Johannes Schul

SONG RECOGNITION BY TEMPORAL CUES IN A GROUP OF CLOSELY RELATED BUSHCRICKET SPECIES (GENUS TETTIGONIA)

Abstract Female phonotaxis of Tettigonia viridissima and T. caudata was investigated on a walking compensator to determine the temporal parameters of the male song used for song recognition, and to compare them with the previously described pulse rate filtering of T. cantans. The T. cantans song is continuous with a ≈30-Hz pulse rate. The T. caudata song has a higher pulse rate (≈40 Hz) and duty cycle than T. cantans and a distinct verse structure. The T. viridissima song is continuous with a double-pulse pattern. While the pulse rate is essential for song recognition in T. cantans, neither pulse rate not verse structure were essential for song recognition in T. caudata: females responded to signals above a minimum duty cycle. T. viridissima females did not require the double-pulse structure, but a single long pulse, equivalent to the duration of the double pulses and interval between them, was effective. Song attractiveness was limited by a minimum duration of the merged double pulse, and by minimum and maximum duration of the interval between them. Pulse rate recognition had little if any importance in either of the species investigated. Thus, the three congeners use different mechanisms for temporal song recognition.

Non-parallel coevolution of sender and receiver in the acoustic communication system of treefrogs

Advertisement calls of closely related species often differ in quantitative features such as the repetition rate of signal units. These differences are important in species recognition. Current models of signal–receiver coevolution predict two possible patterns in the evolution of the mechanism used by receivers to recognize the call: (i) classical sexual selection models (Fisher process, good genes/indirect benefits, direct benefits models) predict that close relatives use qualitatively similar signal recognition mechanisms tuned to different values of a call parameter; and (ii) receiver bias models (hidden preference, pre–existing bias models) predict that if different signal recognition mechanisms are used by sibling species, evidence of an ancestral mechanism will persist in the derived species, and evidence of a pre–existing bias will be detectable in the ancestral species. We describe qualitatively different call recognition mechanisms in sibling species of treefrogs. Whereas Hyla chrysoscelis uses pulse rate to recognize male calls, Hyla versicolor uses absolute measurements of pulse duration and interval duration. We found no evidence of either hidden preferences or pre–existing biases. The results are compared with similar data from katydids (Tettigonia sp.). In both taxa, the data are not adequately explained by current models of signal–receiver coevolution.

Auditory stream segregation in an insect

Auditory stream segregation is the perceptual grouping of the acoustic mixture reaching the ear into coherent representations of sound sources. It has been described in a variety of vertebrates and underlies auditory scene analysis or auditory image formation. Here we describe a phenomenon in an invertebrate that bears an intriguing resemblance to auditory stream segregation observed in vertebrates: in Neoconocephalus retusus (Orthoptera, Tettigoniidae) an auditory interneuron segregates information about bat echolocation calls from background male advertisement songs. This process utilizes differences between the temporal and spectral characteristics of the two stimuli, a mechanism which is similar to those of auditory stream segregation in vertebrates. This similarity suggests that auditory stream segregation is a fundamental feature of auditory perception, widespread from invertebrates to humans.

Neuronal basis of phonotactic behaviour in Tettigonia viridissima : processing of behaviourally relevant signals by auditory afferents and thoracic interneurons

Information transmission in the auditory pathway of Tettigonia viridissima was investigated using song models and artificial stimuli. Receptor cells respond tonically to song models and copy the syllable pattern within a wide intensity range. The omega-neuron responds tonically to soma-ipsilateral stimuli. Contralateral stimuli elicit IPSPs both within dendritic (ipsilateral) and axonal (contralateral) branches, thereby emphasizing directionality. Both AN1 and AN2 respond with tonic, non-adapting responses, precisely copying the syllable pattern of the song. While AN1 is excited by sonic frequencies and inhibited by ultrasonic frequencies, AN2 responds predominantly to ultrasound. The TN1 only responds to the ultrasonic components of the song, with phasic responses, which adapt quickly. In the adapted state, it responds selectively to the time pattern of the conspecific song, but not to the song patterns of two syntopic Tettigonia species. TN2, which has not been described up until now, is tuned to ultrasonic frequencies. Its responses to song models vanish after a few syllables, because of quick adaptation. The morphology is unusual with the axon running contralateral to the input site. The behavioural relevance of auditory interneurons is discussed and compared with the auditory system of crickets.

A quantitative analysis of behavioral selectivity for pulse rise-time in the gray treefrog, Hyla versicolor

Abstract The selectivity of female phonotactic responses to synthetic advertisement calls was tested in choice situations. Preferences based on differences in the linear rise-time of synthetic pulses depended on intensity and carrier frequency. When the carrier frequency was 1.1 kHz, simulating the low-frequency peak in the advertisement call, females preferred alternatives with slower rise-time pulses that differed by 5 ms at playback levels of 75 dB SPL and higher. A rise-time difference of 10 ms was discriminated at 65 dB SPL. When the carrier frequency was 2.2 kHz, simulating the high-frequency peak in the call, females discriminated a 5-ms difference in rise-time only at 85 dB SPL. Females showed no preference when the difference was 10 ms at lower playback levels. The difference in the thresholds (about 15–20 dB) for discriminating differences in rise-time at the two carrier frequencies was greater than the difference in behavioral thresholds for these two frequencies (about 10 dB). This result suggests that rise-time discrimination can be mediated solely by the neural channel mainly tuned to the low-frequency peak in the call. Females probably assess differences in rise-time by comparing the first few pulses of each call rather than by averaging over the entire call.

SELECTIVE PHONOTAXIS IN TETTIGONIA CANTANS AND T. VIRIDISSIMA IN SONG RECOGNITION AND DISCRIMINATION

Abstract The selectivity of female phonotaxis in Tettigonia cantans and T. viridissima was investigated on a Kramer treadmill, with respect to the specific differences in temporal pattern and spectrum of the songs of both species. In choice situations, both species preferred the conspecific song over the heterospecific one. The courses of both species were deflected by about 15–20° from the position of the conspecific song, that of T. viridissima being away from, that of T. cantans in the direction of the heterospecific song. In no-choice situations, song models with the temporal pattern of T. cantans did not attract T. viridissima. Models with the conspecific time pattern but heterospecific spectrum were as attractive as the conspecific model. In contrast, T. cantans was attracted by T. viridissima song presented alone. In choice situations, either spectral or temporal differences were sufficient for discrimination. The preference for the conspecific model gradually disappeared with stepwise reduction of its intensity and was reversed at −12 dB. Acoustic communication alone can serve species isolation in T. viridissima; however, premating isolation in T. cantans must involve other mechanisms. The orientation during the choice situations suggests a serial processing of song recognition and localization for the Tettigonia species.

What determines the tuning of hearing organs and the frequency of calls? A comparative study in the katydid genus Neoconocephalus (Orthoptera, Tettigoniidae)

SUMMARY The calls of five syntopic species of Neoconocephalus varied significantly in their spectral composition. The center-frequency of the narrow-band low-frequency component varied from 7kHz to 15kHz among the five species. Hearing thresholds, as determined from whole nerve recordings, did not vary accordingly among the five species but were lowest in the range from 16kHz to 18kHz in all five species. Iso-intensity response functions were flat for stimulus intensities up to 27dB above threshold, indicating an even distribution of the best frequencies of individual receptor cells. At higher stimulus intensities, the intensity/response functions were steeper at frequencies above 35kHz than at lower frequencies. This suggests the presence of a second receptor cell population for such high frequencies, with 25-30dB higher thresholds. This receptor cell population is interpreted as an adaptation for bat avoidance. The transmission properties of the Neoconocephalus habitat (grassland) had low-pass characteristics for pure tones. Frequencies below 10kHz passed almost unaffected, while attenuation in excess of spherical attenuation increased at higher frequencies. Considering these transmission properties and the tuning of female hearing sensitivity, call frequencies of approximately 9-10kHz should be most effective as communication signals in this group of insects. It is discussed that the frequency of male calls is strongly influenced by bat predation and by the transmission properties of the habitat but is not strongly influenced by the tuning of the female hearing system.

Pulse-rate recognition in an insect: evidence of a role for oscillatory neurons.

Various mechanisms have been proposed as the neural basis for pulse-rate recognition in insects and anurans, including models employing high- and low-pass filters, autocorrelation, and neural resonance. We used the katydid Tettigonia cantans to test these models by measuring female responsiveness on a walking compensator to stimuli varying in temporal pattern. Each model predicts secondary responses to certain stimuli other than the standard conspecific pulse rate. Females responded strongly to stimuli with a pulse-rate equal to half the standard rate, but not to stimuli with double the standard rate. When every second pulse or interval was varied in length, females responded only when the resulting stimuli were rhythmic with respect to the period of the standard signal. These results provide evidence rejecting the use of either high-/low-pass filter networks or autocorrelation mechanisms. We suggest that rate recognition in this species relies on the resonant properties of neurons involved in signal recognition. According to this model, signals with a pulse rate equal to the resonant frequency of the neurons stimulate the female to respond. The results are discussed with regard to both neural and evolutionary implications of resonance as a mechanism for signal recognition.

Developmental plasticity of mating calls enables acoustic communication in diverse environments

Male calls of the katydid Neoconocephalus triops exhibit substantial developmental plasticity in two parameters: (i) calls of winter males are continuous and lack the verse structure of summer calls and (ii) at equal temperatures, summer males produce calls with a substantially higher pulse rate than winter males. We raised female N. triops under conditions that reliably induced either summer or winter phenotype and tested their preferences for the call parameters that differ between summer and winter males. Neither generation was selective for the presence of verses, but females had strong preferences for pulse rates: only a narrow range of pulse rates was attractive. The attractive ranges did not differ between summer and winter females. Both male pulse rate and female preference for pulse rate changed with ambient temperature, but female preference changed more than the male calls. As a result, the summer call was attractive only at 25°C, whereas the slower winter call was attractive only at 20°C. Thus, developmental plasticity of male calls compensates for differences in temperature dependency between calls and preferences and enables the communication system to function in heterogeneous environments. The potential role of call plasticity during the invasion of new habitats is discussed.

Mechanisms and evolution of synchronous chorusing: Emergent properties and adaptive functions in Neoconocephalus katydids (Orthoptera: Tettigoniidae).

Synchronous interactions arise in various animal species that rhythmically broadcast acoustic, vibratory, and visual signals. These interactions are characterized by a coincidence in both rate and phase of the rhythms of neighboring signalers. Theory predicts several ways in which synchronized rhythms may specifically benefit the interacting signalers. However, synchrony may also arise as an emergent property, a default phenomenon that is neither preferred by conspecific receivers evaluating the signals nor advantageous to the signalers themselves. Here, we examine several well-studied cases of acoustic synchrony in Neoconocephalus katydids (Orthoptera: Tettigoniidae), a New World genus wherein males broadcast loud advertisement songs. We report that call synchrony found in N. spiza and N. nebrascensis results from two rather different mechanisms of rhythm adjustment. Moreover, synchrony in the former species appears to represent an incidental byproduct of signal competition between evenly matched males, whereas in the latter species synchrony functions as a specific adaptation in which cooperating males ensure that critical call features can be perceived by females. We discuss the separate evolutionary trajectories that may have led to similar outcomes, synchronous chorusing by advertising males, in these closely related species.