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First published online October 31, 2008
Journal of Experimental Biology 211, 3613-3618 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.023143
Hearing and spatial behavior in Gryllotalpa major Saussure (Orthoptera: Gryllotalpidae)
1 University of Tulsa, Faculty of Biological Sciences, 600 South College, Tulsa,
OK 74104, USA
2 University of Toronto Scarborough, Department of Life Sciences, 1265 Military
Trail, Scarborough, ON, M1C 1A4, Canada
* Author for correspondence (e-mail: dhoward{at}utsc.utoronto.ca)
Accepted 23 September 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: Gryllotalpidae, hearing, spacing, tallgrass prairie
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Forrest, 1983;
Otte, 1992;
Sakaluk et al., 1995). While
other critical factors, such as the availability of forage, refugia and mates
as well as the intensity of predation pressures, are also assumed to influence
the spatial distribution of crickets, most studies have focused on the role of
the calling song in driving the spatial dynamics of orthopteran aggregations
(Farris et al., 1997). As in
most other Orthoptera, the spatial dynamics of advertising male crickets are
often well described but poorly understood. In many cases, these spatial
arrangements are an outcome of the complex interaction of an animal's neural
capacity for detecting neighboring conspecific signalers and the acoustic
interference of environmental factors
(Römer and Bailey,
1986).
Mole crickets (Gryllotalpidae) are a unique ensiferan clade distinguished
from other true crickets by morphological and behavioral adaptations that
allow the group to exploit an almost exclusively subterranean ecological niche
(Ulagaraj, 1975;
Figg and Calvert, 1987). Mole
crickets have large, powerful forelimbs (dactyls) that allow for the
excavation of complex burrow systems beneath the subsoil. Several species in
the group produce advertisement calls from within the burrow complex or from
within an acoustic calling chamber located at the terminus of a burrow at the
soil surface (Bennet-Clark,
1987). While considerable work has been done in describing the
production of acoustic signals in mole crickets
(Bennet-Clark, 1970), fewer
studies have focused on the hearing of species within the group, other than
the morphological description of the tympanum: covered in Gryllotalpa
species and Neocurtilla hexadactyla, exposed in the genus
Scapteriscus and missing in Triamescaptor aoeta
(Hill et al., 2002). One early
study described a high-frequency sensitivity in several gryllotalpid species,
including a 20–30 kHz best frequency in the northern mole cricket
(Neocurtilla hexadactyla), which was attributed to conspecific
calling song detection (Suga,
1968). However, later studies found mole crickets' advertisement
calls to be devoid of high-frequency components
(Bennet-Clark, 1989;
Forrest, 1983), leaving this
high-frequency sensitivity unexplained. Scapteriscus borelli, a mole
cricket species introduced to the southeastern USA that engages in nocturnal
dispersal flights, has been shown to have a bimodal hearing capacity that
includes sensitivity within the range of conspecific advertisement calls (3
kHz) as well as a second sensitivity peak in the ultrasonic range (25 kHz)
that probably allows detection of echolocating bats. Additionally, this flying
mole cricket species exhibited evasive behavior when faced with ultrasonic
stimuli (Mason et al., 1998).
Scapteriscus abbreviatus, a closely related flightless species,
exhibited neither sensitivity to ultrasound nor any behavioral aversion to the
stimulus in the same study.
The prairie mole cricket, Gryllotalpa major Saussure (Orthoptera:
Gryllotalpidae), is a rare endemic of the tallgrass prairie ecosystem of the
south central USA and is currently known only from Oklahoma, Arkansas, Kansas
and Missouri (Walker and Figg,
1990; Vaughn et al.,
1993). Adult males construct an acoustic calling chamber (see
Movie 1 in supplementary material) with an opening to the soil surface that is
used exclusively for producing sexual advertisement calls
(Hill, 1999). Unlike most
gryllotalpids, males produce a long sequence of brief chirps varying from 1.7
to 2.9 per second at a carrier frequency of 
2.0 kHz
(Walker and Figg, 1990;
Hill, 2000) and with harmonic
overtones up to 10 kHz (Hill,
1998). Variation in the call's dominant frequency and syllables
per chirp correlates with male size
(Howard and Hill, 2006).
Females are attracted to the calling aggregation, fly over the sound field
created by the displaying males and then drop to the ground to enter the
selected male's burrow (Walker and Figg,
1990; Hill, 1999;
Howard and Hill, 2006);
however, no information is available on whether or not this flying species is
able to detect the ultrasonic frequencies emitted by echolocating bats.
In the mole cricket genus Scapteriscus, males are attracted to the
calls of other males and are thought to space themselves in a uniform
distribution near the periphery of a nearest neighbor's sound field
(Forrest and Green, 1991).
Scapteriscus females are known to be preferentially attracted to
advertisement calls with greater intensity (SPL), and a male with a louder
call is thought to reduce nearest neighbor distances to ensure that the sound
field of a less attractive male is contained entirely within his own.
Conversely, less attractive males are thought to space themselves further away
from the focal male to increase the likelihood of attracting a female
(Forrest and Green, 1991).
Gryllotalpa major males mate in leks with statistically
significant aggregation at three levels of scale: (1) the lek proper, (2)
subsets of clumped burrows and (3) 1–3 burrows within plots of 3–4
m in diameter (Hill, 1999).
Prairie mole cricket leks form more often on recently burned sites
(Howard and Hill, 2007), and
inter-male spacing among the two or three nearest neighbors is thought to be
influenced by substrate-borne vibrations produced by advertising males
(Hill and Shadley, 1997;
Hill and Shadley, 2001).
However, the richness of harmonic overtones in the calling song, rather than
the call intensity, was the parameter in the airborne component of the call
found to be correlated with distance to the nearest neighbor
(Hill, 1998). The current
study examines spacing in natural aggregations in the context of auditory
sensitivity (under idealized assumptions about sound propagation and
attenuation). We predict the following: (1) G. major hearing will be
tuned to the carrier frequency of the advertisement call; (2) as a flying
species potentially exposed to predation by bats, G. major will
exhibit hearing at ultrasonic frequencies; (3) all advertising G.
major males within a lek will be spaced within the maximum effective
signal range of all other advertising males in the lek and (4) because the
number of harmonic overtones negatively correlates with nearest neighbor
distance (Hill, 1998), some
harmonic content of the calling song will be detectable by neighboring
males.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Acoustic stimuli were tone pulses of 50 ms duration with 1 ms rise/fall times, repeated at 2.0 s–1. Stimulus generation and data acquisition were controlled by a custom program written in Matlab (The Mathworks, Natick, MA, USA). The stimulus pulses were synthesized and output through a D/A interface (National Instruments PCI-6040E, 200 kHz sampling rate, Austin, TX, USA). The tone pulses were attenuated (Tucker Davis PA5; Alachua, FL, USA), amplified (Brüel and Kjaer Type 2706; Nærum, Denmark) and then broadcast through loudspeakers (Realistic piezo tweeter and 4 inch woofer; Fort Worth, TX, USA) placed 50 cm from the position of the cricket preparation. Stimulus levels at the position of the cricket were calibrated using a microphone (Brüel and Kjaer type 4939) and sound level meter (Brüel and Kjaer type 2231). The frequency range of the system was 1–35 kHz, and all sound pressure levels (SPLs) given are in dB (re. 20 µPA).
Data were recorded as averaged summed action potentials (SAP; 50 responses averaged). For each frequency, data were collected by recording responses over a range of intensities beginning at 90 dB SPL and stepping down in 3 or 6 dB increments for at least 40 dB, but always until auditory responses were no longer discernible. Smaller intensity increments (3 dB) were used for frequencies near the auditory sensitivity peaks. Responses were quantified in two ways: (1) by measuring the maximum peak-to-peak amplitude in the SAP and (2) by measuring RMS (root mean square) level of the SAP waveform to derive a measure of the relationship between response amplitude and stimulus intensity (intensity–response or IR curves) at each frequency. The two methods of quantifying response amplitudes gave similar results. Data are shown for RMS level measurements. Frequency tuning was measured both as threshold curves and iso-intensity response curves (stimulus amplitude vs frequency for a constant stimulus level). Threshold intensities at each frequency were interpolated from our IR curves as follows. Response amplitudes were measured over a range of intensities from subthreshold levels (no auditory response) to the maximum available stimulus level. We used the RMS levels of subthreshold neural responses as a measure of background activity. We defined threshold as the frequency at which response amplitude exceeded the average level of background (non-auditory) activity by two standard deviations. These values were obtained by interpolation using the `interp1' function in Matlab. We also calculated thresholds using two different methods for pooling the data: (1) thresholds were calculated for each specimen, using the above procedure, and values averaged for the five individuals and (2) intensity–response data were pooled to create global IR curves, and thresholds were calculated from the pooled data (see Fig. 1). Iso-intensity response curves (Fig. 2) show pooled data.
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The spatial data were then exported and processed into ArcView shapefiles using Trimble Pathfinder Office software (Navigation Ltd) and evaluated using ArcMap 9.0 GIS software (ArcGis 9.0; Environmental Systems Research Institute, Redlands, CA, USA). The Hawth's Spatial Ecology extension (http://www.spatialecology.com/htools) was used to calculate the area of each lek as defined by a minimum convex polygon (MCP) and to produce a matrix of the inter-burrow distances for all data points within the lek. Additionally, each burrow position could be used as the center of a series of concentric buffer zones representing the estimated effective signal range for the frequencies found in the advertisement call of G. major (2–10 kHz).
Spatio-auditory ecology
We combined data on male spacing in the field and estimated call broadcast
range and auditory sensitivity under two idealized assumptions. (1) Broadcast
range can be estimated using the measured intensity of male calls in the
field, while also considering the propagation constraints of spatial
attenuation due to spherical spreading, frequency-dependent excess attenuation
due to temperature and humidity (Beranek
and Ver, 1992), and masking noise levels in a complex or noisy
environment (US Department of Defense,
1983; Römer,
1993). In other words, we did not measure propagation directly,
and our estimates neglect possible effects of absorption by vegetation. These
estimates, therefore, can be taken to represent maximum propagation distances
for prairie mole cricket calls in native habitat. (2) Auditory frequency
tuning curves and hearing intensity thresholds measured in the lab are a
reasonable estimate of sensitivity in the field.
Using published spectral data for the G. major calling song
(Hill, 1998) and relative SPL
estimates for each harmonic overtone of the call
(Hill et al., 2006), the most
conservative maximum signal distance per frequency under ideal conditions was
calculated. An inter-burrow distance matrix, produced within the GIS and
identifying each burrow's linear distance from all others within the
aggregation, was then queried to determine the number of male pairs within the
lek spaced within these maximum signal distances at each of the harmonic
frequencies within the calling song. The percentage of the total paired
interactions that were spaced within these maximum signal distances was then
calculated for each harmonic frequency based upon this queried total.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Iso-intensity response curves (Fig. 2) indicate that the bimodal frequency sensitivity is also evident at suprathreshold stimulus intensities but further suggest that, as stimulus increases, frequency tuning for G. major flattens somewhat to include the harmonic components of the advertisement call. Moreover, at the highest intensities examined (85 dB), the curve shifts toward a lower-frequency sensitivity peak at 1.0 kHz with two additional secondary sensitivity peaks within the curve at 3.5 kHz and 30 kHz. This shift may occur due to the recruitment of responses from subgenual organs.
Spatio-auditory ecology
Overall, the majority (74%) of the 300 males were spaced so that they could
detect the dominant frequency of a neighbor's calling song, but only a small
proportion (<11%) were spaced such that they could detect the highest
frequency components of a neighbor's song
(Table 1). It was more common
for males to be spaced at or beyond the estimated maximum signal range of
other males within small leks (N<7 males), while in larger leks
nearly all males tend to be within the estimated maximum signal range of the
entire lek membership at the dominant frequency.
Only a small proportion of males within the leks surveyed were spaced close enough to potentially detect the higher-frequency components of a neighbor's calling song (based on overlap of concentric circles representing frequency in Fig. 3). At the first harmonic overtone of the advertisement call (4 kHz), 10.7% of males were spaced close enough to potentially interact acoustically with a neighbor, while 8.6% of males could potentially detect a neighbor's second harmonic overtone (6 kHz). Even smaller proportions of advertising males could potentially detect the calling song's two highest harmonic overtones (1.4% at 8 kHz and 0.1% at 10 kHz) (Fig. 4; Table 1). Regardless of the population size of the lek examined, small groups of 2–4 males were consistently seen distributed across the lek in clusters spaced closely enough together to potentially detect some harmonic overtones of the other males in the cluster. A greater proportion of these intra-lek clusters was spaced to allow for the potential detection of the member's second and third harmonics rather than of the fourth and fifth harmonics, and generally only nearest neighbor pairs were spaced close enough to potentially detect these high-frequency call components.
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). While
males do appear to space themselves, at least in larger leks, within earshot
of other singers, our data do not support the prediction that they make use of
higher harmonic components of the call in spacing their burrows on a lek. Even
assuming optimal signal propagation (attenuation due to spherical spreading
and excess attenuation only) (see
Römer, 1993;
Beranek and Ver, 1992), few
individuals were close enough to their nearest neighbor for any call
components above the fundamental to be audible.
A rather narrow sensitivity in the ultrasonic range suggests that G.
major's hearing allows it to avoid echolocating bat predators. Oklahoma
has 22 native bat species (Caire et al.,
1989), and at least five species are common to the area of our two
field sites (W. A. Caire, personal communication). Echolocation frequencies
for two of these, the big brown bat (Epitesicus fuscus) and hoary bat
(Lasiurus cinereus), were measured in Arizona to be as low as 26 kHz
(Fenton and Bell, 1981) but
vary at the population level across North America: E. fuscus,
25–70 kHz, and L. cinereus, 16.9–39 kHz
(Thomas et al., 1987). These
frequencies vary within a species based on the individual, the habitat and
interactions with conspecifics (Obrist,
1995). However, larger bats typically have lower-frequency calls,
and lower frequencies are not suitable for detection of small prey
(Jones, 1999). Measured
echolocation frequencies for both the big brown bat and hoary bat are in the
range of the higher tuned hearing sensitivity in G. major, which is
the largest cricket in North America
(Walker and Figg, 1990) and
apparently an appropriate prey size for larger bats. Thus, ultrasound hearing
should be advantageous in this species, even if a secondary adaptation for
detecting and evading bat predators (Hoy
1992; Mason et al.,
1998).
In a study of Scapteriscus species, signal intensity was the call
component correlated with male spacing
(Forrest and Green, 1991).
Gryllotalpa major call intensity varies among males, but inter-male
distance does not correlate with calling song intensity. Harmonic content of
the calling song does correlate negatively with nearest neighbor distance
(Hill, 1998); however, it is
not known if males can modulate harmonics in the calling song, only that males
advertising at the closest inter-male distances produced songs with a greater
number of harmonic overtones. Regardless, less than 11% of the males in the
current study were spaced at distances to potentially detect the second
harmonic of a calling neighbor. This result lends support to the argument
(Hill and Shadley, 1997;
Hill and Shadley, 2001) that
G. major males space themselves based on cues from the
substrate-borne vibrational component of the calling song. Further, males are
not known to respond to the airborne component of the advertisement signal of
a conspecific neighbor once the acoustic burrow has been constructed and the
male has begun to actively signal (Hill
and Shadley, 2001). Theory suggests, however, that airborne
females, who fly 4–5 m above a lek and then swoop down to a height of
1.5–2 m for a second pass (Howard
and Hill, 2006), can use the harmonic overtones to assess the
proximity of advertising males, as the rapid attenuation of these
higher-frequency call components provides distance cues. A signaler with more
harmonic overtones will be perceived as closer in space than a signaler at an
equal distance and an equally intense calling song, but with fewer harmonics
(Römer and Lewald, 1992).
Thus, males with more harmonic calls may be considered more attractive to
females than males with less spectral richness in the calls.
Two unexpected results of this study require additional data before strong
conclusions can be drawn. Tuning of the hearing within the range of
frequencies in the calling song is broader at higher stimulus levels. Stronger
responses at low frequencies might suggest that vibration receptors are being
recruited and therefore hint at a more important role for substrate components
in this species than we have so far documented. Additionally, males
advertising on leks of less than seven individuals tended to be more broadly
spaced than males on larger leks. In katydids, central nervous processing of
signals allows individuals to filter out all but the two closest singing
conspecifics in a chorus (Römer,
1993), and other orthopterans may attend to only one or two
closest or loudest neighbors (Römer,
1993; Greenfield,
1994) or even sing from less than half the maximum estimated
signal range (Römer and Bailey,
1986). Thus, G. major males may only be interacting with
nearest neighbors, even on leks with the fewest individuals, where we found
inter-burrow distances to be relatively greater.
In many orthopteran groups, the detection of conspecific advertisement song
is known to mediate inter-male spacing and pair formation
(Alexander, 1967;
Morris, 1979;
Latimer and Sippel, 1987;
Snedden and Sakaluk, 1992;
Tuckerman et al., 1992;
Farris et al., 1997;
Forrest et al., 2006), while
signal detection is under constraint by both biological and environmental
factors (Römer, 1993). As
in other ensiferan species, hearing in G. major is tuned to the
carrier frequency of the calling song, and females are phonotactic. While
orthopteran signal transmission and detection in natural environments is well
described (Römer and Bailey,
1986; Römer and Lewald,
1992; Cooley, 2001;
Couldridge and von Staaden,
2004), less is known about which components of the advertisement
call are actually used in inter-male spacing, or in female choice, especially
in G. major. A general trend, however, is for temporal patterns of a
signal to be used in species recognition, while the frequency spectrum is used
more to detect predators, to discriminate mates at close range or to estimate
distance to a conspecific singer (Stumpner
and von Helversen, 2001). This interpretation is supported by
empirical evidence in bushcrickets that prefer louder, harmonic songs, where
the higher harmonics could be useful in estimating distance to the signaler
(Latimer and Sippel, 1987),
and female Teleogryllus oceanicus, who preferred calls with
additional harmonics in two-choice tests between pure tone calls and those
with harmonic overtones (Latimer and
Lewis, 1986).
While distinct from other mole cricket species in aspects of its
advertisement call structure, habitat requirements and lek mating system,
G. major fits a generalized ensiferan model with regard to its
hearing. As in Scapteriscus species
(Mason et al., 1998), G.
major has bimodal hearing sensitivity that allows efficient detection of
conspecific calling songs as well as the ultrasonic signals of potential
microchiropteran predators. This study also supports the hypothesis of a
bimodal signaling system (Hill and
Shadley, 1997) whereby the airborne component targets flying
females and the substrate-borne component provides spacing cues used in
inter-male spacing. Higher-frequency harmonic overtones in the calling song
may be important in female choice as females estimate distance to the nearest
male, but additional study is required before a role is found for harmonic
overtones in nearest neighbor interactions among closely spaced males. Such
studies might test a harmonic-mediated male spacing model by assessing male
response to simulated closely spaced nearest-neighbor calling songs that
exhibit differential harmonic overtones at high amplitude and the
corresponding female preferences for the same.
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Acknowledgments |
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Footnotes |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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