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J Comp Physiol A (1994) 175:303-310 Springer-Verlag 1994 S. Dobler A. Stumpner - K.-G. Heller Sex-specific spectral tuning for the partner's song in the duetting bushcricket Ancistrura nigrovittata ( Orthoptera: Phaneropteridae) Accepted: 18 March 1994 Abstract The song of the male bushcricket Ancistrura tus on the forewings has most probably evolved indepen- nigrovittata consists of a sequence of verses. Each verse dently in males and females (Hartley et al. 1974; Nickle comprises a syllable group, plus, after about 400 ms a and Carlysle 1975; Heller and von Helversen 1986; single syllable serving as a trigger for the female re- Zhantiev and Korsunovskaya 1990). Nevertheless, the sponse song. The carrier frequency of the male song frequency spectra of male and female songs usually are spectrum peaks at around 15 kHz, while the female song similar, even if the elytra of males are larger (Nickle peaks at around 27 kHz. The thresholds of female re- 1976; Hartley and Robinson 1976; Heller and von Hel- sponses to models of male songs are lowest for song fre- versen 1986; Robinson et al. 1986). quencies between 12 and 16 kHz and therefore corre- The songs of many phaneropterid species are relative- spond to the male song spectrum. The threshold curve of ly sharply tuned (Heller and von Helversen 1986; Heller the female response to the trigger syllable at different 1988) and the frequency-spectra often are species specif- frequencies has the same shape as the tuning for the syl- ic. How can this specific information be utilized in intra- lable group. Phonotactic thresholds of male Ancistrura specific communication? Two principle answers may be nigrovittata to synthetic female responses at different given: at first, a narrow frequency spectrum improves the frequencies are lowest between 24 and 28 kHz and there- signal to noise ratio in the auditory system of a receiver, by correspond to the female song spectrum and clearly if its sensitivity is tuned to the sender's song frequency. differ from female response thresholds. Activity of the Second, the receiver could recognize the frequency and tympanic fibre bundle of both sexes is most sensitive be- could discriminate conspecific from heterospecific or tween 15 and 35 kHz and therefore not specifically tuned from predator signals. In the best case, it even could dis- to the partner's song. Individual behavioural thresholds criminate between different conspecific individuals. The have their minimum within 10 dB of the values of tym- most clearly documented example for frequency discrim- panic thresholds. ination comes from flying crickets, which exhibit posi- tive phonotaxis to low-frequency sound, but negative Key words Orthoptera Tettigonioidea phonotaxis to ultrasound (Moiseff et al. 1978; Nolen and Song frequency - Song recognition Sex difference Hoy 1986). Behavioural results from various bushcricket species indicate that frequency is a factor during phono- taxis; however, due to complex frequency spectra or be- Introduction cause the neuronal sensitivity at different frequencies re- mains unknown, in most studies intensity cannot be In several groups of bushcrickets, e.g. in Ephippigerinae ruled out as the factor actually causing response differ- (Bradyporidae) and in Phaneropteridae, not only males ences (Latimer and Sippel 1987; Bailey and Yeoh 1988; produce songs but females respond by a song of their Gwynne and Bailey 1988). Only in the non-flying bush- own ]review: Robinson (1990)]. The stridulatory appara- cricket Leptophyes punctatissima has it been demonstrat- ed that male phonotaxis is more sharply tuned than the S. Dobler j A. Stumpner 2 (~) K.-G. Heller hearing threshold of the whole auditory organ (Hardt lnstitut far Zoologie der Universit~tt Erlangen, Staudtstrasse 5, 1988). For a demonstration of frequency recognition, D-91058 Erlangen, Germany knowledge of auditory tuning is indispensable since the Present addresses: highest sensitivity of the auditory system is not necessar- i EPO Biology, University of Colorado, Campus Box 334, ily tuned to the song frequency ]Bailey and R6mer Boulder, CO 80309, USA 2 I. Zoologisches lnstitut der Universit~tt G6ttingen, (1991); Mason (1991); see also Popov (1990), for cica- Berliner Str. 28, D-37073 G0ttingen, Germany das]. 304 S. Dobler et al.: Frequency recognition in a bushcricket In the acoustic communication system of the bush- Male phonotaxis cricket Ancistrura nigrovittata (Phaneropteridae) the For phonotaxis experiments a male was set on a platform in an an- male produces a song with a complicated temporal pat- echoic room at 30 ~ A microphone registered the male song. A tern (Heller and von Helversen 1986). Its song comprises logic circuit and an interactive microprocessor (Rockwell AIM65) a syllable group, most important for species recognition, triggered a synthetic response with the species specific time delay and a single syllable serving as a trigger for the female of 30 ms from the onset of the single syllable of a male verse. As a synthetic response we used a sine wave signal of 1.5 ms duration reply (Dobler et al. 1994). We will demonstrate that with a short rising and a longer falling ramp, similar to the female males and females of this species in addition produce response song. songs with different carrier frequencies. This raises sev- When a male started singing spontaneously, for a first test a eral questions: is male and female behaviour, which can 32-kHz response of 55 dB was presented. All males immediately be elicited by the partner's song, sex-specifically tuned to started to orient towards the speaker. In following tests, for each frequency suprathreshold intensities were given initially and then the different frequencies? Is the spectral tuning of the fe- the intensity was decreased in 6-dB steps. If the male showed any male's behaviour identical for the two elements of the type of phonotactic response the position of the speaker was male song, which obviously serve different functions? changed by about 90 ~ before the next stimulus was given. The dis- How do behavioural tuning and auditory tuning corre- tance between speaker and male was always kept at 20 cm. If the male ceased to orient towards the speaker, the intensity was raised late? by 6 dB and then the threshold was determined in decreasing 3-dB steps. A maximum of 15 artificial responses was presented per in- tensity. As threshold we took the lowest value, at which at least Material and methods three out of five trials evoked a phonotactic reaction. Animals and song analysis Electrophysiology The experimental animals were captured as nymphs in southern Yugoslavia (Makedonia) and northern Greece. In the laboratory For recordings of tympanic activity the animals were briefly an- the animals were kept at room temperature in natural daylight. aesthetized with CO 2 and mounted dorsal side up on a free stand- Adult females and males were kept acoustically isolated from each ing holder using a resin-wax mixture. The frontal legs were fixed other. The songs were recorded on magnetic tape, digitized and at right angles to the body's long axis and sagittal plane. The tibiae evaluated on a FFT-spectrum analyzer (MEDAV). For details see were oriented perpendicularly to the femur and the tarsi attached Dobler et al. (1994). to a thin wire. The preparation was located in a sound-damped chamber 38 cm away from a piezoelectric speaker. The stimuli consisted of bandpass filtered white noise (Kemo VBF9), which Female response behaviour was amplitude modulated under computer control (AIM 65): a stimulus had a 35 ms duration with 1.5-ms rising and falling ramp Female response behaviour was analysed in a computer-controlled and was repeated every 0.5 s. Calibration was done as for be- setup (von Helversen 1979). The female was placed in a small havioural experiments, intensity was varied in 3-dB steps. For ex- cage within an anechoic chamber at 30 ~ After a stimulus com- amining the effect of sound from different directions the whole an- prising a verse was broadcast via a speaker within the chamber, imal was reoriented and a new recording was made each time. the computer monitored female responses for a defined time be- Summed action potentials of the tympanic fibre bundle were tore generating the next verse. The stimuli consisted of bandpass recorded in the distal half of the femur through a small hole cut in filtered white noise (KEMO VBFS, VBF9; 48 dB per octave). All the dorsal cuticle. The potentials were recorded by a suction elec- intensity values are given in dB SPL (re. to 2.10 5 Pa). For more trode with a thin, flexible tip (opening around 0.1 ram). The least details of the setup see Dobler et al. (1994). intensity at which activity of the nerve coupled to the stimulus To establish the behavioural threshold at different frequencies (judged by means of headphones and oscilloscope) was defined as the following stimulus program was used: the first stimulus of any threshold. For reliability of this method see Stumpner and Heller frequency started with the lowest (in any case subthreshold) inten- (1992). sity. Seven verses were presented and any responses between the trigger syllable and the beginning of the next syllable group were monitored. If no reply was registered, then after a 15-s pause a 3 Results dB louder stimulus was given. If a female answered, however, the stimulus intensity was decreased in 3-dB steps until the female stopped responding. Then the intensity was increased again until a Frequency spectra of male song predefined number of stimuli had been presented. The lowest an- and female response song swered intensity of each decreasing intensity scan was assigned one point and the number of points were cumulatively displayed in The calling song of male consists of a se- dependence of intensity.

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