Gryllus Texensis) in Response to Acoustic Calls from Conspecific Males

J Insect Behav DOI 10.1007/s10905-013-9375-7

Phonotactic Behavior of Male Field Crickets (Gryllus texensis) in Response to Acoustic Calls From Conspecific Males

Thomas M. McCarthy & John Keyes & William H. Cade

Revised: 17 December 2012 /Accepted: 10 January 2013 # Springer Science+Business Media New York 2013

Abstract We studied male phonotactic behaviors elicited by acoustic cues that simulate conspecific male songs in the field cricket, Gryllus texensis. Males exhibited significant positive phonotaxis in response to the simulated song stimuli, but showed no such response to atypical song stimuli. We found no significant relationship between males’ own calling behavior and their phonotactic responses to the stimuli. Analyses indicated that larger males exhibited greater phonotactic responses, which may indicate a greater tendency to engage in aggressive interactions if size is an indicator of fighting ability. Male phonotactic responses were significantly weaker than those exhibited by females, and adult males did not exhibit stronger responses with increasing age as has been documented for females. Observed sex differences in the strengths of phonotactic responses may reflect differences in the fitness-payoffs of responding. That is, females are under strong selection pressure to respond to male songs and subsequently mate. In contrast, males responding to acoustic signals from other males need not precisely locate the signaler but would likely move to areas where females are likely to be found. Alternatively, males might benefit from avoiding areas with calling males and establish- ing their own calling stations away from competing males.

Keywords Acoustic signal . phonotaxis . mating system . behavioral strategy . size . orthoptera

Introduction

A major goal in behavioral ecology is to understand the selective forces that influence the evolution of mating systems. Much effort has gone into increasing our

T. M. McCarthy (*) Department of Biology, Utica College, 1600 Burrstone Road, Utica, NY 13502, USA e-mail: [email protected]

J. Keyes : W. H. Cade Department of Biological Sciences, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada J Insect Behav understanding of mating processes, and how mating decisions translate into fitness benefits (Bateson 1983; Andersson 1994). Signalling and communication are impor- tant components of many mating systems; signalling facilitates the exchange of information between individuals (both potential mates and rivals) engaged in interactions that influence reproductive success. Acoustic signals are among the most extensively studied sexually-selected traits, especially in insects, anurans and birds (Andersson 1994; Gerhardt and Huber 2002). There are likely to be strong selection pressures on both the signallers and the receivers of acoustic cues (Andersson 1994; Johnstone 1997; Gerhardt and Huber 2002): signallers must attract mates and dissuade rivals, while those receiving acoustic cues must assess the signal in order to determine either the quality of the signaller as a mate, or the level of risk imposed by the signaller if they are competitive rivals for mates. In crickets, characteristics of males’ acoustic songs have been studied extensively (e.g. Souroukis et al. 1992; Ciceran et al. 1994; Gray and Cade 1999a; Gray and Eckhardt 2001). Additionally, numerous studies have examined females’ phonotactic responses. Male songs generally elicit a positive phonotactic response from females, such that, females move in the direction of the calling male. These studies have shown that females prefer some songs to others (Wagner et al. 1995; Hedrick and Weber 1998; Gray and Cade 1999b; Wagner and Reiser 2000), and that their preferences can be context-dependent (Cade 1979a; Prosser et al. 1997; Lickman et al. 1998). In contrast to the attention focused on female responses, very few studies have considered short-range phonotactic responses of male crickets (e.g. Kiflawi and Gray 2000; Leonard and Hedrick 2009; Jang 2011). Several lines of evidence suggest that males respond to acoustic cues, including: long-range phonotaxis (Cade 1989), spatial distribution patterns of males (French et al. 1986; Cade and Cade 1992; Souroukis and Cade 1993), and alternative male mating strategies (Cade 1981; Cade and Cade 1992; Rowell and Cade 1993; Walker and Cade 2003; Zuk et al. 2006). Consequently, male calls may serve a dual function: to attract females and repel competing males (Alexander 1975; Otte 1977; Cade 1979b; Andersson 1994). However, non-calling males may also eavesdrop (McGregor 1993) and exploit an individual’s calls by employing a satellite- male mating strategy (Cade 1979b, 1981; Cade and Cade 1992; Rowell and Cade 1993; Zuk et al. 2006). These hypotheses lead to specific predictions that can be experimen- tally tested: (1) if competing males are deterred by a calling male, or if calling serves as a mechanism for spatially distributing individuals, then negative phonotaxis should be observed. In various cricket species, calling males are separated by distances greater than 1 m in the field (Cade 1979b). (2) If competing males act agonistically toward a calling male, then positive phonotaxis should be observed. Here, positive male phonotaxis could be an indicator of aggression or competitive ability in a fight (Leonard and Hedrick 2009). Individuals that move toward a calling male might attempt to displace that male and occupy his calling territory. (3) If competing males employ a satellite mating strategy, then either positive phonotaxis or increased non-directed movements should be observed. Positive male phonotaxis may be a mechanism to increase the efficacy of satellite behavior. Silent males roaming in the vicinity of a calling male (Rowell and Cade 1993) should encounter more receptive females than those randomly moving through the environment. Our goals for this study were to determine whether male field crickets exhibit phonotactic responses as has been observed for conspecific females, and if so, J Insect Behav examine the ontogeny of the response and determine whether there is a relationship between male calling behaviors and male phonotaxis. We addressed these questions by exposing individual crickets to synthetic acoustic stimuli and observing their behavioral responses in the laboratory. We predicted that male crickets would exhibit phonotaxis in response to the calls of other males. We also predicted that the responses of males might be state-dependent and change with the age of the individual, as is the case for females (Cade 1979a; Prosser et al. 1997; also see Jang 2011 (G. rubens males)). Finally, we predicted that there would be a significant relationship between calling and phonotactic behaviors of individual males that might reflect alternative male mating strategies. Specifically, we hypothesized that males with low calling rates may adopt satellite mating strategies, and so we predicted that they would have strong positive phonotactic responses. Alternatively, males with high calling rates might respond by moving away from the stimulus source (negative phonotaxis) as a means of spacing between calling males, or by moving toward the stimulus (positive phonotaxis) to displace the calling male.

Methods

We studied phonotactic behaviors of male field crickets, Gryllus texensis (Cade and Otte 2000). Crickets were raised in laboratory cultures originally founded by wild- caught specimens from Austin, Texas; cultures were supplemented with wild-caught crickets annually (Lickman et al. 1998). The cultures were maintained on a reversed 14:10 light:dark cycle. Individuals were isolated in 500 ml plastic containers and provided food and water prior to use in experiments.

Experiment 1: Calling Behavior and Phonotaxis

Measuring Calling Behavior

We removed newly molted adults from the laboratory cultures daily (within 24 h of adult molt). Adults remained isolated for 7–8 days prior to observations to ensure that they were functionally mature. Males produce spermatophores and begin to call within 3–6 days postecdysis (Cade 1981), and the mean age of females at first mating is 3.6 days (Solymar and Cade 1990). We recorded the calling behavior for each male (N=81) over the course of a single night following the isolation period. This should be a reliable indicator of an individual’s propensity to call as previous work indicated calling duration is a repeatable behavior (Bertram et al. 2007) and is relatively consistent across contexts (Cade 1991). Furthermore, song characteristics do not appear to be condition-dependent (Gray and Eckhardt 2001; but see Hedrick 2005). Observing males for a single night, rather than over multiple nights, allowed us to maximize the number of males that could be observed. Males’ containers were distributed such that they were separated by at least 50 cm during the calling assays (see Cade 1981). Observations were divided into 5-min intervals over the course of 11 h (132 intervals per male): 30 min before the lights turned off, 10 h of darkness, and 30 min after the lights turned on. We recorded whether a male called during a given interval and calculated the proportion of intervals that each male called during J Insect Behav the night. Observations were conducted at temperatures between 25 and 28 °C, and a red-filtered light source was used for illumination.

Measuring Phonotaxis

We assayed the phonotactic responses of crickets using the Kugel apparatus. The Kugel (described in greater detail by Wagner et al. 1995; Prosser et al. 1997) is a device that measures the relative speed and direction of a cricket’s movement in response to acoustic stimuli played through a series of speakers. Briefly, the device is comprised of a circular anechoic chamber (47.5 cm high, 101.9 cm in diameter) with four speakers (linear, B-4- 5 JO, full range frequency) set at 90° intervals around the wall of the chamber. A plastic sphere (16.2 cm in diameter) floats on a column of air in the center of the chamber, and sensors record the displacement of the sphere as the cricket moves (see Figure 2 in Wagner et al. 1995). Each speaker is 35.4 cm from the sphere. Crickets were tethered in position at the top of the sphere during trials. Tethering was accomplished by mounting a small block of wood to the pronotum using a drop of hot beeswax. A pin running from the wood block to a horizontal arm located above maintained the position of the crickets at the top of the sphere. Crickets were able to rotate 360° and had limited vertical movement. Cricket locomotion (i.e. sphere displacement) was assessed relative to the active speaker and data were stored as vector scores, incorporating both direction and speed of movement. The total vector score (Doherty and Pires 1987) for a stimulus was calculated as Σ cos (vector angle) x vector length. The movements of crickets responding to acoustic stimuli were recorded between 24 and 72 h of the calling observations. We assayed the phonotactic responses of males (N= 72; nine males died prior to phonotactic assays) and females (N=49) on the Kugel apparatus. Females were the same age as males and provided a comparison for the responses of males. We recorded the weight and pronotum width of each cricket prior to tethering. Following Prosser et al. (1997), crickets were allowed 5 min of silence to acclimate once positioned on top of the sphere.Wethenpresentedcricketswithastandard pre-trial song array. This exposed the crickets to the stimuli they would experience during the trial and controlled for any period of latency to respond (Prosser et al. 1997). We presented two types of synthetic, acoustic stimuli to the crickets: (1) a simulated song with characteristics within the range of the species, and (2) an atypical signal. Single artificial acoustic pulses, constructed and described by Gray and Cade (2000), were used to assemble the synthetic stimuli. While the pulse rate of the simulated song was adjusted for temperature variations (25 °C: 80.0 pulses/sec; 26 °C: 83.5 pulses/sec) (see Souroukis et al. 1992;GrayandCade2000), the other characteristics did not change between trials (35–55 pulses/trill, 85–265 msec inter-trill interval). Theatypicalstimulus(20.0pulses/ sec, 1 pulse/trill, 3,199–3,200 msec inter-trill intervals) was intended to represent a period of relative silence, lacking recognizable song stimuli (a ‘click’sound was broadcast). Since some studies suggest random movements by satellite males within close range of calling males (Rowell and Cade 1993), we were interested in whether the song stimulus could influence a male’sactivitylevel(i.e.ageneralphonoresponse) without eliciting a directional response (phonotaxis). Thus, we could test for changes in activity without depend- ing on directional movements by comparing the responses to the song versus the atypical stimuli with the assumption being that crickets would exhibit baseline activity levels during atypical broadcasts. J Insect Behav

In the pre-trial array, each stimulus was broadcast twice for 60 s with 30-sec intervals between stimuli. During the experimental trials, the song stimulus was broadcast 10 times and the atypical stimulus was broadcast 5 times to each cricket. The presentation order was not changed between crickets but the speakers through which the stimuli were broadcast were randomized for each cricket. Broadcasts lasted for 120 s during the trial phase, with 30-sec intervals between stimuli. The intensity of the stimuli was approximately 83 dB at the position of the cricket, as measured by a General Radio (1565-D) sound-level meter.

Statistical Analyses

Data for the probability of response, total distance moved, and net vector scores for each cricket were non-normally distributed and standard transforms did not sufficiently alter the distributions. Thus, we used nonparametric Wilcoxon signed rank tests to compare individuals’ responses across stimuli using the raw data. Following Prosser et al. (1997), raw data were ranked and the ranked data (except for probability-of-response data) were normally distributed (Kolmogorov-Smirnov, Lilliefors test). Ranked data were used in subsequent analyses to make comparisons between the sexes (MANCOVAwith pronotum width and mass as covariates, followed by univariate F tests), and to examine any relationships between measures of size, calling, and phonotactic responses (correlation and multiple regression analyses). Since the ranked probability-of-response data were still not normally distributed, gender comparisons were made using Mann–Whitney U tests. Two levels of analysis were carried out on male phonotactic responses: the first examined all males, while the second examined only calling males. This was to ensure that non-calling males did not mask any underlying patterns.

Experiment 2: Ontogeny of Male Phonotaxis

Measuring Phonotaxis

We assayed phonotactic responses of male crickets (N=27) at five different ages using the Kugel apparatus in order to examine ontological shifts in behavior. Large juvenile (late instar) males were removed from the laboratory cultures, isolated in 500 ml plastic containers, and provided food and water. After acclimating for at least 48 h, the phonotactic responses of juvenile males were assayed using the Kugel apparatus. We recorded the weight and pronotum width of each cricket prior to tethering. Followingatrial,juvenile males were returned to their containers and checked daily until they molted to the adult stage. Phonotactic responses were then measured in the adult males at 1, 4, 10 and 21 days following the ultimate molt. We again recorded the weight and pronotum width of each cricket prior to tethering on day 1, and recorded only weight measurements for subsequent trials. The testing procedure and acoustic stimuli used in this experiment were identical to those described above.

Statistical Analyses

We assayed males prior to their ultimate molt because juveniles do not have fully developed auditory tympana (Fig. 1). Consequently, we did not expect phonotactic J Insect Behav

Fig. 1 Photographs showing the small inner (top row) and large outer (bottom row) auditory tympana, indicated by arrows,onthe tibia of the anterior legs of juvenile (left column) and adult (right column) crickets

responses prior to adulthood. Thus, the movements of crickets as juveniles could serve as a comparison for their responses as each individual aged. Data for the net vector scores for each cricket were non-normally distributed. Thus, we used nonparametric tests to compare responses to different stimuli and at different ages. Preliminary analyses indicated that males’ responses did not differ significantly between the adult ages of 1 and 4 days, or between the ages of 10 and 21 days, for either acoustic stimulus. Therefore, we collapsed these ages into two categories: reproductively immature adults (ages 1 and 4 days) and reproductively mature adults (ages 10 and 21 days). We felt that these were biologically relevant categories since males produce spermatophores and begin to call within 3–6 days postecdysis (Cade 1981). Preliminary tests (multiple regression analyses) also indicated that there were no body-size effects (mass, pronotum width), so these measurements were dropped from the final analyses. We used Wilcoxon signed rank tests to compare individuals’ responses to stimuli at each age. We used nonparametric Friedman tests to compare individuals’ responses to each stimulus as they aged, with post-hoc Wilcoxon signed rank tests to compare responses between age categories.

Results

Experiment 1: Calling Behavior and Phonotaxis

Calling Behavior

A majority of the males (60 of 81) called during their observation period. Many of the males (41.9 %) began calling early in the night within 4 h of the onset of darkness (Fig. 2a). The number of calling males increased throughout the night, with 46.9 % of J Insect Behav

Fig. 2 Numbers of males that (a) first began to call, and (b) were observed calling, during a given time period. Open bars indicate periods that the lights were on: ‘Dusk’ refers to the 30- min period before the lights turned off; ‘Dawn’ refers to the 30-min period after the lights turned on. (N=81)

the males calling during the last 30 min with the lights on (Fig. 2b). While a few males called for several hours, most called for less than 2 h (Fig. 3). Male size was not related to calling behavior. Student’s t-tests indicated that calling and non-calling males (59 and 19 individuals, respectively(nosizedatawere recorded for three crickets)) did not differ significantly for either pronotum width (mean ± SD mm; calling: 5.13±0.41; non-calling: 5.00±0.48; separate variance t27.0=1.07, P=0.29) or mass (mean ± SD g; calling: 0.39±0.06; non-calling: 0.37± 0.09; separate variance t24.1=0.89, P=0.38). Furthermore, multiple regression analyses using only the calling males did not indicate any relationships between a male’s size and when that male began calling (pronotum width: partial r=−0.155, P=0.46; mass: partial r=0.006, P=0.98) or the calling duration exhibited by that male (pronotum width: partial r=−0.039, P=0.85; mass: partial r=0.085, P=0.69).

Calling & Phonotaxis

The simulated song stimulus elicited phonotactic responses from males. Individual males were equally likely to be active during presentations of either type of stimulus (Fig. 4a; Wilcoxon signed rank test: z=−0.30, P=0.77), and activity levels were positively correlated between stimulus types (N=72, Pearson r=0.77, P=0.001). That is, males that moved longer distances during song presentations also tended to move longer distances during atypical presentations. J Insect Behav

Fig. 3 Numbers of males exhibiting the given calling durations. (N=81)

The responses of males differed between stimulus types. The song stimulus elicited longer total walking distances (Fig. 4b; Wilcoxon signed rank test: z=−4.58, P=0.001) and greater directional movement toward the active speaker (Fig. 4c; Wilcoxon signed rank test: z=−4.29, P=0.001) than the atypical stimulus. The low net vector scores, which do not significantly differ from zero (one-sample t-test: t71=0.82, P=0.42), indicated that the atypical stimulus did not elicit directional responses from the males. Multiple regression analyses, using only the calling males, indicated a positive relationship between males’ pronotum width and their net vector scores during presentations of the song stimulus (Table 1). We found no significant relationships between males’ responses to either stimulus type and their calling durations, or the time at which calling began (Table 1). When we included both calling and non-calling males in the analyses, and therefore dropped calling duration and calling-start times from the models, again we found that the only significant relationship was between pronotum width and net vector scores during the song presentations (Table 2a).

Comparing Phonotaxis of Females and Males

Males and females had similar qualitative responses to the stimuli, but differed in the relative strengths of their responses. Like the males, female activity levels were positively correlated between stimulus types (N=49, Pearson r=0.67, P=0.001). However, females were more responsive to the stimuli than males (MANCOVA: Wilks’ Lambda=0.860, F4,114=4.65, P=0.002). As with the males, females were equally likely to be active during presentations of either stimulus type (Fig. 4a; Wilcoxon signed rank test: z=−0.46, P=0.64), but females were more likely to J Insect Behav

Fig. 4 Mean values (± SD) of responses by male and female crickets to ‘song’ and ‘atypical’ acoustic stimuli: (a) Proportion of trials in which crickets were active; (b) total distance (Kugel sphere displacement) crickets moved during trials; (c) net vector score for phonotactic response relative to active speaker. See text for statistical analyses. (Males: N=72, solid bars; Females: N=49, open bars) exhibit a response than males (Fig. 4a; song stimulus: Mann–Whitney U=2450.5, P=0.001; atypical stimulus: Mann–Whitney U=2324.5, P=0.001). Females also walked longer distances during presentations of the song stimulus than during presentations of the atypical stimulus (Fig. 4b; Wilcoxon signed rank test: z=−5.45, P=0.001), with greater directional movement toward the active speaker (Fig. 4c; Wilcoxon signed rank test: z=−5.61, P=0.001). Females were significantly more phonotactic than males during song presentations (Fig. 4b, c). That is, females moved greater distances (univariate F test: F=11.40, P=0.001) and had higher vector scores than males (univariate F test: F=17.21, P=0.001). J Insect Behav

Table 1 Multiple regression analyses examining factors potentially influencing phonotactic responses of calling males during presentations of ‘song’ and ‘atypical’ acoustic stimuli (N=54)

Pronotum Mass Calling duration Onset calling

Partial rp Partial rp Partial rp Partial rp

Song Total distance 0.15 0.50 0.19 0.39 −0.20 0.22 −0.02 0.90 Net vector score 0.59 0.009 −0.35 0.11 0.01 0.98 −0.04 0.78 Atypical Total distance 0.28 0.21 −0.02 0.93 −0.11 0.50 0.07 0.64 Net vector score −0.03 0.91 0.08 0.73 −0.02 0.89 0.11 0.53

The relatively low net vector scores produced by females during presentations of the atypical stimulus, which do not significantly differ from zero (one-sample t-test: t48=1.84, P=0.07) or from those of the males (Fig. 4c;univariateF test: F=0.79,P=0.38),indicated that the atypical stimulus did not elicit phonotactic (directional) responses. There was a nonsignificant trend for females to move greater distances than males during presentations of the atypical stimulus (Fig. 4b;univariateF test: F=2.25, P=0.14). This trend, along with the significant differences in the probability of being active and on the strengths of response to song stimuli, suggests that females are generally more active than males. Females were significantly larger than males in terms of both pronotum width (mean ± SD mm; females: 5.25±0.36; males 5.09±0.43; separate variance t-test: t113.4 =2.17,P=0.03) and mass (mean ± SD g; females: 0.46±0.09; males 0.38±0.07; separate variance

Table 2 Multiple regression analyses examining affects of size characteristics on the phonotactic responses of (a) males (N=72) and (b) females (N=49) during presentations of ‘song’ and ‘atypical’ acoustic stimuli

Pronotum Mass

Partial rp Partial rp

(a) Males Song Total Distance 0.20 0.27 0.13 0.47 Net Vector Score 0.54 0.003 −0.24 0.18 Atypical Total Distance 0.21 0.27 0.07 0.71 Net Vector Score −0.13 0.51 0.16 0.42 (b) Females Song Total Distance −0.25 0.14 0.32 0.06 Net Vector Score −0.13 0.46 0.25 0.16 Atypical Total Distance −0.15 0.40 0.06 0.74 Net Vector Score 0.01 0.98 0.15 0.39 J Insect Behav

t-test: t86.2=5.19,P=0.001). Nevertheless, the differences in activity cannot be adequately explained by size differences since neither of these characteristics had significant effects as covariates in the MANCOVA comparing the responses of the sexes (pronotum: Wilks’ Lambda=0.954, F4,114=1.36, P=0.25; mass: Wilks’ Lambda=0.931, F4,114=2.10, P=0.09). In contrast to the males, we also failed to find any significant relationships between these size characteristics and females’ responses in the multiple regression analyses (Table 2).

Experiment 2: Ontogeny of Male Phonotaxis

The responses of males differed between stimulus types and changed with age (Fig. 5). Males’ responses to the atypical stimulus did not change with the age of the individual (Friedman test: χ2 =0.22, df=2, P=0.90). In contrast, males’ phonotactic responses to the song stimulus increased as they reached adulthood (Friedman test: χ2=13.4, df=2, P=0.001). The low net vector scores exhibited by the males during the juvenile stage did not significantly differ from zero for either the song stimulus (one-sample t-test: t26=−1.48, P=0.15) or the atypical stimulus (t26=−0.53, P=0.60), nor did the responses to the different stimuli differ significantly from each other (Wilcoxon signed rank test: z=−0.70, P=0.49). Following ecdysis to the adult stage, functionally immature males increased their phonotactic responses when presented with the song stimulus relative to their responses as juveniles (Wilcoxon signed rank test: z=−3.00, P=0.003). Male responses to the song stimuli did not differ significantly between the immature and sexually mature adult stages (Wilcoxon signed rank test: z=−0.70, P=0.49). Adult males were significantly more responsive to the song stimulus than the atypical stimulus both immediately after ecdysis (Wilcoxon signed rank test: z=−2.98, P=0.003) and as they became functionally mature (Wilcoxon signed rank test: z=−3.32, P=0.001).

Fig. 5 Net vector scores elicited by the song and atypical acoustic stimuli for individual males in three age categories. Each box shows the median, quartiles, and extreme values; outliers have been omitted from the plot for the purpose of scale. See text for statistical analyses. (N=27) J Insect Behav

Discussion

We monitored the calling characteristics and phonotactic responses of male Texas field crickets. Although we found no significant relationship between the nightly calling duration of male calling behavior and their phonotactic responses, there were significant relationships between body size and phonotaxis. Males and females did not display any directional movements in response to presentations of the atypical stimulus. In contrast, the song stimulus elicited positive phonotactic responses from both sexes. By comparing the sexes we found that males were less likely to respond to stimuli and had weaker responses than did females. Juvenile males exhibited no phonotactic responses, but began to exhibit phonotaxis upon reaching the adult stage. Our observations of male G. texensis calling behaviors generally agree with those of previous studies using this species. The distribution of call durations among males is highly skewed as demonstrated in this and previous studies (Cade 1991; Cade and Cade 1992; Bertram 2000; 2002a; Bertram et al. 2007). Cade and Cade (1992) found that large G. texensis males called longer than smaller males in low-density con- ditions, and that mating frequency was correlated with calling duration, but these relationships disappeared under higher population densities. Most males (~75 %) in our study called, and many of those began calling within a few hours of dark. This period corresponds with the peak times for dispersal flights (Cade 1979c, 1989); thus, calling during this period might attract large numbers of potential mates. It should be noted, however, that our study and other studies found considerable individual variation in when males began to call. The average calling start-times may also vary between the spring and fall breeding seasons. Bertram (2000, 2002b) found that the average calling start-time was within 4 h of dark. This variation in start-times (and other calling characteristics), both individually and seasonally, may be explained in part by selective pressures imparted by the acoustically orienting parasitoid fly, Ormia ochracea (Cade 1975; Gray and Cade 1999b; Bertram 2002b; Walker and Cade 2003). Flies are most active just after sunset, suggesting that the risk of being attacked by the flies decreases over the course of the night (Cade et al. 1996; Bertram et al. 2004). We found that the numbers of calling males increased throughout the night with the greatest numbers of males calling near dawn (also see French and Cade 1987). We found that both males and females were positively phonotactic in response to the simulated song stimulus, and neither sex exhibited directional movement in response to the atypical stimulus. The lack of phonotaxis during atypical presentations supported our prediction that this stimulus would not be a recognizable acoustic signal for this species. Previous work (Cade 1989) found a similar pattern when examining the long-range attraction of crickets to calling males in the field; both flying males and females were attracted to sites with calling males relative to silent control sites, but significantly more females were attracted than males. Regardless of the sex of the receiver, a male call is a cue indicating a potential resource. Selection pressure(s) should favor phonotactic responses in both males and females, though the responses may vary qualitatively and quantitatively given differences in the potential fitness benefits and costs of responding to acoustic calls for each sex. Given the findings of previous studies and the biology of the species, we expected the strong positive female phonotaxis to the song stimulus—females should J Insect Behav respond to male songs because successful reproduction for females requires encounters with males. While the song stimulus elicited stronger positive phonotactic responses from females, males also moved toward the song stimulus. Even though a male’s mating success does not require encounters with other males his fitness can be affected by such encounters. Male reproductive success could increase as a result of positive phonotaxis if rival calling males can be displaced through fights (calling site as a resource). Leonard and Hedrick (2009) found G. integer males likely to win fights with other males were also more likely to approach male song stimuli. Alternatively, responding males may alsobenefitbyemployingasatellite-male mating strategy (possible increased encounter rates with females). A third, non- adaptive explanation for male phonotaxis is that it is a consequence of a genetic correlation between the sexes: gene expression leading to adaptive phonotaxis by females may also lead to non-adaptive responses in males. So while the behavioral response is the same for males and females (i.e. positive phonotaxis), differences in the vigor of the response likely reflect probable differences in strengths and types of selection pressures acting on each sex. This might also explain correlated observations, such as, why females increase their phonotactic responses as they age (Cade 1979a; Prosser et al. 1997; Lickman et al. 1998) but this pattern was absent in males in the current study. Jang (2011) found that male G. rubens decreased phonotactic activity as they aged. Given the potential fitness implications of phonotaxis, we predicted that there might be a relationship between male calling and male phonotaxis. The males with little or no calling behavior might be expected to exhibit greater phonotactic responses; our assumption being that this would lead to corresponding satellite- male mating tactics in the field. Alternatively, males with high calling rates may exhibit strong phontaxis: negative phonotaxis may be indicative of spacing between males, whereas positive phonotaxis may signal aggression between calling males. We found no evidence to support any of these predictions, suggesting there is no relationship between male calling and phonotaxis. However, while seemingly unlike- ly given our data, we cannot fully discount this hypothesis since the focus of this experiment was male phonotaxis itself rather than the behavioral interactions between individuals following phonotaxis. As with previous studies (e.g. Prosser et al. 1997), we found no relationship between body size and phonotaxis in females. Again, regardless of the size of the female, successful reproduction requires encounters with males. We would not expect female phonotaxis to vary with body size. In contrast, large males often have a mating advantage in many systems (Roff 1992). For example, large males are more aggressive and competitively superior to small males during agonistic interactions in G. texensis (Dixon and Cade 1986), G. bimaculatus (Simmons 1986) and G. pennsylvanicus (Souroukis et al. 1992). We found a positive relationship between male size (pronotum width) and directional movement toward the active speaker. Larger males had greater phonotactic responses. A similar pattern was observed in male house crickets, Acheta domesticus, within an arena; larger males were more likely than smaller males to make contact with the active speaker (Kiflawi and Gray 2000). This can be interpreted as size-mediated aggression. In G. bimaculatus, calling is positively correlated with the proportion of fights a male wins (Simmons 1986), and more aggressive males have higher mating J Insect Behav success (Tachon et al. 1999). Thus, large males may benefit from approaching and initiating agonistic encounters with calling males, whereas this would be a costly strategy for smaller males that are likely to lose the fight. Consequently, we would expect selection to favor larger, more aggressive males that could establish territories and begin calling as the females are flying.

Acknowledgments We wish to thank B. Kitsch, E. Jeffery, and J. Sloan for their help in maintaining cricket cultures and aiding in the experiments. T. McDonald, D. Gray and S. Walker provided invaluable assistance and advice in using the Kugel apparatus. Several anonymous reviewers provided insightful comments that improved the paper. We also wish to thank the Natural Sciences and Engineering Research Council (grant to W.H.C.), the University of Lethbridge, and Utica College (Leadership Fund to T.M.M.) for support and funding.

References

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