Journal of Behavior, Vol. 2, No. 1, 1989

Sexual Selection at Varying Population Densities in Male Field Crickets, veletis and G. pennsylvanicus

B. Wade French z and William H. Cade l

Accepted 10 September 1987; revised 4 March 1988

Male field crickets call and attract females or they silently search for females in the vicinity of calling males. At high population densities, fewer calling sites are available, defense of calling sites is costly, and an increased proportion of matings should result from searching behavior. To test these predictions, indi- vidually marked field crickets, Gryllus veletis and G. pennsylvanicus, were observed for 10 h nightly in large outdoor arenas at relatively high and low densities (20 : 20 and 5 : 5, males and females). Data were gathered on body weight, calling duration, movement, and mating frequency for individual crick- ets. These observations showed that variance in male mating success was sig- nificantly greater at a low density in G. pennsylvanicus, and calling duration correlated with mating success at this density. Direct selection on a trait was estimated as the partial regression coefficient (selection gradient, {3 ') and the total selection was estimated (direct and indirect selection on correlated traits) as the covariance (standardized intensity of selection, s ') of the trait on the relative mating success. Direct selection favored increased movement at a high density in G. veletis, and direct and total selection favored increased calling duration at a low density in G. pennsylvanicus. Most other comparisons were not statistically significant. The data are discussed in terms of density-depen- dent fluctuations in sexual selection on correlated male traits.

KEY WORDS: Gryllus; sexual selection; density dependence; mating success; acoustical sig- nals; alternative reproductive behavior; searching behavior; correlated traits; ; .

~Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3AI.

105 0892-7553/89/0100-0 [ 05506.00/0 ~c~ 1989 Plenum Publishing Corporation 106 French and Cade

INTRODUCTION Male field crickets compete for females by acoustical signaling, searching silently in the vicinity of calling males, and fighting with other males over access to calling sites and females (Alexander, 1961, 1975; Burk, 1983; Cade, 1979a; Dixon and Cade, 1986; Otte, 1977 Simmons, 1986a,b). The factors influencing male mating success should vary in importance with increased pop- ulation density: the average number of matings resulting from female attraction to calling song should decline, while the success of searching should increase, and the amount of physical aggression should decline. Density-dependent fluc- tuations in the mating success derived from calling or searching for females may result from decreased availability of calling sites, increased cost of defense, and increased probability of random contacts between males and females at higher densities. There are, however, no data available from field populations of crickets to test these predictions on relative mating success. This study was intended to measure individual male mating success in the field crickets, Gryllus veletis and G. pennsylvanicus (Orthoptera: Gryllidae), under field conditions and at different levels of population density. Data are presented on correlations among mating success and male calling, movement, and body size (a compo- nent of fighting ability). Also, a new method of estimating relative selection pressures on correlated traits is used to analyze data (Arnold, 1983, 1985; Arnold and Wade, 1984; Lande and Arnold, 1983).

METHODS

G. veletis is sexually mature from May until early August, whereas G. pennsylvanicus is sexually mature from late July until November (Alexander, 1968). These species were studied at Brock University, St. Catharines, Ontario, from June to September 1983 and 1984. Outdoor Arenas. All studies were carried out in outdoor arenas placed in natural habitats. The two arenas used to study G. pennsylvanicus and G. veletis were constructed of 1-m-high aluminum siding and measured 13 x 13 m in 1983 and 12 x 12 m in 1984. (Different arenas were used in the 2 years because of new construction by the university.) The 1983 and 1984 arenas were divided into 25 2.6 x 2.6-m and 25 2.4 x 2.4-m quadrats, respectively. Each quadrat was assigned a number and marked with flags. Artificial burrows (approximately 4-8 cm deep and 4 cm wide) were excavated in the center of each quadrat and covered with wood blocks. Rocks were placed along the walls of bordering quadrats. Grass in the arena was initially mowed to a height of about 4 cm and watered routinely. The grass reached a height of 6 cm during an observation period, but the crickets were easily observed. Collecting and Marking Crickets. G, veletis and G. pennsylvanicus were Sexual Selection in Male Field Crickets 107 collected as nymphs and adults in and near St. Catharines. Crickets were brought into the laboratory and kept at 25-30~ and 30-35% relative humidity. The photoperiod was 12 h light and 12 h clark [for techniques on housing crickets see Cade and Wyatt (1984)]. Individual male and female crickets were marked distinctively with small dots of enamel paint on the pronotum or numbers on the pronotum using Liquid Paper and India ink (Walker and Wineriter, 1981). Sexually mature crickets were weighed to the nearest 0.1 mg with an Oertling R20 electronic balance. Observations and Data Collection. A series of observations began when males and females were released into the center of the arena or under randomly chosen burrows at 3-5 h before sunset. Two densities were used in G. veletis and G. pennsylvanicus: a relatively low density of 5 males and 5 females and a relatively high density of 20 males and 20 females. These densities and sex ratios are within the ranges observed for these species (Alexander and Meral, 1967; Cade, 1981a, unpublished data; French et at., 1986). After an introduc- tion the density was maintained at approximately (see Results) the initial level by adding previously collected crickets that were held for 1 to 5 days in the laboratory. Various trials for the same species at the same densities were per- formed. These trials were as follows: one sample of 10 nights at the high density and two samples of 10 nights each at the low density for G. veletis and two samples of 10 and 19 nights at the high density and one sample of 9 nights at the low density for G. pennsytvanicus. A long (19 nights) trial was included to study aspects of age variation. These results are reported elsewhere (French, t986). Female crickets mate several times (Sakaluk and Cade, 1980), and the length of all trials was sufficient to ensure that almost all females had mated. Arenas were observed from 2200 to 1000 h, or from approximately 1.5 to 3 h past sunset to 2 to 3 h past sunrise: These times included the period of greatest cricket mating activity (French and Cade, t987). At the beginning of each night a complete survey of the arena was conducted with a headlamp to ascertain each cricket's location. Crickets were readily visible under the shelters or in the grass, and their locations were marked on maps of the arena. A com- plete survey of the arena was made at least every hour, and a new map used to indicate cricket positions. Calling behavior was ascribed to a particular male if he was the only one in a given location at the previous and following hourly check of cricket loca- tions. Partial checks of cricket locations were carried out between the hourly intervals if there was any uncertainty about a caller's identity. If more than one male was in a shelter, the male which had his wings still raised in the calling position was scored as calling. Males disturbed in this way usually resumed singing after a few minutes. If calling was heard from a position where no male had been observed previously, we identified that male and marked his position on the map. Every' 5 rain we recorded the production of calling song for males 108 French and Cade of known identity. A male's hourly calling time was computed as the number of scores x 5 min. This interval scoring was the shortest possible that allowed for recording of behavior at the high density. Matings were considered to have occurred when a copulation was observed or a female possessed a fresh spermatophore while in the immediate presence of a single male. Recently attached spermatophores are milk-white in color but turn brown in 30 min-1 h (French and Cade, personal observation). Two males were in the immediate presence of a mated female on 18 occasions, and the male "guarding" the female was assigned the mating. Loher and Rence (1978) described guarding behavior in the field cricket, Teleogryllus commodus, and very similar behavior occurs in the species studied here. The linear distance on a map between hourly locations of individual males was used as an estimate of the distance traveled by males. This method of measuring locomotion assumes straight-line movement and is undoubtedly an underestimate of actual movement. Points separated by less than 1 cm on the arena maps were considered as no movement by crickets. Crickets therefore traveled at least 60 cm in the arena before movement was scored. This was the minimum detectable displacement allowed for transferring a cricket's location in the arena to its location on the arena map. Analyses. The nightly totals of time calling, movement, and mating were determined and averaged for each mate. Crickets occasionally escaped from the arena and only males that were in the arena for at least 2 nights were used in the analyses. G. veletis males were in the arena for 8.2 (SD = 2.4) and 8.3 (SD = 2.5) nights on average at high and low densities, respectively. G. penn- sylvanicus males spent 11.2 (SD = 5.9) and 6.0 (SD -- 2.8) nights in the arena on average at high and low densities. Data were transformed to natural loga- rithms to meet assumptions of parametric tests. Replicates at the same density were combined for statistical analysis. Sample means were based on individual cricket means and used to test for significant differences between densities. If sample variances were significantly different between densities, t tests based on separate variance estimates were used (Zar, 1984). Pearson's product-moment correlation analyses were used to test for relationships among mean number of matings per night and mean calling duration per night, mean distance moved per night, and body weight. Several models have been developed for estimating relative selection pres- sures acting on phenotypic characters (Arnold, 1985; Arnold and Wade, 1984; Manly, 1985; Sutherland, t985a; Endler, 1986). Price (1970) and Lande and Arnold (1983) showed that the total intensity of selection acting on a trait is equivalent to the standardized selection differential. Total selection includes selection operating directly on a trait and the indirect forces of selection on other, correlated traits. This value is the within-generation shift in mean phen- otypic value in units of standard deviation due to the direct and indirect effects Sexual Selection in Male Field Crickets 109 of selection. The standardized selection differential is calculated as the covari- ance between relative mating success and a standardized variable and is also called the "coefficient of selection" (Falconer, 1981). Significance was deter- mined using parametric correlation coefficients since the covariance between two variables is directly proportional to the correlation coefficient between the variables. In addition, Lande and Arnold (1983) showed that the intensity of selection acting directly on a trait can be estimated as the partial regression coefficients (/3') of relative mating success on the standardized variables hold- ing all other characters constant. This measure excludes indirect selection on other traits. Direct selection is called the "selection gradient." Significance for the selection gradients was determined by t tests. Phenotypic characters were standardized to have sample means of 0 and variances of 1. In contrast, relative mating success was not transformed to nat- ural logarithms but only standardized to have means of 1 (Zar, 1984). The fitness component, relative mating success, was not log transformed because transformation leads to erroneous values for selection intensities (Lande and Arnold, 1983). Also, by standardizing only to means of 1, the variation in relative fitness is unaffected. The squared multiple correlation coefficient (R 2) represents the variation in relative fitness explained by the independent varia- bles. Significance levels for R 2 were assessed with ANOVAs.

RESULTS Calling. Mean calling durations per night for individual G. veletis at low and high population densities are shown in Fig. 1A. Low densities were com- bined since there were no significant differences in replicate means (t = 0.55, df = 9, P > 0.05) or variances (Fmax = 1.5, df = 4,5, P > 0.05). Males at the low density called on average 1.8 h (SD = 1.2) and males at the high density called 1.3 h (SD = 1.1) per night, and the means are not significantly different (t = 1.31, df = 35, P > 0.05). Figure 1B shows mean calling durations per night for individual G. penn- sylvanicus at low and high densities. Males at the low density called 0.8 h (SD = 0.7) on average per night. High densities were combined since there were no significant differences in replicate means (t -- 1.16, df = 46, P > 0.05) or variances (Fmax = 1.2, df = 26,20, P > 0.05). Males at the high density called 1.1 h (SD = 1.0) on average per night. These means are not significantly different (t = !.01, df = 53, P > 0.05). The shape of the distribution of calling times is indicative of the type of selection operating on calling males and the occurrence of noncalling or satellite male behavior (Cade and Wyatt, 1984). For G. veletis at the low density, the distribution of individual mean calling durations was not significantly skewed (skewness test, gl = 0.9, P > 0.05) and was not significantly different from a 110 French and Cade

Gryllus veletis A

Low Density [] High Density

151 I B

I 10-5_I 3

1

0.25 1.25 2.25 3.25 4:25 Mean Calling Duration per Night (h) Fig. 1. Mean calling duration per night (h) for G. veletis and G. pennsylvanicus males at low and high population densities.

normal distribution (Kolmogorov-Smirnov, Dma x 0.02, df = 10, P > 0.05). For G. veletis at the high density, the distribution of individual mean calling durations was not significantly skewed (g~ = 0.3, P > 0.05) and was not significantly different from a normal distribution (Dmax = 0.1, df = 25, P > 0.05). Regarding G. pennsylvanicus at the tow density, the distribution of indi- vidual mean calling durations was not significantly skewed (gl = -0.2, P > 0.05) and was not significantly different from a normal distribution (DmaS = 0.1, df = 25, P > 0.05). For G. pennsylvanicus at the high density, the dis- tribution of individual mean calling durations was significantly skewed (g~ = 1.1, P < 0.05) and significantly different from a normal distribution (D ..... = 0.1, df = 48, P < 0.05). Sexual Selection in Male Field Crickets 111

Movement. An example of movement by an individual male in 1 night at the low density is shown in Fig. 2. This map shows measures used in calculat- ing nightly movement. Quadrats 1-5, 6, 11, 16, and 21 were omitted since the male did not enter this area. Figure 3A shows the mean movement per night for individual G. veletis males at low and high population densities. Low densities were combined since there were no significant differences in replicate means (t = -0.43, df = 9, P

m ...... Wooden Block C) ...... Rock

O ...... Cricket

--~ ...... Assumed Direction ...... 60 cm

9 10 (08oo) o II II II II

5.7

12 13 14 15

II II II II

17 18 19 20 II II II II,(070C f~ 0 2.4 22 23 24 25

(0100 - 0300) (2200- 2400) m4m m 4.8m W

2.7 m

o o

Fig. 2. An example of a G. veletis male's nightly movement at the high density in the outdoor arena. Arrows show the path of locomotion. 112 French and Cade

Gryllus veletis A

5

3

0 lo - GryUus pennsyIvanicus

7~ Hl~g_hDensit'r i

6-

2

1.0 5.0 9.0 13,0 t7.0 21.0 25.0 Mean Movement per Night (m)

Fig, 3. Mean movement per night (m) for G. veletis and G. penn- sylvanicus males at low and high population densities.

> 0.05) or variances (Fm,x = 1.2, df = 5,4, P > 0.05). Males at the low density moved on average 10.2 m (SD = 6.7) and males at the high density 9.5 m (SD = 4.3) per night, and the difference was not significant (t = -0.16, df = 35, P > 0.05). Figure 3B shows the mean movement per night for G. pennsyIvanicus males at low and high densities. Males at the low density moved 10.7 m (SD = 5.4) on average per night. High densities were combined since there were no sig- nificant differences in replicate means (t = -1.16, df = 46, P > 0.05) or variances (Fmax = 1.28, df = 26,20, P > 0.05). Males at the high density moved 8.7 m (SD = 5.3) on average per night, and the means are not signifi- cantly different (t = -t.50, df = 53, P > 0.05). Sexual Selection in Male Field Crickets 113

Matings. The mean numbers of matings per night for individual G. veletis males at low and high densities are shown in Fig. 4A. Low densities were combined since there were no significant differences in replicate means (t = 1.28, df = 9, P > 0.05) or variances (Fm~x = 1.58, df = 5,4, P > 0.05). At the low density males mated on average 0.2 (SD = 0.2) and at the high density 0.3 (SD = 0.2) times per night. These means are not significantly different (t = 0.89, df = 35, P > 0.05). Variance in male mating success, a measure of the opportunity for sexual selection, was identical (0.05) at low and high dens- ities in G. veletis. Figure 4B shows the mean number of matings per night for G. pennsylvan- icus at low and high densities. Males at the low density mated 0.4 (SD = 0.4)

Gryttus vetetis A LOw Density [-I High Density m

5

~~- 141 Gryllus pennsylvanicus B

Low DensitZ [7] Z -~l _H~h Density ,

10

6

2

0,05 0.25 0.45 0,65 0.85 1,05 1.25 Mean Number of Matings per Night Fig. 4, Mean number of matings per night for G, veletis and G. pennsylvanicus males at low and high population densities. 114 French and Cade times on average per night. High densities were combined since there were no significant differences in replicate means (t = -0.39, df = 46, P > 0.05) or variances (Fma • = 1.38, df = 26,20, P > 0.05). Males at the high density mated 0.3 (SD = 0.2) times on average per night. Variances in male mating success were 0.18 and 0.06 at low and high densities in G. pennsylvanicus, and these values are significantly different (Fmax = 2.62, df = 6,47, P < 0.05). A t test based on a separate variance estimate was used and showed that the means are not significantly different (t = -0.89, df = 6.7, P > 0.05). Behavioral and Phenotypic Correlations. Correlation coefficients between mean number of matings per night and mean calling duration per night, mean movement per night, and body weight for G. veletis and G. pennsylvanicus at low and high population densities are shown in Table I. A significant positive correlation was found between mean calling duration and mean mating fre- quency at the low density in G. pennsylvanicus. All other comparisons were nonsignificant. Average body weights were 431.7 mg (SD =- 72.0, range = 305.0-542.7) and 421.4 (SD = 63.1, range = 304.4-525.0) for G. veletis at low and high densities and 522.4 (SD = 85.8, range = 415.0-637.0) and 459.9 (SD = 92.0, range = 325.6-733.5) for G. pennsylvanicus at low and high densities. Intensities of Selection. The selection intensities and directional selection gradients operating on mean calling duration per night, mean distance moved per night, and body weight are shown in Table II for G. veletis and G. penn- sylvanicus at low and high population densities. For G. veletis, intensities of selection acting directly and indirectly on mean calling duration per night, mean movement per night, and body weight were not significantly different at low or high densities (r = 0.29, 0.31, and 0.10 and -0.31, -0.34, and -0.44; P > 0.05). Directional selection gradients on mean calling duration per night, mean movement per night, and body weight were not significant at the low density (t = -1.05, -1.87, and -0.95; P > 0.05). Also, directional selection on mean weight and mean calling duration was not significant at the high density (t =

Table I. Parametric Correlation Coefficients Among Mean Number of Matings per Night, Mean Calling Duration per Night, Mean Movement per Night, and Body Weight for G. pennsylvanicus and G. veletis at Low and High Population Densities

G. veletis G. pennsylvanicus

Variable Low High Low High

Mating-calling -0.36 0.32 0.83 r 0.09 Mating-movement 0.25 0.37 0.72 -0,11 Mating-weight -0.47 0,05 -0.12 - 0,23

"P < 0.05. Sexual Selection in Male Field Crickets 115

Table II. The Standardized Selection Differentials (s'), Selection Gradients (/3' + SE), and Multiple Correlation Coefficients (R 2) for Mean Calling Duration per Night, Mean Movement per Night, and Body Weight for Gryllus veletis and G. pennsylvanicus Males at Low and High Population Densities

Species Density Variable s' j3' SE R z

G. veletis Low Calling -0.26 -0.33 0.32 0.46 Movement -0.28 -0.49 0.26 Weight -0.37 -0.27 0.28

High Calling 0.20 0.16 0.13 0.24 Movement 0.21 0.29 0.15 " Weight 0.07 0.22 0.15

G. pennsylvanicus Low Calling 0.86" 0.79 0.29 a 0.86 Movement 0.70 0.28 0.29 Weight -0.20 -0.44 0.23 High Calling 0.07 0.08 0.15 0.08 Movement -0.15 -0.19 0.15 Weight 0.20 0.20 0.15

~'P < 0.05.

1.28 and 1.48; P > 0.05). There was, however, significant selection pressure acting directly on mean movement per night at high density (t = 2.0, P < 0.05). Variation explained (R 2) in relative mating success by mean calling duration per night, mean displacement per night, and individual weight was not significant at the low or high density (ANOVAs, F = 2.01, df = 3,7 and F = 2.28, df = 3,22; P > 0.05). For G. pennsylvanicus, intensities of selection acting directly and indirectly on mean movement per night and individual weight were not significant at low or high densities or on mean calling duration per night at the high density (r = 0.66, -0.18, -0.15, and 0.21 ; P > 0.05). Mean calling duration per night at the high density was also not significant (r = 0.07, P > 0.05), but there was significant direct and indirect selection pressure on mean calling duration per night at the low density (r = 0.81, P < 0.05). Directional selection gradients on mean movement per night and individual weight were not significant at the tow density and high density, and mean calling duration per night at the high density was not significantly different (t = 0.96, -1.88, -1.30, 1.37, and 0.50; P > 0.05). There was, however, significant selection pressure acting directly on mean calling duration per night at the low density (t = 2.71, P < 0.05). Variation in relative mating success explained by mean calling duration per night, mean displacement per night, and individual weight was not signifi- 116 French and Cade

cant at the low density or high density (ANOVAs, F = 6.27, df = 3,3 and F = 1.24, df = 3,22; P > 0.05).

DISCUSSION Cricket calling songs attract females and function in territorial spacing (Cade, 1981a). As the local population density increases, the costs of territorial defense should increase, the proportion of matings resulting from male search- ing should increase, and males might be expected to reduce the amount of time spent calling and increase the searching time (Alexander, 1975). There were no differences in mean calling duration or mean movement per night, however, between high and low densities for G. veletis or G. pennsylvanicus males. In contrast, reduced calling with increased density was observed in the field crick- ets G. bimaculatus in the laboratory (Simmons, 1986b) and G. integer in the field (Cade and Wyatt, 1984) and in field populations of the grasshopper, Lig- urotettix coquilletti (Greenfield and Shelly, 1985). G. bimaculatus and G. inte- ger are cricket species with a high frequency of long-winged, flying individuals (Alexander, 1968; Cade, 1979b). Local density fluctuates greatly in G. integer and similar macropterous species as crickets migrate through an area (Cade, 1979a, 1981a). Selection may have favored the ability in male G. bimaculatus and G. integer to change calling behavior in response to fluctuations in density within male lifetimes. Under conditions where population density varies and is unpredictable, individuals may change their behavior in response to the present conditions. Long-winged crickets may also migrate to other populations. Long- winged individuals occur very infrequently in G. veletis and G. pennsylvanicus (Alexander, 1968), and population density may not fluctuate as greatly as in the long-winged species. These species may not therefore have had a history of selection for male ability to change calling duration in response to fluctuations in density. Relatively stable levels of poPulation density for G. veletis and G. pennsylvanicus may also account for failure of males to vary amount of move- ment with density, but comparable data from other cricket species are not avail- able. It is also possible that the densities used here were not high or low enough to obtain the predicted result. In L. coquilletti, Otte and Joern (1975) showed that males increase the amount of movement with increasing population dens- ities. The frequency distributions of mean calling durations can be used to infer directional or stabilizing selection against calling. For example, calling duration in laboratory and field samples of G. integer is highly biased toward noncalling and may result from selection against calling by Euphasiopteryx ochracea (Dip- tera; Tachinidae), an acoustically orienting parasitoid (Cade, 1975). Calling durations were normally distributed without significant skewness in laboratory studies on G. veletis and G. pennsylvanicus, and these species are not parasi- Sexual Selection in Male Field Crickets 117 tized by acoustically orienting in Ontario (Cade and Wyatt, 1984). The shape of the distributions were normal and without significant skewness in low- and high-density samples of G. veletis and at the low density in G. pennsylvan- icus. These results agree with frequency distributions from laboratory studies of G. veletis and G. pennsylvanicus and suggest that stabilizing selection is operating on calling duration in these cricket species. Significant skewness char- acterized calling duration at the high density in G. pennsylvanicus, but the shift toward less calling was not as great as that observed in G. integer. The differ- ence between the frequency distribution of high-density and that of low-density G. pennsylvanicus may reflect the tendency of crickets to reduce calling with increased density, although there was no mean reduction in calling in the high- density G. pennsylvanicus males. Mean mating frequency per night for individual males did not differ between densities in G. veletis or G. pennsylvanicus, and there was no difference between densities in G. veletis mating frequency variance. Low-density G. pennsylvan- icus had a significantly greater variance than the high density of this species. Mating success variance is a general measure of the opportunity of sexual selec- tion in males (Arnold and Wade, 1984; Trail, 1985; Wade, 1979; Wade and Arnold, 1980). Our data on variance suggest that sexual selection is more intense at low rather than high densities in G. pennsylvanicus. Mating frequency and calling duration were significantly and highly cor- related in the low but not the high density of G. pennsylvanicus. There were no correlations between mating success and weight or movement at either den- sity of G. pennsylvanicus. Higher variance and thus sexual selection intensity in the low-density sample of G. pennsylvanicus may result partly from selection on nightly calling duration. Data are rare from invertebrates on the relationship between male traits and mating success in field populations. One exception is Greenfield and Shelly's (1985) demonstration that increased calling correlated with increased mating frequency in the grasshopper, Lo coquilletti. Standardized selection intensities and directional selection gradients on mean calling duration, weight, and movement allow a more precise analysis of factors contributing to variance in male mating success. A significant positive selection intensity and standardized selection gradient were found for nightly calling duration at the low density in G. pennsylvanicus. Selection intensity was high and is a measure of direct selection on calling duration, as well as indirect selection on calling through selection on correlated traits. Direct selection on calling duration as measured by the directional selection gradient was also high, however, and very close to the standardized selection intensity in magnitude. Similarity in values for standardized selection intensity and directional selection gradient indicates that most of the selection on calling is direct and not corre- lated with other male traits. A high multiple correlation coefficient for G. penn- sylvanicus at the low density suggests that much of the variation in male mating 118 French and Cade success is due to calling duration, but this correlation coefficient was only close to significance. Male-male aggression and other factors not studied here may be responsible for some variation in male mating success. Selection intensities were nonsignificant for weight, movement, and calling in G. veletis at both densities. The directional selection gradient was significant and positive only for movement in the high-density sample. The estimate for direct selection on movement was low, however, and apparently insufficient to lead to any differences in the variance of male mating success between high- and low-density samples Nonsignificant selection intensities and significant directional selection gradients suggest that positive selection for movement is countered by selection on some correlated trait Weight and movement showed a strong inverse correlation in G. veletis at the high density (French, 1986), and our measurement of the directional selection gradient for weight in this popu- lation was very close to significance. Large males have increased mating suc- cess in laboratory populations of G. bimaculatus (Simmons, 1986b) and selection for weight may have nullified selection for male movement in the G. veletis high-density sample. The predicted increased proportion of matings resulting from searching rather than calling with increasing density (Alexander, 1975) is partly supported by our study. There was direct selection for male movement at the high density but not at the low density in G. veletis, and direct selection for calling was present only at the low density in G. pennsylvanicus. Our data do not distin- guish between male competition and female choice, and many other factors such as age, level of aggression, sperm competition, and random variation may influence sexual selection on males (Backus and Cade, 1986; Dixon and Cade, 1986; Endler, 1986; Koenig and Albano, 1986; Manly, 1985; Sakaluk, 1986; Sutherland, 1985b; Zuk, 1986) The importance of selection as an evolutionary force has received much attention (for recent reviews see Endler, 1986; Grant, 1985; Manly, 1985). The relative importance of selection should depend on/the/ type of selection operating and our data were intended to measure the intensity of sexual selection on male traits. If sexual selection is a very strong and constant force, it should lead to the rapid deterioration of additive genetic variation underlying traits important to male fitness (for examples see Taylor and Williams, 1982; Williams, 1975). Laboratory studies, however, often show significant amounts of additive genetic variation underlying male traits (reviewed by Cade, 1984). In G. integer, for example, the amount of male calling and male body size have significant amounts of additive genetic variation (Cade, 1981b; McGowan, 1986) Male body size also shows heritable variation in G. bimaculatus (Simmons, 1987). Density-dependent and other fluctuations in selection may be responsible for the maintenance of much of the genetic variation underlying traits important in fitness (Clarke, 1979; Kojima, 1971). The data presented here show that sexual Sexual Selection in Male Field Crickets 119

selection on male G. veletis and G. pennsylvanicus was often relaxed and varied in intensity between populations and species. In contrast, data from vertebrate populations suggest that sexual selection is often an intense force (for examples see Howard, 1983; Price, 1984; and Searcy, 1979). Fewer data are available to evaluate the intensity of sexual selection in field populations of , but some data show intense levels of sexual selection on males and females (Thorn- hill, 1976; Gwynne, 1984; see also references given by Thornhill and Alcock, 1983). More information is needed from a variety of species, however, to eval- uate the importance of selection in natural populations (Endler, 1986) and to understand more clearly the effects of selection on aditive genetic variation.

ACKNOWLEDGMENTS

This research was completed in partial fulfillment of the requirements for the M.Sc. degree in biological sciences (B.W.F.) and was funded by Natural Sciences and Engineering Research Council of Canada Grant A6174 (to W.H.C.). We thank R. D. Alexander, T. Burk, R. W. Knapton, R. D. Morris, S. Sakaluk, J. J. Flint, and M. Zuk for comments on an early version of the manuscript. J. W. Chardine and Y. Vrbik helped with statistical analyses.

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