are effective acoustic aposematic or mimetic signals, conferring protection against aerial hawking bats. Clicks are produced in response to bat echolocation calls. Palatable, silent non-tiger-moth species with bat-detecting ears fly away from distant bats and effect erratic flight maneuvers or stop flying in response to the calls of bats nearby. These flight responses are also an effective defense. We tested the hypotheses that sound-producing tiger moths (i) do not exhibit the reduction in flight time typical of silent, palatable moth species when presented with ultrasound simulating bat echolocation calls and (ii) exhibit more flight activity than silent, palatable species both in the presence and absence of ultrasound. We found that sound-producing tiger moths did not significantly reduce flight activity to bat-like sounds and that silent tiger moths and other noctuoid species did. We also found that sound-producing tiger moths flew significantly more than did silent species in both the presence and the absence of ultrasound. The benefits of acoustic aposematism may allow sound producers to spend more time aloft than silent species and thereby improve their chances of successful reproduction.
Experiments were conducted at Queen’s University Biological Station (QUBS) near Chaffey’s Lock, Ontario, Canada (43° 34′N, 79°15′W), between 18 June and 29 July 2003. Male moths were captured from fluorescent and mercury vapour light traps and identified to species using criteria in Ward et al. (1974), Covell (1984), and Riotte (1992). The general auditory sensitivity of all species used in the present study to the peak frequencies of sympatric echolocating bats is similar (Fullard and Barclay 1980; Fullard and Dawson 1999; J.H. Fullard, unpublished data). All are active at night (Fullard and Napoleone 2001; J.H. Fullard, unpublished data) and thus potential prey for the eight species of insectivorous vespertilionid bats found at and around QUBS (five residential, three migratory). Six species were selected from the family Arctiidae (= Noctuidae: Arctiinae; Lafontaine and Fibiger 2006), and one each from the families Noctuidae and Notodontidae (Table 1). A priori power analysis of the paired sample one-tailed t tests producing significant results reported in Fullard et al. (2003) indicated that sample sizes of five or six individuals per species produced an estimated power of 0.82 or 0.89, respectively (for details see Fullard et al. 2004). For all species (see Table 1), save Ctenucha virginica, we used 6 individuals; for C. virginica we used 5 individuals. One-tailed tests and a priori power analyses are appropriate given the first hypothesis being tested (i.e., that sound-producing tiger moths exhibit reduced flight cessation relative to silent species rather than simply differ in response to bat-like sounds from silent species). When there was no a priori reason to use a one-tailed test, two-tailed tests were employed.
The method for quantifying the moth flight acoustic startle response of Fullard et al. (2003, 2004) was used in this study and is briefly described here. Each night, within a screen tent positioned in partially open, mixed deciduous forest, three moths of three different species were placed in individual, visually isolated screen chambers (half cylinders, 15.2 cm high × 6.5 cm radius) and videotaped for 6 h with a near-infrared camera between 2200 and 0400 (for a discussion of the validity of using cylinders to measure flight refer to Fullard and Napoleone 2001 and Soutar and Fullard 2004). Moths were exposed to simulated sympatric bat calls (based on those described for the big brown bat, Eptesicus fuscus (Beauvois, 1796), by Surlykke and Moss 2000) consisting of 25 kHz, 10 ms synthesized tones amplified to 94 dB peSPL (relative to a continual tone at 25 kHz; intensity of E. fuscus calls for prey 1 m from bat; Kick and Simmons 1984) and broadcast at a rate of 12.5·s–1 from a speaker mounted 60 cm from the moths to ensure an equal intensity sound field. Sounds of this frequency, intensity, and duty cycle induce the flight reduction in silent eared moths sympatric with bats and elicit clicks in the sound-producing species tested here (Fullard 1979; Fullard and Fenton 1977; Fullard et al. 2003).
Nightly observation periods were randomly divided into thirty-six 10 min bins of which half were designated “sound” and half “no-sound”. During sound bins, pulses were delivered to the moths for 1 min followed by 1 min of silence. During no-sound bins, moths were exposed to the same playback equipment (and attendant electronic noise) as during sound bins but without the synthetic bat pulses. Moths were deemed to be “in-flight” if they were observed moving about their cage while they were flapping their wings and “not flying” if stationary. We therefore classified both actual flight and wing fluttering accompanied by walking as “in-flight”. Total flight time within each sound bin was recorded. For further details refer to Fullard and Napoleone (2001) and Fullard et al. (2003, 2004). Flight times were scored blind to the moths’ species identification, sound or no-sound bins, and acoustic class (sound-producing species or silent species).