Acoustic Aposematism and Evasive Action in Select Chemically Defended Arctiine (Lepidoptera: Erebidae) Species: Nonchalant or Not?

Tiger moths (Erebidae: Arctiinae) have experienced intense selective pressure from echolocating, insectivorous bats for over 65 million years. One outcome has been the evolution of acoustic signals that advertise the presence of toxins sequestered from the moths’ larval host plants, i.e. acoustic aposematism. Little is known about the effectiveness of tiger moth anti-bat sounds in their natural environments. We used multiple infrared cameras to reconstruct bat-moth interactions in three-dimensional (3-D) space to examine how functional sound-producing organs called tymbals affect predation of two chemically defended tiger moth species: Pygarctia roseicapitis (Arctiini) and Cisthene martini (Lithosiini). P. roseicapitis and C. martini with intact tymbals were 1.8 and 1.6 times less likely to be captured by bats relative to those rendered silent. 3-D flight path and acoustic analyses indicated that bats actively avoided capturing sound-producing moths. Clicking behavior differed between the two tiger moth species, with P. roseicapitis responding in an earlier phase of bat attack. Evasive flight behavior in response to bat attacks was markedly different between the two tiger moth species. P. roseicapitis frequently paired evasive dives with aposematic sound production. C. martini were considerably more nonchalant and employed evasion in fewer interactions. Our results show that acoustic aposematism is effective at deterring bat predation in a natural context and that this strategy is likely to be the ancestral function of tymbal organs within the Arctiinae.     

Convergent evolution of anti-bat sounds

Bats and their insect prey rely on acoustic sensing in predator prey encounters—echolocation in bats, tympanic hearing in moths. Some insects also emit sounds for bat defense. Here, we describe a previously unknown sound-producing organ in Geometrid moths—a prothoracic tymbal in the orange beggar moth (Eubaphe unicolor) that generates bursts of ultrasonic clicks in response to tactile stimulation and playback of a bat echolocation attack sequence. Using scanning electron microscopy and high-speed videography, we demonstrate that E. unicolor and phylogenetically distant tiger moths have evolved serially homologous thoracic tymbal organs with fundamentally similar functional morphology, a striking example of convergent evolution. We compared E. unicolor clicks to that of five sympatric tiger moths and found that 9 of 13 E. unicolor clicking parameters were within the range of sympatric tiger moths. Remaining differences may result from the small size of the E. unicolor tymbal. Four of the five sympatric clicking tiger moth species were unpalatable to bats (0–20 % eaten), whereas E. unicolor was palatable to bats (86 % eaten). Based on these results, we hypothesize that E. unicolor evolved tymbal organs that mimic the sounds produced by toxic tiger moths when attacked by echolocating bats.     

Arctiid moth clicks can degrade the accuracy of range difference discrimination in echolocating big brown bats,Eptesicus fuscus

Summary Four big brown bats (Eptesicus fuscus) born and raised in captivity were trained using the Yes/No psychophysical method to report whether a virtual sonar target was at a standard distance or not. At threshold bats were able to detect a minimum range difference of 6 mm (a t of 36 s).Following threshold determinations, a click burst 1.8 ms long containing 5 pulses from the ruby tiger moth, Phragmatobia fuliginosa (Arctiidae), was presented randomly after each phantom echo. The sound energy of the click burst was -4 dB relative to that of the phantom echo. Clicks presented for the very first time could startle naive bats to different degrees depending on the individual.The bats' performance deteriorated by as much as 4000% when the click burst started within a window of about 1.5 ms before the phantom echo (Fig. 4). Even when one of ten phantom echoes was preceded by a click burst, the range difference discrimination worsened by 200% (Fig. 9). Hence, clicks falling within the 1.5 ms time window seem to interfere with the bat's neural timing mechanism.The clicks of arctiid moths appear to serve 3 functions: they can startle naive bats, interfere with range difference determinations, or they can signal the moth's distastefulness, as shown in earlier studies.Abbreviations peSPL peak equivalent sound pressure level - sd standard deviation - FM frequency modulation - CF constant frequency - EPROM erasable programmable read only memory     

Acoustic irradiation produced by flying moths (Lepidoptera,Noctuidae)

Characteristics of acoustic waves accompanying the flight of noctuid moths (Noctuidae) were measured. The low-frequency part of the spectrum is formed of a series of up to 17 harmonics of the wingbeat frequency (30–50 Hz) with a general tendency toward the decrease in the spectral density and the increase in the sound frequency. The root-mean-square level of the sound pressure from flapping wings was found to be 70–78 dB SPL. Besides low-frequency components, the flight of moths was accompanied by short ultrasonic pulses, which appeared with every wingbeat. Most of the spectral energy was concentrated within a range of 7–150 kHz with the main peaks at 60–110 kHz. The short-term pulses were divided into two or more subpulses with different spectra. The high-frequency pulses were produced at two phases of the wingbeat cycle: during the pronation of the wings at the highest point and at the beginning of their upward movement from the lowest point. In most of the specimens tested, the peak amplitude of sounds varied from 55 to 65 dB SPL at a distance of 6 cm from the insect body. However, in nine noctuid species, no high-frequency acoustic components were recorded. In these experiments, the acoustic flow from the flying moth within a frequency range of 2 to 20 kHz did not exceed the self-noise level of the microphone amplifier (RMS 18 dB SPL). Probable mechanisms of the high frequency acoustic emission during flight, the effect of these sounds on the auditory sensitivity of moths, and the possibility of their self-revealing to insectivorous bats are discussed. In addition, spectral characteristics of the moth echolocation clicks were more precisely determined within the higher frequency range (>100 kHz).     

Tiger moths and the threat of bats: decision-making based on the activity of a single sensory neuron

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.     

Discrimination of fluttering targets by the FM-batPipistrellus stenopterus?

Summary Five bats of the speciesPipistrellus stenopterus were trained in a two-alternative forced-choice procedure to discriminate between two fluttering targets. The positive target simulated an insect with a 50 Hz wingbeat rate. The negative target was varied between 0 and 48 Hz.The bats were able to discriminate a target with 41 Hz from a target with 50 Hz with 75% correct choices. In the discrimination task, they typically emitted echolocation calls of 2–4 ms duration sweeping from 60 kHz to 30 kHz. The duty cycle (i.e. fraction of time filled with echolocation sounds) increased when the targets fluttered, but was always lower than 3%.The performance ofP. stenopterus in discriminating fluttering targets is comparable to that of bats emitting longer sounds with constant-frequency (CF) components and a higher duty cycle. The FM-sounds ofP. stenopterus are short compared with the period of the fluttering targets, and therefore make it difficult for the animal to measure the time interval between two acoustic glints. Other cues may be prominent, such as the frequency modulation by Doppler shifts from the moving blades.     

Acoustic imaging in bat sonar: Echolocation signals and the evolution of echolocation

Summary Echolocating bats behave as though they perceive the crosscorrelation functions between their sonar transmissions and echoes as images of targets, at least with respect to perception of target range, horizontal direction, and shape. These data imply that bats use a multi-dimensional acoustic imaging system for echolocation with broadband, usually frequencymodulated signals. The perceptual structure of the echolocation signals used by different species of bats was investigated using the crosscorrelation functions between emitted signals and returning echoes as indices of perceptual acuity.Thebandwidth andaverage period of echolocation signals are identified as the principal acoustic features of broadband sonar waveforms that determine the quality of target perceptions. The multiple-harmonic structure of echolocation sounds, which is characteristic of the broadband signals of the majority of species of bats, yields a lower average period (separation of peaks in the crosscorrelation function) than would be expected from the average frequency of the signal as a whole, sharpening target localization.The frequency-modulation of the harmonics in the sonar sounds of bats reduces the heights of side-peaks in the crosscorrelation functions of the signals, promoting sharp, unambiguous determination of target position, and leads to the well-known coupling of perception of range and velocity for moving targets. The shapes of the frequency sweeps and bandwidths of frequency modulation contribute to reducing this range-velocity coupling. Harmonic organization nearly eliminates range-velocity coupling.The use of multiple-harmonics and fairly broad frequency modulation in sonar signals yields especially sharp resolution of target position to reject clutter interference. Such signals are commonly used by bats in cluttered environments. Very broad frequency sweeps with fewer harmonics may accomplish the same effect, but the low signal periodicity contributed by harmonic structure is an important factor in banishing side-peaks in the crosscorrelation function from perception.Abbreviations ACR autocorrelation function - AMB ambiguity diagram - CF constant frequency - FM frequency modulated - LFM linear frequency sweep - LPM linear period sweep - XCR crosscorrelation function     

Detection of sonar signals in the presence of pulses of masking noise by the echolocating bat,Eptesicus fuscus

Summary Two big brown bats (Eptesicus fuscus) were trained to report the presence or absence of a virtual sonar target. The bats' sensitivity to transient masking was investigated by adding 5 ms pulses of white noise delayed from 0 to 16 ms relative to the target echo. When signal and masker occurred simultaneously, the bats required a signal energy to noise spectrum level ratio of 35 dB for 50% probability of detection. When the masker was delayed by 2 ms or more there was no significant masking and echo energy could be reduced by 30 dB for the same probability of detection. The average duration of the most energetic sonar signal of each trial was measured to be 1.7 ms and 2.4 ms for the two bats, but a simple relation between detection performance and pulse duration was not found.In a different experiment the masking noise pulses coincided with the echo, and the duration of the masker was varied from 2 to 37.5 ms. The duration of the masker had little or no effect on the probability of detection.The findings are consistent with an aural integration time constant of about 2 ms, which is comparable to the duration of the cries. This is an order of magnitude less than found in backward masking experiments with humans and may be an adaptation to the special constraints of echolocation. The short time of sensitivity to masking may indicate that the broad band clicks of arctiid moths produced as a countermeasure to bat predation are unlikely to function by masking the echo of the moth.Abbreviations SPL sound pressure level - SD standard deviation - SE standard error - BW bandwidth     

Clicks and Communication: The Behavioural and Social Contexts of Hector's Dolphin Vocalizations

Hector's dolphins (Cephalorhynchus hectori) have a simple vocal repertoire, consisting almost entirely of ultrasonic clicks. They produce no whistles, and very few audible sounds. To examine acoustic communication in this species I analysed the relationship between click types and behaviour. The proportion of complex click types was greater in large groups, suggesting that these sounds have social significance. Clicks having 2 peaks in their time envelope and two frequency peaks were strongly associated with behaviours indicative of feeding. High pulse rate sounds, in which the repetition rate of ultrasonic clicks was audible as a “cry”, were most strongly associated with aerial behaviours. These data suggest that echo-location is not the sole function of Hector's dolphin clicks, and that echo-location and communication are likely to be closely linked. I hypothesize that dolphins may have the ability to gather information from the echoes of each other's sonar pulses. This may reduce the need for a large number of vocal signals, and may explain the apparent simplicity of the acoustic repertoires of some odontocetes.     

Defensive Sound Production in the Tobacco Hornworm, Manduca sexta (Bombycoidea: Sphingidae)

The tobacco hornworm (Manduca sexta) is a model organism extensively studied for many aspects of its biology, including its anti-predator strategies. We report on a novel component of this caterpillar’s defence repertoire: sound production. Late instar caterpillars produce discrete clicking sounds in response to disturbance. Click trains range in duration from 0.3–20.0 s (mean 3.3 ± 4.8 s) and contain 2–41 clicks (mean 7.1 ± 9.5). Sounds are broadband with a dominant frequency of 29.8 ± 4.9 kHz. We investigated the mechanism of sound production by selectively ablating three identified sets of ridges on the mandibles, and determined that ridges on the inner face strike the outer and incisor ridges on the opposing mandible to produce multi-component clicks. We tested the hypothesis that clicks function in defence using simulated attacks with blunt forceps. In single attack trials 77% of larvae produced sound and this increased to 100% in sequential attacks. Clicks preceded or accompanied regurgitation in 93% of multiple attack trials, indicating that sound production may function in acoustic aposematism. Sound production is also accompanied by other behaviours including directed thrashing, head curling, and biting, suggesting that sounds may also function as a general warning of unprofitability.     

Ultrasound detection in fish--a parallel to the sonar-mediated detection of bats by ultrasound-sensitive insects?

Most bats use ultrasonic sonar signals, or cries, to locate prey. Many of their insect prey species have evolved an ability to hear and respond to these signals, and studies clearly demonstrate the survival value associated with this ability. Like bats, toothed whales locate prey by emitting ultrasonic sonar signals, or clicks. As a parallel to the insect prey of bats, it would seem obvious to assume that some fish species likewise are capable of sensing the ultrasonic clicks of their odontocete predators. As judged from classical fish audiometry literature, this seems not to be the case, however, and although in recent years some fishes have been proven responsive to ultrasound, examination of ecological and acoustic differences reveals that conclusions on ultrasound-mediated insect escape behavior are not immediately applicable to fish. This has the consequence that future experiments on fish ultrasound detection should not be looking for observations directly parallel to those observed in the bat-insect interactions.     

Acoustic response to a pheromonal cue in the arctiid moth Pyrrharctia Isabella

ABSTRACT. The acoustic properties of the clicks emitted in response to male courtship pheromone by female Pyrrharctia Isabella (J. E. Smith) (Lepidoptera: Arctiidae) have been investigated. Extracts of male scent organs or synthetic male pheromones can be applied to a glass rod and used to stimulate females to produce these sounds. Power spectra, sound pressure readings, and oscillographic analyses show that the acoustic signals elicited by male extracts or synthetic male pheromones are not distinguishable from those produced in response to disturbance (handling). This is the first reported example of a sound produced by a female moth in a sexual context, and is the first reported example in moths of an acoustic response to a pheromonal stimulus.     

Keeping returns optimal: gain control exerted through sensitivity adjustments in the harbour porpoise auditory system

Animals that use echolocation (biosonar) listen to acoustic signals with a large range of intensities, because echo levels vary with the fourth power of the animal's distance to the target. In man-made sonar, engineers apply automatic gain control to stabilize the echo energy levels, thereby rendering them independent of distance to the target. Both toothed whales and bats vary the level of their echolocation clicks to compensate for the distance-related energy loss. By monitoring the auditory brainstem response (ABR) during a psychophysical task, we found that a harbour porpoise (Phocoena phocoena), in addition to adjusting the sound level of the outgoing signals up to 5.4 dB, also reduces its ABR threshold by 6 dB when the target distance doubles. This self-induced threshold shift increases the dynamic range of the biosonar system and compensates for half of the variation of energy that is caused by changes in the distance to the target. In combination with an increased source level as a function of target range, this helps the porpoise to maintain a stable echo-evoked ABR amplitude irrespective of target range, and is therefore probably an important tool enabling porpoises to efficiently analyse and classify received echoes.     

Rapid jamming avoidance in biosonar

The sonar systems of bats and dolphins are in many ways superior to man-made sonar and radar systems, and considerable effort has been devoted to understanding the signal-processing strategies underlying these capabilities. A major feature determining the efficiency of sonar systems is the sensitivity to noise and jamming signals. Previous studies indicated that echolocating bats may adjust their signal structure to avoid jamming ('jamming avoidance response'; JAR). However, these studies relied on behavioural correlations and not controlled experiments. Here, we provide the first experimental evidence for JAR in bats. We presented bats (Tadarida brasiliensis) with 'playback stimuli' consisting of recorded echolocation calls at one of six frequencies. The bats exhibited a JAR by shifting their call frequency away from the presented playback frequency. When the approaching bats were challenged by an abrupt change in the playback stimulus, they responded by shifting their call frequencies upwards, away from the playback. Interestingly, even bats initially calling below the playback's frequency shifted their frequencies upwards, 'jumping' over the playback frequency. These spectral shifts in the bats' calls occurred often within less than 200 ms, in the first echolocation call emitted after the stimulus switch-suggesting that rapid jamming avoidance is important for the bat.     

Hawkmoths produce anti-bat ultrasound

Bats and moths have been engaged in aerial warfare for nearly 65 Myr. This arms race has produced a suite of counter-adaptations in moths, including bat-detecting ears. One set of defensive strategies involves the active production of sound; tiger moths'' ultrasonic replies to bat attack have been shown to startle bats, warn the predators of bad taste and jam their biosonar. Here, we report that hawkmoths in the Choerocampina produce entirely ultrasonic sounds in response to tactile stimulation and the playback of biosonar attack sequences. Males do so by grating modified scraper scales on the outer surface of the genital valves against the inner margin of the last abdominal tergum. Preliminary data indicate that females also produce ultrasound to touch and playback of echolocation attack, but they do so with an entirely different mechanism. The anti-bat function of these sounds is unknown but might include startling, cross-family acoustic mimicry, warning of unprofitability or physical defence and/or jamming of echolocation. Hawkmoths present a novel and tractable system to study both the function and evolution of anti-bat defences.     

Echolocation behavior of the Japanese horseshoe bat in pursuit of fluttering prey

Echolocation sounds of Rhinolophus ferrumequinum nippon as they approached a fluttering moth (Goniocraspidum pryeri) were investigated using an on-board telemetry microphone (Telemike). In 40?% of the successful moth-capture flights, the moth exhibited distinctive evasive flight behavior, but the bat pursued the moth by following its flight path. When the distance to the moth was approximately 3-4?m, the bats increased the duration of the pulses to 65-95?ms, which is 2-3 times longer than those during landing flight (30-40?ms). The mean of 5.8 long pulses were emitted before the final buzz phase of moth capture, without strengthening the sound pressure level. The mean duration of long pulses (79.9?±?7.9?ms) corresponded to three times the fluttering period of G. pryeri (26.5?×?3?=?79.5?ms). These findings indicate that the bats adjust the pulse duration to increase the number of temporal repetitions of fluttering information rather than to produce more intense sonar sounds to receive fine insect echoes. The bats exhibited Doppler-shift compensation for echoes returning from large static objects ahead, but not for echoes from target moths, even though the bats were focused on capturing the moths. Furthermore, the echoes of the Telemike recordings from target moths showed spectral glints of approximately 1-1.5?kHz caused by the fluttering of the moths but not amplitude glints because of the highly acoustical attenuation of ultrasound in the air, suggesting that spectral information may be more robust than amplitude information in echoes during moth capturing flight.     

Classification of insects by echolocating greater horseshoe bats

Summary Echolocating greater horseshoe bats (Rhinolophus ferrumequinum) detect insects by concentrating on the characteristic amplitude- and frequency modulation pattern fluttering insects impose on the returning echoes. This study shows that horseshoe bats can also further analyse insect echoes and thus recognize and categorize the kind of insect they are echolocating.Four greater horseshoe bats were trained in a twoalternative forced-choice procedure to choose the echo of one particular insect species turning its side towards the bat (Fig. 1). The bats were able to discriminate with over 90% correct choices between the reward-positive echo and the echoes of other insect species all fluttering with exactly the same wingbeat rate (Fig. 4).When the angular orientation of the reward-positive insect was changed (Fig. 2), the bats still preferred these unknown echoes over echoes from other insect species (Fig. 5) without any further training. Because the untrained bats did not show any prey preference, this indicates that the bats were able to perform an aspect-anglein-dependent classification of insects.Finally we tested what parameters in the echo were responsible for species recognition. It turned out that the bats especially used the small echo-modulations in between glints as a source of information (Fig. 7). Neither the amplitudenor the frequencymodulation of the echoes alone was sufficient for recognition of the insect species (Fig. 8). Bats performed a pattern recognition task based on complex computations of several acoustic parameters, an ability which might be termed cognitive.Abbreviations AM amplitude modulation - CF constant frequency - FM frequency modulation - S+ positive stimulus - S- negative stimulus     

Acoustic courtship communication inSyntomeida epilais Wlk. (Lepidoptera: Arctiidae,Ctenuchinae)

The polka-dot wasp moth,Syntomedia epilais Wlk. (Lepidoptera, Arctiidae, Ctenuchinae), engages in extensive acoustic signaling during courtship. The signals, which are produced by both sexes, consist of sexually dimorphic trains of ultrasonic clicks. Field and laboratory behavioral experiments demonstrated that (1) sound production from both the male and the female is required for mating success, (2) femaleS. epilais show a higher level of acoustic responsiveness to male acoustic signals than to female signals, and (3) males use the female acoustic signals to locate their potential mates. It is suggested that the existence of the system is made possible by a reduction in bat predation pressure, perhaps the result of extreme distastefulness combined with the distinctive acoustic image that flying ctenuchines present to foraging bats.     

Frequency tracking and Doppler shift compensation in response to an artificial CF/FM echolocation sound in the CF/FM bat,Noctilio albiventris

Summary Bats of the speciesNoctilio albiventris emit short-constant frequency/frequency modulated (short-CF/FM) pulses with a CF component frequency at about 75 kHz. Bats sitting on a stationary platform were trained to discriminate target distance by means of echolocation. Loud, free-running artificial pulses, simulating the bat's natural CF/FM echolocation sounds or with systematic modifications in the frequency of the sounds, were presented to the bats during the discrimination trials. When the CF component of the artificial CF/FM sound was between 72 and 77 kHz, the bats shifted the frequency of the CF component of their own echolocation sounds toward that of the artificial pulse, tracking the frequency of the artificial CF component.Bats flying within a large laboratory flight cage were also presented with artificial pulses. Bats in flight lower the frequency of their emitted pulses to compensate for Doppler shifts caused by their own flight speed and systematically shift the frequency of their emitted CF component so that the echo CF frequency returns close to that of the CF component of the artificial CF/FM pulse, over the frequency range where tracking occurs.Abbreviations CF constant frequency - FM frequency modulation     

Adaptive behavior for texture discrimination by the free-flying big brown bat, <Emphasis Type="Italic">Eptesicus fuscus</Emphasis>

This study examined behavioral strategies for texture discrimination by echolocation in free-flying bats. Big brown bats, Eptesicus fuscus, were trained to discriminate a smooth 16 mm diameter object (S+) from a size-matched textured object (S−), both of which were tethered in random locations in a flight room. The bat’s three-dimensional flight path was reconstructed using stereo images from high-speed video recordings, and the bat’s sonar vocalizations were recorded for each trial and analyzed off-line. A microphone array permitted reconstruction of the sonar beam pattern, allowing us to study the bat’s directional gaze and inspection of the objects. Bats learned the discrimination, but performance varied with S−. In acoustic studies of the objects, the S+ and S− stimuli were ensonified with frequency-modulated sonar pulses. Mean intensity differences between S+ and S− were within 4 dB. Performance data, combined with analyses of echo recordings, suggest that the big brown bat listens to changes in sound spectra from echo to echo to discriminate between objects. Bats adapted their sonar calls as they inspected the stimuli, and their sonar behavior resembled that of animals foraging for insects. Analysis of sonar beam-directing behavior in certain trials clearly showed that the bat sequentially inspected S+ and S−.