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Long-range sound distribution and the calling song of the cicada Beameria venosa (uhler) (hemiptera: cicadidae).

Male cicadas attract mates through production of a species-specific calling song. The sound-production system of cicadas has been modeled as a Helmholtz resonator (Bennet-Clark and Young, 1992). Sound is produced when the timbal muscles buckle the timbals, which are chitinous membranes at the base of the abdomen, generating changes in pressure in the abdominal cavity and exiting through the tympana (Young, 1990; Bennet-Clark and Young, 1992; Fonseca and Popov, 1994; Bennet-Clark, 1998). Sound produced by timbals can be modified by several anatomical structures such as shape and position of the abdomen, timbal covers, opercula, and tensor muscles that alter quality and intensity of sound emitted (e.g., Pringle, 1954; Young, 1990; Bennet-Clark and Young, 1992, 1998; Fonseca and Popov, 1994; Bennet-Clark, 1998; Fonseca and Bennet-Clark, 1998). In addition, it has been shown recently that these structures can be altered to produce adaptations to specific communicatory pressures (Oberdorster and Grant, 2007).

One might expect natural selection on communicatory systems to maximize the effective range and directionality of signals (Alexander, 1967). Because insects are small animals, high resonant frequencies produced by small sound-producing structures would potentially limit their communicatory range. Therefore, production of a directional sound maybe advantageous to an insect (Bennet-Clark, 1971, 1975). In fact, Bennet-Clark (1971) showed that the directional sound produced by a mole cricket (Gryllotalpa vineae) has a greater range than a sound sphere produced by the same species.

At a short distance (15-20 cm) from the insect, sound fields produced by the cicadas Fidicina rang (Aidley, 1969), Cystosoma saundersii (Doolan, 1981; MacNally and Young, 1981; Bennet-Clark, 1998), Cicada atrata (Jiang, 1989), and Tympanistalna gastrica (Fonseca, 1993; Michelsen and Fonseca, 2000) are asymmetrical. Radiation patterns near the source are influenced by structures that radiate sound into the environment (Young, 1990; Bennet-Clark and Young, 1992; Fonseca and Popov, 1994; BennetClark, 1998). However, symmetry of the sound-radiation pattern at greater distances from a singing cicada has not been investigated.

Beameria venosa is one of the smallest cicadas in North America north of Mexico with a body length of 11-13 mm (Lawson, 1920). Habitat of B. venosa has been described as desert grassland (Davis, 1921) and dry, rocky hilltops, or hillsides with sparse vegetation (Beamer, 1928). Recently, it has been shown to inhabit a wide range of habitats as long as grass is present (Phillips and Sanborn, 2007). The small size of the species means it will produce a high-frequency call (Daniel et al., 1993; Bennet-Clark and Young, 1994; Oberdorster and Grant, 2007). Although the near-field pattern of sound radiation by B. venosa has not been measured, the high-frequency call is well suited for determining the long-range sound field. Because high-frequency calls attenuate rapidly, determining when the signal is masked by level of background noise is facilitated for high frequencies. As a result, measurements of distance from source of the sound and mapping of the long-range sound field is possible. The present study was performed to analyze the call of B. venosa and to determine if its long-range sound field is directional.

MATERIALS AND METHODS--The calling song of B. venosa was recorded on 64-mm (0.25-inch) audio tape at a speed of 19 cm/s with an Uher 4000 Report Monitor tape deck (Uher Werke, Munchen, Germany) and an Electro-Voice RE55 dynamic microphone (ElectroVoice, Inc., Buchanan, Michigan). The recorder and microphone have a frequency response to 25 and 20 kHz, respectively. The microphone was placed as close as possible to the calling animal to decrease background noise while recording. Recordings were analyzed with SoundScope/16 (GW Instruments, Somerville, Massachusetts) on a Macintosh computer digitizing songs at a sampling rate of 44.1 kHz and using a narrow-band FFT to determine frequency spectra. For purposes of this research, a sound pulse includes the initial pulse produced by the timbal plate and smaller pulses produced by the timbal ribs. Specimens were recorded in Barber Co., Kansas, Dona Ana, Hidalgo, Otero, and Roosevelt counties, New Mexico, and Kimble, Travis and Val Verde counties, Texas.

A population of singing B. venosa encountered near U.S. Highway 80, ca. 15 km N Rodeo, Hidalgo Co., New Mexico (32[degrees]04'N, 108[degrees]59'W, 1,300 m elevation) was used to determine the long-range, song-broadcast pattern of the calling song. A distance corresponding to the human-auditory threshold for the carrier frequency of the calling song was determined by a null method. Location was marked with a marble and angle from the singing animal was recorded with a hand-held compass located directly above the animal. Measurements at the distance of auditory threshold were made at ca. 20[degrees]-intervals where vegetation permitted. After completing 360[degrees], distance from animal to each point was measured. Additional distances were measured from other animals in the same habitat to verify accuracy of the method.

Several precautions were taken to prevent variability in measurements. The same ear was oriented perpendicularly to the animal at each location, while the other ear was plugged. Increasingly smaller movements were made toward and away from the source of sound to locate precisely the distance of auditory threshold. Acoustic interference from other B. venosa was prevented by physically relocating any individual whose sound field overlapped the sound field of the experimental animal to a distance where it was inaudible during experiments.

An equation represented by the distributed points was determined using the computer program Mathematica (Wolfram Research, Champaign, Illinois). The unknown center and radius of the circle were calculated by minimizing the sum-of-squares deviation [summation][([P.sub.i][C.sup.2][-r.sup.2]).sup.2], where [P.sub.i]C = distance of the ith point from the unknown center C, and r = unknown radius. Statistical analysis was performed using InStat 3.0a for the Macintosh (GraphPad Software, San Diego, California). Body measurements were made with Vernier calipers. Statistics are reported as mean [+ or -] SD.

RESULTS--Beameria venosa is a solitary singing cicada. When calling, wings are partially spread while the abdomen is simultaneously extended and arched. Arching of the abdomen produces a space between the opercula and abdomen during production of the song. Animals tend to stay in grass when calling, but also call from elevated perches, such as Lairea or fence posts, when grasses are limited. It appears the species preferentially calls from grass as a means to facilitate avoidance of predators. Beameria venosa will drop from the perch into grass when disturbed. Cryptic coloration and congregated stems of grass makes locating an animal at the base of a grass plant or within a grassy area difficult.

The calling song of B. venosa is a continuous train of sound pulses with a frequency range with major energy spread over ca. 2.5 kHz around the peak (Fig. 1). Expansion of the time wave of the call shows sound pulses are produced at a rate of 385 [+ or -] 29.5 (n = 15, range 340-430) pulses/s. The power spectrum shows a mean peak frequency of 16.95 [+ or -] 1.76 kHz (n = 15, range 14.77-20.48 kHz).

Broadcast-range experiments were performed in a field ca. 15 km N Rodeo, Hidalgo Co., New Mexico. Natural vegetation in the area is a grama-tobosa shrubsteppe dominated by black grama (Bouteloua eriopoda), tobosa (Hilaria mutica), and creosotebush (Larrea divaricata; Kuchler, 1964). Although B. venosa generally is associated with grass, the experimental location was heavily grazed and B. venosa were singing from virtually all species of plants. Number of animals present and elevational variation in the habitat meant only one complete pattern of sound distribution could be completed.

[FIGURE 1 OMITTED]

The animal used to construct the broadcast coverage diagram (Fig. 2) was singing from a weedy plant 8.25 cm above the ground. The animal was oriented nearly vertically with midline of the dorsal side directed to a compass orientation of 195[degrees]. Each point in Fig. 2 corresponds to distance of auditory threshold. Mean distance of the 15 points surrounding the animal is 18.62 [+ or -] 2.43 m and is not significantly different from mean distance of seven measurements made from three other animals (19.95 [+ or -] 2.33 m; t = 1.228, P = 0.233) used to verify the method. Experimental and verification samples do not differ in standard deviations (F = 1.092, P = 0.490) and both were sampled from Gaussian distributions (experimental group KS = 0.192, P > 0.10; verification group KS = 0.213, P > 0.10). These data suggest the procedure used to determine threshold distance is accurate and reproducible.

[FIGURE 2 OMITTED]

The broadcast pattern is about spherical (Fig. 2). Deviations from a perfectly spherical pattern are the result of differential interference and sound absorption due to varying number, size, and type of vegetation between source of sound and receiver resulting in the non-circular distribution of the points observed. Large L. divaricata in the habitat act as major points of scattering and absorption of sound. The entire sound sphere is shifted along the approximate axis of wind so that the animal producing the sound is not centrally located within the sound sphere.

Mathematica determined the center of points to be at 142[degrees]55' at a distance of 3.17 m from the origin (the animal). Radius of the best-fit circle was 18.39 m. Figure 2 was constructed by the Mathematica program to illustrate distribution of experimental points and the best-fit circle for those points. If the environment was homogeneous and the shift in sound field was due to wind alone, the expected sound field would be along the axis of the wind. During experiments, wind was moving from ca. 340 to 160[degrees] at a speed <8 km/h. Deviation from the calculated value probably was caused by the inhomogeneous environment combined with slight variations in direction of wind. The calculated radius (18.39 m) is about equal to the mean radius determined in experiments (18.62 [+ or -] 2.43 m).

DISCUSSION--The calling song of B. venosa was described by Beamer (1928) as shrill, could only be heard for ca. 1 m, and was of little value in collecting insects. The high frequency of the call is a result of the small body size of the species. Dominant frequency of a cicada call is determined by resonant frequency of the timbal and abdominal air cavity (Pringle, 1954; Bennet-Clark and Young, 1992; Young and BennetClark, 1995). Thus, the small size of B. venosa (length of body 11.88 [+ or -] 0.77 mm, n = 10) and its timbal will produce a high-frequency call. Predicted dominant frequency based on size and the regression analysis provided by Bennet-Clark and Young (1994) was 15.41 kHz. This value is within one standard deviation of the dominant frequency determined here for B. venosa (16.96 [+ or -] 1.76 kHz), within the range of measured values (14.77-20.48 kHz), and well within the 95% confidence interval for the regression analysis.

Postural changes seen in calling B. venosa have been observed in many species of cicadas and probably function to increase intensity of the call by bringing the abdominal air sacs into resonance with the timbals (Pringle, 1954; Young, 1990; Bennet-Clark and Young, 1992). Intensity of the calling song of B. venosa was reported to be 85.2 dB at 50 cm (Sanborn and Phillips, 1995).

The long-range broadcast pattern of B. venosa was about spherical or non-directional at an average distance >18 m. Previous studies of sound fields of cicadas measured intensity of song close (15-20 cm) to animals and determined these sound fields to be asymmetrical (Aidley, 1969; Doolan, 1981; MacNally and Young, 1981; Jiang, 1989; Fonseca, 1993; Bennet Clark and Young, 1998; Michelsen and Fonseca, 2000). Patterns of radiation near the source are influenced by structures that radiate sound into the environment (Young, 1990; Bennet-Clark and Young, 1992; Fonseca and Popov, 1994; Bennet-Clark, 1998) and asymmetries in these near-animal measurements have been attributed to the timbal (Aidley, 1969), absorption of sound by musculature (MacNally and Young, 1981; Jiang, 1989), or absorption by the tympana or cuticle (Fonseca and Bennet-Clark, 1998). However, the long-range broadcast pattern does not appear to be influenced by morphologically produced asymmetries in the sound field near the animal.

Propagation of sound in any environment is affected by geometric spreading, reflection, absorption, refraction, and diffraction (Michelsen, 1978). The broadcast pattern of B. venosa (Fig. 2) shows the sound field was asymmetric around the insect. The asymmetry was about along the axis of the wind (Fig. 2). Distortion by wind is slight because wind was of a low velocity and the sound receiver was elevated above the source of the sound. The animal was singing from a height of 8.25 cm, while the receiver was ca. 175 cm above the ground. By elevating the receiver above the source, the magnitude of the shadow zone was decreased (Schilling et al., 1947; Embleton et al., 1976).

Although distribution of points around the singing animal was about circular (Fig. 2), the broadcast pattern was not perfectly circular due to effects of vegetation on transmission of sound. Presence of vegetation increases attenuation of sound (Wiener and Keast, 1959; Aylor, 1971; Linskens et al., 1976; Yamada et al., 1977; Wiley and Richards, 1978; Martens and Michelsen, 1981; Michelsen and Larsen, 1983). Plants act as a diffraction grating (Michelsen and Larsen, 1983) and as a source of reflection, scattering, and absorption of energy from sound (Weiner and Keast, 1959; Aylor, 1971; Linskens et al., 1976; Yamada et al., 1977). Absorption of energy from sound increases with increases in density of mass of plants (Aylor, 1971; Linskens et al., 1976) and is proportional to the square root of frequency of sound (Yamada et al., 1977). High frequency of the calling song of B. venosa is, therefore, highly susceptible to absorption and disruption by vegetation. Different number, density, and distribution of plants at each point between the singing B. venosa and the receiver were responsible for variation in distance between measured points. Shorter distances were reported where density of plant material caused greater scattering, absorption, or both, of energy from the sound of the call.

Wind and temperature are the most important atmospheric heterogeneities with respect to acoustics (Wiener and Keast, 1959). Broadcast pattern of B. venosa was affected by wind as well as vegetation (Fig. 2).

Wind affects sound waves by altering the path of transmission. Upwind from a source of sound, the sound waves are deflected upward decreasing the apparent distance that sound travels. Upward bending of sound waves produces a shadow zone where intensity is decreased. Downwind from the source, sound waves are deflected down (Ingard, 1953; Wiener and Keast, 1959), while sound waves traveling at right angles to the wind are unaffected (Ingard, 1953). Wind produces an asymmetric sound field with greater intensity for a given distance measured downwind from the source (Weiner and Keast, 1959), as seen in the sound field of B. venosa (Fig. 2).

Communicatory range, or linear distance from the source where sound can be used for communication, is influenced by sound level of source, background noise at receiver, rate of attenuation with distance, and sensitivity of receiver (Waser and Waser, 1977). The most-effective step an animal can take to increase range of its call is to move above 1 m in height (Marten et al., 1977). Beameria venosa can increase its range of communication by moving to an elevated singing perch. Generally, B. venosa is associated with grass and primarily calls from elevated perches on blades of grass. We also observed it singing from elevated perches, such as Lamrea and fence posts and it has been reported to sing from thistles (Davis, 1921) and Juniperus (Heath, 1978). Thus, B. venosa employs behaviors that increase the broadcast range of its call.

Sounds used in communication systems evolve to carry frequency, intensity, and time components a required distance, not necessarily the longest distance (Michelsen and Larsen, 1983). A selective advantage in producing a high-frequency call is a potential reduction in predation. Acoustically signaling animals advertise their location to conspecifics and to potential predators. Thus, predation can be decreased by evolving a call that is beyond the limit of auditory sensitivity of potential predators.

Although ants are predators of B. venosa as the cicadas emerge (Whitford and Jackson, 2007), birds and lizards represent primary potential predators of B. venosa. Auditory sensitivity of birds generally is 200-10,000 Hz. The avian auditory system has maximal acuity of 2-4 kHz, with auditory sensitivity decreasing rapidly toward extreme frequencies. With the exception of owls and parrots, birds do not seem to have a useful sensitivity to sounds >10 kHz (Pumphrey, 1961). By producing a mating call at 17 kHz, B. venosa is acoustically imperceptible to birds. Cryptic coloration has the effect of further reducing avian predation.

As a grassland animal, B. venosa potentially is susceptible to terrestrial-vertebrate predators. Lizards probably represent the major predatory threat to B. venosa. Several authors (e.g., Knowlton, 1934; Fautin, 1947; Paulissen et al., 2006) describe cicadas as prey of lizards. Three genera, Cnemidophorus, Crotaphytus, and Sceloporus, are likely candidates as predators due to their distribution and feeding habits. We have observed multiple species of Cnemidophorus and Sceloporus in the same location as B. venosa and lizards of these genera probably represent the primary predatory threat to cicadas.

Wever (1978) assembled auditory data for many lizards. Extrapolation of data presented gave an estimate of the levels of sound pressure necessary to elicit a nervous response in several species from the genera of interest. Data were converted to a reference of 2 by [10.sup.-4] dynes/[cm.sup.2] and mean threshold pressure for a cochlear response in each genus was calculated from the species present. In the field, B. venosa produces a calling song at an intensity of 85.2 dB at 50 cm (n = 21, range 81.7-88.3 dB; Sanborn and Phillips, 1995). Levels of sound pressure of 98.5 dB, 138.7 dB, and 123.8 dB at 50 cm from source of the sound are estimated to be necessary to elicit a nervous response in Cnemidophorus, Crotaphytus, and Sceloporus, respectively. These values probably are underestimates as cochlear response decreases rapidly at high frequency in lizards (Wever, 1978).

From the calculations, no potential lizard predator would be capable of hearing B. venosa. In addition, behavioral acuity of animals to a source of sound shows a rapid loss of sensitivity at higher frequencies although potential function of the cochlea continues (Wever, 1978). Because B. venosa sings at such a high frequency, lizards are restricted to visual cues, olfactory cues, or both, to hunt these insects as prey. The high-frequency call of B. venosa is of a selective advantage as well as a physical necessity.

Associate Editor was Jerry L. Cook.

We thank H. Porta of the Department of Mathematics, University of Illinois at Urbana-Champaign, for assistance with data analysis and T. Petrino-Lin for producing the resumen. Two anonymous reviewers made suggestions that improved the manuscript.

Submitted l5, January 2008. Accepted 25 July 2008.

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ALLEN F. SANBORN, * JAMES E. HEATH, AND MAXINE S. HEATH

Barry University, School of Natural and Health Sciences, 11300 AT Second Avenue, Miami Shores, FL 33161 (AFS) University of Illinois, Department of Physiology and Biophysics, 524 Burrill Hall, 407 South Goodwin Avenue, Urbana, IL 61801 (JEH, MSH)

Present address of JEH and MSH: 104 Hummingbird Circle, Buchanan Dam, TX 78609

* Correspondent: asanborn@mail.barry.edu
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