© 2001 by The Society for Integrative and Comparative Biology
Talking Back: Sending Soil Vibration Signals to Lekking Prairie Mole Cricket Males1
1 Faculty of Biological Sciences, The University of Tulsa, Oklahoma 74104
2 Faculty of Mechanical Engineering, The University of Tulsa, Oklahoma 74104
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The male prairie mole cricket, Gryllotalpa major, native of the tallgrass prairie of the south central U.S., constructs a specialized acoustical burrow in the spring in the prairie soil from which he generates an airborne calling song that attracts flying females for mating. Males do not phonorespond to manipulations involving playbacks of airborne sounds. At the same time, vibrations with the same temporal scale and pattern as the airborne signal are produced in the substrate through an unknown mechanism. These ground vibrations can be distinguished from background vibrations in the soil at distances up to 3 m depending on soil conditions and conditions that control the background vibration environment (e.g., wind, highway traffic). We hypothesize that males use the vibration component of the call as information for spacing as they form display arenas, or leks. We used modified field recordings of soil vibrations from singing males with an electromechanical vibration exciter to simulate the vibration component of a calling song in playback experiments. Airborne sounds of males were monitored for two minutes before and two minutes after the introduction of the ground vibration stimulus with a tape recorder microphone placed 20 cm from the burrow opening. Males did respond to the manipulation experiment; although, we observed individual variation in the level of response.
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INTRODUCTION |
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SYNOPSISINTRODUCTIONGENERAL METHODSCHARACTERISTICS OF THE CALLING...SYNTHESIZING A PLAYBACK CALLFIELD MANIPULATIONS OF MALES...SUMMARY AND CONCLUSIONSReferences |
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Animal communication requires a sender to transmit information to a receiver, and the receiver to use the information to formulate a response. The information is packaged in the form of a signal (Bradbury and Vehrencamp, 1998
). Before an event can be classified as a signal, its role in a communication pathway must be confirmed. Merely demonstrating that a sound is species-specific and stereotyped does not support its classification as a signal unless it can also be demonstrated that the sound is functionally significant (Doherty and Gerhardt, 1984). Our interest in communication through vibration originated with attempts to classify signals in a rare grasslands insect.
The prairie mole cricket, Gryllotalpa major, is the largest North American cricket. Walker and Figg (1990) reported that individuals could be as long as 5 cm and weigh as much as 2.6 g. Like other members of the family Gryllotalpidae, prairie mole cricket males construct specialized acoustic burrows in which they sit and produce loud, long-range airborne calling songs to target flying females for mating (Walker and Figg, 1990). Callings songs are characterized by a regular pattern of long chirps with a dominant frequency about 2 kHz and up to five harmonics (Hill, 2000) and are audible up to 400 m away from the male's burrow (Walker and Figg, 1990). Displaying males are, thus, in a fixed position in the burrow, and mobile females are free to choose a mate based on characteristics of the calling song.
Prairie mole cricket males exhibit classical lek, or arena-based, mating behavior (Hill, 1999), even though aggregations of Orthopteran insects have typically not been classified as leks (Höglund and Alatalo, 1995). Males make their spectacular acoustic displays from burrows that are aggregated on larger sites, and even within these aggregations the burrows are clumped in two smaller levels of interactive groups (Hill, 1999). This differs from other Orthopteran aggregations in general, where individuals are regularly spaced (Cade, 1981; Farris et al., 1997), especially at the closest nearest-neighbor distances (Campbell, 1990), even when aggregated on another scale (Shaw et al., 1982; Bailey and Thiele, 1983).
Prairie mole cricket males are also one of possibly four species world-wide that produce a chirping advertisement call, while others produce trills or no call at all (Nevo and Blondheim, 1972; Walker and Figg, 1990; Broza et al., 1998; Hill, 2000). Males alternate their chirps in a chorus (Greenfield and Shaw, 1983) with nearest neighbors, and both chirp rate and harmonic content of the call are correlated with distance to the nearest calling male (Hill, 1998). Trilling species, which might not hear during their own calls (Greenfield and Minckley, 1993; Greenfield, 1997), and thus would be without the potential to assess others' signals during the interval between chirps, are not known to interact with near neighbors or be sensitive to substrate vibrations or disturbance during calling (Forrest, 1991). The introduced mole crickets in the United States (Scapteriscus species) form mating "sprees" with no spatial aggregation or arena-based displays (Walker, 1983). We have been interested in how the constraints of environment and phylogeny (Römer, 1993) have led to this communication and mating system in prairie mole crickets (see also Desutter-Grandcolas, 1997a, b). Why leks form in this species continues to be investigated (Hill, 1999), but how do they form? How do males space their burrows in these nested aggregations? What cues are they using, and are these cues signals?
Manipulation of prairie mole cricket males in the field requires delivery of a realistic signal directed toward the calling male. Anecdotal comments from field workers between 1987 and 1991 and published sampling methods (Mehlhop-Cifelli, 1990) reported that prairie mole cricket males did not respond to playbacks of airborne calling songs. Walker (1983) suggested that mole crickets in sprees probably do not phonorespond but sing at the same time in response to environmental conditions. In phonoresponse tests in 1994, prairie mole cricket males did not change call rate, temporal pattern of the call, or call intensity—they ignored the airborne stimulus. In 1995, knowing that males were extremely sensitive to substrate vibration from footfalls (unlike members of the genus Scapteriscus where trilling males are not disturbed by workers walking through the aggregations), we used the methods developed by Lewis and Narins (1985) for studying the white-lipped frog, Leptodactylus albilabris, to examine the possibility of communication between calling males through soil vibration. Our objectives were the following: (1) to determine if calling males can generate ground vibration of sufficient magnitude at typical burrow-to-burrow distances to be distinguishable above background ground vibration levels; (2) to determine if calling males respond to ground vibration excitation having temporal scale and pattern similar to the ground vibration component of the calling song; and (3) to examine the possibility of soil vibration communication by males as a factor in burrow spacing.
We have reported on work on the first objective (Hill and Shadley, 1997), but technical problems remained before progress could be made on the second and third objectives. In this paper we present our continuing efforts in three sections: (1) a review of characteristics of the airborne and ground vibration components of the prairie mole cricket calling song; (2) a solution to the problem of simulating the call for playback purposes; and (3) a report on early experimental trials to measure response of males to the simulation. Lastly, we outline future work to address the issue of male spacing.
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GENERAL METHODS |
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Burrows of a natural population of G. major males were located in a tallgrass prairie meadow in Craig County, Oklahoma, USA, (36°37'N, 95°16'W) by listening to the male's airborne advertisement calls. Initial experimental work focused on monitoring and recording airborne and ground vibration components of the calling song generated by the calling male. Later, the scope was expanded to include transmitting ground vibrations to the calling male and recording the insect's response. We wanted to manipulate males to respond to a signal.
For monitoring and recording ground vibration signals, a geophone (Oyo Geospace Model GS-200 DM) was buried under about 20 cm of soil in a vertical orientation at one meter from the burrow opening of a focal male in a direction perpendicular to the longitudinal axis of the burrow. This transducer, which has a sensitivity of 15.0 to 19.0 V-sec-m–1 (Volt seconds per meter, or Volts per unit of velocity) at frequencies above 40 Hz, was connected by above-ground cables in turn with an amplifier/filter designed for the project, a Bruel and Kjaer Model 2203 Sound Level Meter, and a TASCAM Model DA-PI digital audio tape (DAT) recorder to record any vibrations that resulted from the males' advertisement calls. All activity was monitored with headphones. The through-system sensitivity of this measurement system was 4.3 to 5.5 x 104 V-sec-m–1 at frequencies above 40 Hz at the amplifier/attenuator setting typically used in the experiments. The DAT recorder and sound level meter were also used to record the airborne component of the calling song using a Bruel and Kjaer 25 mm diameter Model 4145 condenser microphone. Recorded vibrations were analyzed using wave file editors Goldwave (Craig, 1999) and Cool Edit 2000 (Johnston, 2000), and Fast Fourier Transform (FFT) analyzer SpectraPLUS (Sound Technology Inc., 1999).
Additional equipment was used to generate soil vibrations in playback experiments. We used a Sony Model TCD-D7 digital audio tape recorder for the playback of the vibration stimulus. A SoundStream two-channel, 100-watt automobile amplifier Model SouUSA100 powered by a 12 V automobile battery linked the recorder to the vibration exciter, a Ling Dynamic Systems Inc. Model V-203, 17.8N peak sine thrust, d.c. to 13 kHz frequency range, electrodynamic vibration generator with added 0.318 kg inertial load.
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CHARACTERISTICS OF THE CALLING SONG |
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The airborne component of the calling song
Figure 1 shows a sample of the amplitude of the airborne component of the male cricket's calling song versus time, measured at a distance of about one meter from the burrow opening. The call consists of loosely periodic bursts of sound separated by brief periods of quiet. Figure 2 shows a 1.35 Hz bandwidth spectrum analysis of the sample. The maximum amplitude of the call in this sample corresponds to a sound pressure level of slightly over 80 dB (re 2 x 10–5 Pa) and is at a frequency of about 2 kHz as has been noted previously (Walker and Figg, 1990
). A feature much lower in level in Figure 2 is centered at about 100 Hz and shows several narrow band sounds, the most prominent of which is at 100 Hz, from the cricket calling song and other sources. The 2 kHz sound surely propagates through the air to burrows neighboring the calling male. But, because of the very high attenuation of high frequency signals through soil, 2 kHz sounds propagating through the soil at the Craig County site very quickly fall below background noise levels. The low frequency component, however, has a chance to be detected above background noise levels if the propagation distance is not too great.
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The ground vibration component of the calling song
Figure 3 shows a sample of the velocity amplitude of the ground vibration component of the calling song versus time with the geophone buried about 20 cm deep in the soil and about one meter from a calling male. This sample consists of approximately periodic bursts of vibration energy separated by quieter periods. The bursts in this sample are not so sharply defined as they were in the airborne sample, and the quieter periods in this sample are filled with background noise. Background noise that is a significant component of the total ground vibration is typical at distances of more than a few centimeters from the calling male.
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A velocity amplitude spectrum of the ground vibration one meter from a calling male is shown in Figure 4. A cluster of several large-amplitude, narrow-band spikes can be observed in this spectrum at frequencies from about 50 Hz to 300 Hz. Amplitudes of these narrow-band spikes for geophone locations about one meter from the calling male are typically in the range of a few µm/sec rms (Hill and Shadley, 1997
). Some of these represent the low frequency component of the calling song. There is another spike of narrow-band energy at about 2 kHz. This spike represents the ground vibration analog of the 2 kHz sound pressure spike observed in Figure 2. However, attenuation of high frequency sound is much stronger in the soil than in the air. For this reason, most of the high-frequency energy that reaches the geophone likely first propagates through the air to a point above the geophone and then down through the soil to the geophone.
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A recording was made of the background ground vibration in the absence of calling crickets over a time period when obvious high-background-noise sources such as train and truck traffic were not in the immediate vicinity. Figure 5 shows a ground vibration spectrum of such a background noise sample. The background noise spectrum (fine dark line) is shown in this figure superimposed on the calling song spectrum (broader gray line) originally shown in Figure 4. The background noise spectrum shows a low level broadband mound in the frequency range 20 to about 300 Hz. But, of more importance are the strong narrow-band spikes at 60 Hz, 120 Hz, 180 Hz, 240 Hz and 300 Hz. It is not uncommon for recordings from electrical transducers to contain noise at 60 Hz and multiples of 60 Hz. In outdoor measurement, these are often due to emissions from electrical power transmission lines. Similar narrow-band electrical noise spikes can occur also in laboratory data reduction due to AC power influences. In comparing the noise spectrum with the calling song spectrum, it appears likely that most or all of the energy in the calling song spectrum at 60 Hz, 300 Hz, and perhaps also some of the other harmonics of 60 Hz, is really due to electrical noise sources and not to the calling male mole cricket.
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SYNTHESIZING A PLAYBACK CALL |
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Deriving the source signal
A step of some complexity in planning playback experiments is deriving the ground vibration source signal, i.e., the stimulus that is to be put on recorder tape to play back (in this research to the calling male cricket). The source signal cannot be simply a ground vibration recording of the call made at some convenient distance. All the instruments used to make and playback the recording have characteristics that shape the signal that finally reaches the cricket. In addition, the interaction between the vibration exciter and the soil, and the propagation of the vibration signal through the soil are greatly affected by the properties of the soil. Instrument and soil characteristics have to be accounted for in the source signal.
The objective is to derive a source signal for the playback tape that, when played through the ground vibration generation system, will yield a recording at the geophone location distance d1 away that is exactly the same as the signal recorded at distance d2 from a calling cricket. Then to simulate the ground vibration propagation from cricket-to-cricket over distance d2, the vibration exciter is placed at distance d1 from the cricket burrow and the stimulus source signal is played through the vibration exciter system.
Under ideal conditions, the source stimulus can be derived exactly using four signals related by the following equation:
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where q(t) is the stimulus source signal needed on the playback tape, Xd(
) is the Fourier transform (Meirovitch, 1986) of the signal desired at the measurement point (the signal corresponding to a calling male recorded at a pre-selected distance), X() is the Fourier transform of a known, but arbitrary, signal that has fairly uniform representation of all frequencies of interest, Y() is the Fourier transform of the system response to X() (the signal recorded at the geophone when X() is played through the system), and 
–1 denotes the inverse Fourier transform. In eq. 1, the product X()Y()–1 is the inverse of the function often referred to as the "frequency response" of the system. An advantage of the method is that the source signal obtained produces the desired result in both frequency and phase.
The method was attempted in the present research using the Fast Fourier Transform (FFT) (Meirovitch, 1986) in a procedure developed by Shadley (1998) for aircraft flight simulator sound system applications. But, if in eq. 1 the signal desired at the measurement point, Xd(), is a ground vibration recording of an advertisement call, the signal is, to some degree, contaminated by noise from sources other than the cricket. Y() is similarly affected by background noise. Sources of ground vibration noise at the site include highway traffic, train traffic, aircraft, wind-induced motion of plant roots, and various creatures inhabiting the near-surface layers of the soil. Eq. 1 shows that if background noise represents substantial components of Xd() and Y(), then it becomes a substantial component of the source signal, q(t), too. The problem is particularly acute in mid-to-high frequency ranges (above about 300 Hz) where signal strengths of Xd() and Y() are typically low due to rapidly increasing ground vibration attenuation with increasing frequency.
A simpler, but less precise method for deriving the source signal was employed that reduced (but, did not eliminate) the influence of the background noise recorded in Xd() and Y(). Figure 6 shows frequency characteristics of an input signal to the vibration exciter and the geophone response to the input signal. The vibration exciter was placed directly on the soil cleared of vegetation and debris, and the geophone was placed with its sensitive axis vertical under 20 cm of soil one meter distant from the vibration exciter. The exciter input signal was a "Maximum Length Sequence" (MLS) (Rife and Vanderkooy, 1989). An MLS was used as the stimulus for evaluating the frequency response characteristics of the ground vibration generating and measuring system because it has strong representation in nearly every frequency of interest and has a great deal of overall energy. The lowest curve in Figure 6 is the system response to the MLS at a distance of one meter from the vibration exciter. This is the signal that actually gets imprinted on the recording tape of the digital recorder that is connected to the geophone output. The system response signal divided by the input signal represents the frequency response of the system. The system includes the playback recorder, playback signal amplifier, vibration exciter, soil path from exciter to geophone, geophone, and recorder.
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The system response to the MLS signal falls off rapidly at frequencies below and above a frequency of about 100 Hz. For frequencies above 100 Hz the most important factor in the frequency response behavior is the increasingly strong attenuation characteristics of the soil at increasing frequency (Hill and Shadley, 1997
). Some of the rapid decline in the system response at decreasing frequencies below 100 Hz can be attributed to the characteristics of the geophone. The heavy line in Figure 6 represents the geophone response to a mechanical excitation having the same velocity amplitude at every frequency. The response curve was calculated based on manufacturer's specifications. This response is nearly constant at high frequencies, but falls off rather sharply at frequencies below about 40 Hz. Perhaps a more important reason for the sharply declining system response at low frequencies is that the vibration exciter is too small to generate low frequency (long wavelength) energy very efficiently. Because the system response falls off sharply at both low and high frequencies, using our recorded ground vibration sample as input to the vibration exciter would yield a ground vibration signal at a distance of one meter that would be lacking the needed low and high frequency information. The remedy employed in this research was to boost the low and high frequency ends of the source signal spectrum. The solid line in Figure 7 shows the filter characteristics selected to shape the spectrum of the source signal. The filter boosts the signal at frequencies below 100 Hz at a rate of 60 dB per decade to 30 Hz, and above 100 Hz at the same rate to a frequency of 500 Hz. No ground vibration data from calling males has been observed below about 30 Hz, and exciter power limitations would prevent continuing the filter ramp much beyond 500 Hz. The wavy line shows the result of applying the filter to a sample of white noise. The filter was applied to the sample wave file using the wave file editing program Cool Edit 2000. The same filter was applied to the wave file containing a sample of the ground vibration component of the calling song to generate the source signal, i.e., the signal put on the playback recorder tape for input to the vibration exciter. One disadvantage of this approach to deriving the source signal is that the signal amplitude as a function of frequency is only roughly approximated. Another disadvantage is that no attempt is made by the method to derive the phase of the source signal as a function of frequency: source signal phase is the same as the phase of the calling song ground vibration recording.
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Simulation of the calling song ground vibration
A playback signal was derived that is based on a ground vibration field recording of a calling male, and modified to increase signal intensity at frequencies below and above 100 Hz. Figure 8 shows a spectrum of ground vibration measured one meter from the vibration exciter as it processed the playback signal. This spectrum (fine dark line) can be compared in Figure 8 to the original vibration spectrum that was recorded one meter from a calling male (broader gray line) from which the playback signal was derived. Narrow-band spikes at 60 Hz and 300 Hz can be ignored in the comparison since they are likely to be due to electrical noise and not to the cricket.
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Comparison of the two ground vibration signals in Figure 8 shows that amplitudes at frequencies below and above a frequency of about 75 Hz (47 Hz, 90 Hz, 100 Hz, 111 Hz, 139 Hz, 203 Hz) are lower in the vibration exciter-generated spectrum than in the cricket-generated spectrum. In listening to the two ground vibration signals using headphones, the two signals sound quite similar, except that the vibration exciter-generated signal does not sound as rich in the higher harmonics. It may be asking too much to require that the two signals have exactly the same amplitude at all frequencies. The sound propagation properties of the soil change from day to day depending on temperature and moisture content. Also, the vibration exciter puts very little sound energy into the air. It may be that a significant component of the higher frequency narrow-band energy from the cricket is reaching the geophone transducer through an air-soil path instead of strictly through the soil path to which propagation from the vibration exciter is largely restricted. In previous research at the Craig County site it was found that velocity level in soil vibration at 142.5 Hz fell off at 44 dB per decade increase in distance as compared with only 20 dB per decade increase in distance for particle velocity level in air (Hill and Shadley, 1997
). A loudspeaker was added to the signal generating equipment to reduce differences between the two spectra in Figure 8 at higher frequencies. |
FIELD MANIPULATIONS OF MALES WITH VIBRATION STIMULI |
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Generating soil vibration in playback experiments
The vibration exciter and geophone were placed near the burrow with burrow opening, exciter and geophone occupying the three corners of an equilateral triangle having sides one meter in length. The earth under the vibration exciter was cleared of vegetation and debris and the exciter was placed directly on the cleared earth.
Recordings of the airborne responses of the focal male to the playback experiment were made by placing a Panasonic Model RQ-L307 minicassette recorder (with integral microphone) loaded with Maxell tape at a distance of 20 cm (outside the near-field of sound for the dominant frequency of the species) from the burrow entrance, since the mouth of the burrow represents the system's "radiator" or acoustic piston, rather than the wing surface of the male inside the burrow (Bennet-Clark, 1995, 1998). Recorders were aligned with the long axis of the focal male's body (Michelsen and Nocke, 1974), since the exact location of the individual is known while he calls from his burrow. A speaker (Optimus mid-range speaker, 90 Hz to 20 kHz frequency range, 50 watt) was added to the playback system to add an audible stimulus synchronized with the vibration stimulus.
Manipulating a focal male
We monitored ground vibrations generated by the calling male through headphones. Power was then increased to the vibration exciter until the vibration signals at the geophone from the calling male and from the vibration exciter were about the same as judged by ear through the headphones. Subjective descriptions of the response were noted and the airborne calling song of the focal male was recorded with the minicassette recorder for later analysis in the laboratory. The recording was sampled at 30-sec intervals for two minutes before and two minutes after introduction of the vibration exciter stimulus and for two minutes after the audible stimulus was added, following Westcott (1997). Digitized wave files were created using SIGNAL software (Beeman, 1996) and used to generate oscillographs, sonographs and frequency spectra from which we measured the following parameters at each of the intervals: chirp (pulse) rate, chirp duration, interchirp interval, duty cycle, syllables per chirp, and maximum amplitude of the sample. Data were statistically analyzed with SIGMASTAT (Jandel Scientific Software, 1995).
Calling male responses to playback experiments
During the spring mating seasons in 1998, 1999 and 2000 a total of eight trials of this experiment were conducted to study responses of male mole crickets to calling song ground vibration simulations. In 1998 and 1999, observations were limited to interruptions in the call and subjective evaluations of adjustments in the call due to the introduction of the ground vibration stimulus. Recordings of the four males from the 2000 population were analyzed with SIGNAL.
Many factors appeared to affect the responses of the crickets to the vibration exciter, e.g., environmental factors such as soil temperature and wind speed, the time of the experiment relative to the time frames of the daily calling window and annual calling cycle, and cricket-to-cricket variation. There was individual variation in male responses, which are summarized in Table 1. The first four rows in Table 1 contain data from the 1998 and 1999 experiments. The last four rows pertain to the 2000 experiments.
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Male #67 in 1998 was played the vibration stimulus alone. He fell silent for the duration of the stimulus but began calling rhythmically once the stimulus was removed. In 1999, #69 fell silent immediately after the vibration source was switched on but resumed calling with a "stuttering," non-rhythmic pattern and attempted to alternate its call with the simulated call played through the shaker. When the audible sound stimulus was added, #69 ceased calling within 2 sec and was silent for 75 sec after which it repeated the stuttering call. Male #73 behaved similarly, but after the initial period of silence when the audible sound was added, his song was much slower. Male #127 stopped calling altogether when the vibration exciter was activated and did not resume.
Male #115 was alternating his calling song with a near neighbor in 2000 when we introduced the vibration stimulus. The vibration completely inhibited the neighbor, who ceased calling and did not resume that night. After the audible stimulus was added, #115 ceased calling altogether. There was a slight, but insignificant, decline (Fig. 9) in the chirp rate of #115 (216 to 198 chirps/min) from 2 min before to 2 min after the vibration stimulus, which resulted in a slightly slower chirp rate than that of the vibration stimulus (214.2 chirps/min). The temporal spacing of the call of #115 varied little over the 4-min monitoring period (Fig. 10), but a significant difference in chirp duration was seen between the samples taken at 90 sec before and 120 sec after the vibration stimulus was applied (Mann-Whitney Rank Sum Test: P = 0.026). Other intervals showed P-values just above significance and lower power of the t-tests when compared with the sample from 120 sec after the stimulus. The call at that point was highly variable (Fig. 10), and four t-tests comparing that sample with others failed the normality test.
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Male #121 was little affected by the vibration stimulus alone (Fig. 9), but introduction of the audible sound caused him to "skip" pulses, which resulted in an overall lower chirp rate at the end (146 chirps/min) than the beginning of manipulation (166 chirps/min). Male #122 was little affected by the manipulation but "stuttered" at the 60-sec mark after the audible sound was added. Male #129 had an increase in chirp rate from 128 to 154 chirps/min from 90 to 30 sec before the vibration stimulus, but his highest calling rate was after the audible stimulus had been applied for 120 sec (192 chirps/min). As with #115, this rate more closely approximates that of the artificial source. The pairwise analyses of chirp duration for #129 show significant variation before the vibration stimulus was applied: 90 sec vs. 60 sec before (t-test: P = 0.006), 60 sec vs. 30 sec before (t-test: P = 0.005). Yet, pairs of samples taken during the two minutes after the vibration stimulus was introduced did not differ from each other, and did not differ from the sample taken 30 sec before the stimulus.
The manipulations did not affect call amplitude, but they did have an impact on the duty cycle, or percent of the time a male actually called from the beginning of one chirp to the beginning of the next (Fig. 10). Although #129's call varied little with respect to pacing, #121 and #122 both had longer intervals between chirps after the audible sound was introduced.
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SUMMARY AND CONCLUSIONS |
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Male prairie mole crickets create ground vibrations that are detectable above background noise in the substrate at the Craig County, Oklahoma site over typical interburrow distances. Stridulation with the file-and-scraper mechanism used to produce the calling song (Hill, 2000
) is sufficient to set up the vibrations, much as a similar mechanism in the leaf-cutter ant permits it to send signals to its nest-mates through the soil (Masters et al., 1983). While the airborne call of the prairie mole cricket has its energy peak at about 2 kHz, analysis of the transmitted ground vibration component indicated a series of very narrow band energy spikes in the spectra in the range 30 to about 300 Hz. This shift away from high frequencies in the ground vibration spectrum is due in large part to the very high attenuation rate for high-frequency ground vibration at the site.
A vibration exciter was used to simulate calling song ground vibrations in playback experiments to examine vibration as a possible communication channel between calling males. The source signal for the vibration exciter was based on a recorded sample of the ground vibration component of the calling song, filtered to boost high and low frequencies in accordance with the system frequency response characteristics. The resulting simulations produced a ground vibration at a distance of one meter from the vibration exciter that was similar to the recorded calling song sample, but mid- and high-frequency components in the simulation were somewhat low in level. A likely explanation for this result is that for points as far away as one meter or so, the mid- and high-frequency components of the cricket-generated calling song are taking an air-soil path instead of the strictly soil path to which propagation from the vibration exciter is largely confined. A loudspeaker was added to the playback system to fill in some of the higher frequency components. Although we were not completely satisfied with the frequency spectrum of our simulated call at a distance of one meter, it should be noted that Brownell and Farley (1979) found that scorpions were insensitive to the mechanism used to produce a stimulus, as long as the intensity and temporal pattern of the stimulus were similar to those produced by prey. In our experiments the parameters of the simulated call fell within the normal range of variation for these characteristics.
Males responded to the simulated calling song, but in ways that are not yet predictable. Some males fell silent as soon as the vibration stimulus was turned on. Others responded more to the added audible stimulus. Still others appeared to ignore the artificial calling song stimuli. Males singing under natural conditions do not modify the call intensity, but do alter calling rate in response to competition from neighbors (Hill, 1998). Similar behavior was found in some of the trials conducted with the simulated calling songs. Another typical response was that males initially attempt to alternate chirps with the simulated call. This response is also a feature of males calling under natural conditions (Hill, 2000).
Whether prairie mole cricket males use soil vibration communication in burrow spacing or for other purposes is not yet clear. Bushcrickets that produce airborne and substrate-borne signals with a similar temporal pattern (Keuper and Kühne, 1983), much like G. major does, seem to space themselves based on information provided in this multimodal call (Schatral et al., 1985). Even though bushcrickets produce airborne and substrate-borne signals, they may actually use vibration signals as the major communication channel because of the restrictions imposed on airborne calling by their biotope (Kalmring et al., 1997), and therefore vibration would be the primary cue used for male spacing.
It is likely that many natural movements and calling activities of animals set up airborne sound and substrate-borne vibration simultaneously (Gogala, 1985), but whether or not these are true signals depends on the environment and adaptations of animals to communication in that environment (Keuper et al., 1985). Of particular interest is why the prairie mole cricket males aggregate their burrows on a lek, produce chirping calls and respond to vibration when most members of their family do not. It is possible that G. major represents some transitional evolutionary stage where some members recognize signals that are merely noise to others. Or, competition for audible space could be driving male members to use signals differently. Still, information in vibration signals can be assessed by calling males in the silent periods between their chirps, whereas relatives that produce a trilling song may not be able to utilize vibration as a communication channel.
Additional research is needed on the responses of prairie mole cricket males to ground vibration and airborne components of the calling song to better understand the roles of these stimuli in prairie mole cricket behavior. We know that males do not alter their calls in response to simulated airborne calls, but some do in response to vibration stimuli. Whether or not repeated application of substrate vibration will cause males to abandon and relocate burrows has yet to be established. Certainly, we plan to use the simulated ground and air components of the calling song described here, with perhaps some improvements, in additional playback experiments designed to better establish patterns and ranges of responses of calling males to the calling song of a neighbor. What we expect to learn in these manipulations should help us to better understand the relationship between the prairie mole cricket and its environment.
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FOOTNOTES |
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1 From the Symposium Vibration as a Communication Channel presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 3–7 January 2001, at Chicago, Illinois.
2 E-mail: john-shadley{at}utulsa.edu
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References |
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Bailey, W. J., and D. R. Thiele. 1983. Male spacing behavior in the Tettigoniidae: An experimental approach. In D. T. Gwynne and G. K. Morris (eds.), Orthopteran mating systems: Sexual competition in a diverse group of insects, pp. 163–184. Westview Press, Boulder, Colorado.
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