Behavioral Ecology
doi:10.1093/beheco/arm017
Advance Access publication 19 March 2007

Sexual selection in a moth: effect of symmetry
on male mating success in the wild
Chiharu Koshio,a Makoto Muraji,a Haruki Tatsuta,b and Shin-ichi Kudoa
Department of Biology, Naruto University of Education, Takashima, Naruto 772-8502, Japan,b
Laboratory of Ecological Risk Assessment, National Institute for Environmental Studies, 16-2,
Onogawa, Tsukuba, Ibaraki 305-8506, Japana

Sexual selection is generally caused by female choice and male–male competition. In female choice process, female preference isfavored indirectly and/or directly by sexual selection. In indirect selection, females expressing the preference might gain indirect
genetic benefits. In direct selection, females expressing the preference might gain direct benefits or avoid male-imposed costs.
The white-tailed zygaenid moth Elcysma westwoodii is monandrous, and males often gather around a female to mate with her,suggesting a high opportunity for sexual selection on male traits. We quantified phenotypic selection on male morphology in thisspecies in the field. The morphological characters analyzed included body weight, antenna length, forewing length, hind winglength, hind wing tail length, genital clasper length, and the fluctuating asymmetry (FA) of these bilateral traits. In E. westwoodii,
selection favored males with more symmetric genital claspers, as well as longer and more symmetrical hind wings and antennae.
Negative correlations between FA and size were also detected in the clasper and the antenna. Our results suggest that FAs of maletraits, in particular the genital clasper, may have indirect and direct influences on mating success. During a copulatory attempt,
an E. westwoodii male will try to grasp the female’s abdominal tip with his claspers but often fail to do so because of the female’s
reluctance to mate. The female abdominal tips are smooth and strongly sclerotized and could thus be difficult for males to grasp.
We hypothesize that more symmetrical male claspers are more efficient in overcoming female reluctance. Key words: fluctuating
asymmetry, genitalia, Lepidoptera, phenotypic selection, sexual conflict, sexual selection. [Behav Ecol 18:571–578 (2007)]

exual selection is generally caused by 2 processes: competition between members of one sex for members of the
opposite sex (usually male–male competition) and preferenceby members of one sex for certain members of the opposite
sex (usually female choice). These 2 processes often act together (Andersson 1994). Any trait that enhances mating andfertilization opportunities in either of these 2 processes will befavored by sexual selection. In female choice process, female
preference is favored indirectly and/or directly by sexual selection. In indirect selection, females expressing the preference might gain indirect genetic benefits through Fisher’s
runaway process or ‘‘good gene’’ process. In direct selection,
females expressing the preference might gain resources orother direct benefits or avoid male-imposed costs such as reduced longevity. Such female avoidance or minimization of
direct cost imposed by males can evolve under sexual conflictor sexually antagonistic coevolution (Holland and Rice 1998;
Chapman et al. 2003; Arnqvist and Rowe 2005). Sexual conflict occurs when the genetic interests of males and females
differ with each other. Females often suffer significant direct
costs of matings (Arnqvist and Rowe 2005). Therefore, female
traits that facilitate resistance to unwanted matings evolve,
and as a result, exaggerated male traits also evolve to overcome
resistant females (i.e., chase-away sexual selection; Holland
and Rice 1998).
Female resistance is also interpreted from the perspectiveof indirect selection instead of sexual conflict; it evolved andis maintained as a result of selection to screen among malesof different genetic quality (Eberhard 1996; Cordero andEberhard 2003), although such indirect genetic benefits are

S

Address correspondence to C. Koshio. E-mail: koshio@naruto-u.
ac.jp.
Received 15 July 2006; revised 31 January 2007; accepted 2 February
2007.
 The Author 2007. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: journals.permissions@oxfordjournals.org

expected to be a weak force because of the costs of resistance(Chapman et al. 2003). In the elephant seal Mirounga angustirostris, for example, females increase the probability of mating with a high-ranking male by simply rejecting all copulatory
attempts during early estrus and thus inciting male competition (Cox and Le Boeuf 1977). Such female incitation of maleaggression has also been reported in several insect species(Thornhill and Alcock 1983).
The white-tailed zygaenid moth Elcysma westwoodii has amonandrous mating system. The male has a pair of extended
forceps-like claspers of the genitalia (see Figure 1c). Forcepslike claspers are also known in the closely related species,
Elcysma dohertyi (Elwes 1890; Horie K, personal communications) as well as in several species of the related genera,
Achelura and Agalope (Owada 1992; Owada et al. 1999). Such
grasping devices are common among insects and otheranimals such as amphibians, and sexual conflict may haveplayed an important role in the evolution of these devices(see Arnqvist and Rowe 2005). In E. westwoodii, courting males
try to grasp the female abdominal tip with their claspers while
fluttering their wings, but females are often reluctant to mate
and refuse male attempts by bending their abdomens andwalking away (Koshio and Hidaka 1995). Many males sometimes gather around a single female to copulate, suggestinga strong opportunity for sexual selection. It is likely that thefemale’s resistance behavior is relevant to sexual selection favoring males with the ability to overcome reluctant females(Koshio and Hidaka 1995). Here we try to understand sexual
selection on male morphological traits including fluctuating
asymmetry (FA).
FA is defined as small, random deviations from perfectsymmetry of bilateral traits (Van Valen 1962; Markow 1995). FA
is thought to indicate an individual’s ability to buffer against
genetic or environmental stress during development andthus to be an index of the individual’s developmental stability(Palmer and Strobeck 1986, 1992; Palmer 1994). Several

Behavioral Ecology

572

Figure 1
Measurements made for thesexual selection analysis (a):forewing (F), hind wing (H),hind wing tail (T), antennae(A), and clasper length (C).
Ventral view of a copulating pair(b): a male grasps the female
abdomen with his claspers.

reviews refer to the relationship between FA and fitness indicators especially in the context of sexual selection (see Swaddle
2003). The role of FA in the sexual selection process, however,is a controversial topic and the subject of considerable debate(Møller and Swaddle 1997; Palmer 1999; Swaddle 2003). In
many cases, relationships between FA and fitness are argued
on the basis of the association between FA and developmental
instability related to genetic quality. By contrast, some of the
known relationships between fitness and FA arise from the direct influences of asymmetry (see Swaddle 2003). In maledung flies, for example, symmetrical forelegs enhance mating
success possibly due to mechanically constraining their ability
to grip resistant females (Allen and Simmons 1996). In birds,
asymmetry in wings and tails has aerodynamic costs (Balmford
et al. 1993; Thomas 1993; Evans et al. 1994).
In the present study, we quantify phenotypic selection on
males of the white-tailed zygaenid moth E. westwoodii by targeting morphological traits, including FA, during mating
episodes using selection gradient analysis (Lande and Arnold
1983; Brodie et al. 1995). In examining phenotypic selection
in natural populations, this method is particularly effective for
identifying target characters of selection and quantifying themode and degree of selection when selection acts simultaneously on a set of characters.
MATERIALS AND METHODS
Insects
Elcysma westwoodii is usually monandrous (Koshio and Hidaka
1995) and protandrous (Koshio C, Muraji M, Kudo S, unpublished data). It is univoltine, and larvae depend on plants
belonging to the family Rosaceae, for example, cherries,
plums, and peaches. Adult emergence and breeding occur
from late September to early October. Adults are diurnal;
pheromone release by females settled on leaves, male searching flights, and mating all occur in the early morning (Koshio
and Hidaka 1995). Copulation usually lasts for several hours
after which the females takeoff to search for oviposition sites.
Oviposition occurs in the afternoon (Koshio 1996).
Mating experiments
Our field experiments were carried out in Tokushima city
(344#N, 13434#E), Tokushima Prefecture, Shikoku, Japan.
Elcysma westwoodii breeds every year in this area, which is host
to many cherry trees.

Despite the high density of moths, finding naturally
emerged virgin females calling males on cherry trees is very
difficult. Thus, we conducted female-setting experiments inthe field to observe sufficient mating events for analysis.
We collected pupae in September 1997 in and around thestudy area and kept them in plastic cages under natural conditions until emergence. After emergence, we kept males andfemales separately. Males were released into the field the following morning, whereas females were reared on water until
the mating experiments were started.
The mating experiments were carried out between 0530
and 0900 h from 21 September until 8 October 1997; thiscovered almost the entire mating period. Our experiments
were designed to analyze sexual selection only after males
had landed on leaves with females and started a copulatory
attempt. Thus, selection during the male searching flight andorientation to females were not the subjects of this study.
We placed virgin females (21.4 6 11.7 [standard deviation]
females per day, n ¼ 364 in total) on the underside of hostleaves in the observation area. These females always stayed on
the leaves, probably calling males. No male came to approachto 171 out of 364 females. The remaining 193 females wereapproached by, on average, 1.61 6 1.22 males. First arriving
males did not always mate females. Females approached bymales were collected irrespective of their mating status at
the end of mating period in that morning. We collected all
males that approached and attempted to copulate with thefemales (for mating behavior, see Koshio and Hidaka 1995).
The males were divided into 2 classes: males that achieved
copulation and males that failed to copulate. Nonmating males
were caught by net as they flew away from females. Once
copulation was established, we kept the pair in a plastic cage
until the completion of copulation. Both mating and nonmating males were killed by freezing and kept for trait measurements at 20 C.
Measurement of traits
For the sexual selection analysis, we measured 6 traits on
mating and nonmating males: body weight and the lengthsof antennae, of forewings and hind wings, of hind wing tails,
and of claspers, respectively (Figure 1). Males were weighedjust after collection using an electric balance with an accuracyof 0.01 mg (Sartorius AG, Goettingen, Germany). Wings,
antennae, and claspers were detached from the body before
measurement. Wings and antennae were carefully extended

Koshio et al. • Sexual selection in a mothand held between 2 transparent sheets to be unfolded andmeasured using calipers with an accuracy of 0.01 mm. Forewing length was measured from the small wart on the base to
the distalmost end of the wing (F: Figure 1a). Hind winglength was also measured from the base of the frenulum, aprojection that couples with the forewing, to the distal end of
the tail (H: Figure 1a). Tail length was measured from the junction of the M2 and M3 veins to the distal end of the tail (T: Figure1a). Clasper lengths were measured from the base of inner side
to the tip (C: Figure 1a) using a stereoscopic microscope (103)with micrometer.
We also investigated the degree of FA on 5 bilateral traits:antennae, forewings and hind wings, tail of hind wings, andclaspers. To quantify measurement errors (see below), all individuals were measured 3 times. We did not reposition thembetween repeated measures to avoid damaging them. For eachindividual paired trait, we calculated signed asymmetry as themean value of 3 measures of left minus right value (L  R).
FA analysis
FA is, by definition, small, and it can therefore have a magnitude similar to that of measurement errors. Thus, we evaluated measurement errors according to Palmer and Strobeck(1986) and Palmer (1994). A mixed-model 2-way analysis ofvariance (ANOVA) was performed for all individuals thatwere measured 3 times, with ‘‘side’’ as a fixed factor and‘‘individuals’’ as a random factor. The side mean square wasdivided by the interaction mean square to obtain an F value
indicating whether significant directional asymmetry was present. The interaction mean square divided by the error meansquare yielded F and P values, indicating whether the asymmetry was significantly larger than the measurement error.
Kurtosis was calculated to test for the presence of platykurtosis, an indication of antisymmetry.
The results of the ANOVA indicated that each asymmetry
was measured with sufficient precision for the subsequent
analysis (antenna FA: F297,1192 ¼ 3.87; forewing FA: F307,1232 ¼
4.48; hind wing FA: F286,1148 ¼ 7.15; tail FA: F284,1140 ¼ 373.74;
clasper FA: F308,1236 ¼ 40.02; P , 0.001 for all traits). Directional asymmetry, the side effect in the ANOVA, was not
significant for any trait. Kurtosis was positive for all traits, indicating no evidence of antisymmetry. Therefore, we used theasymmetry (a mean value of the 3 R  L measures) of these 5bilateral traits for subsequent selection analysis as true FA.
The Spearman’s rank correlation coefficients among 11
traits, including 6 trait sizes and 5 unsigned FAs, were calculated for the entire male sample. The significance levels of all
coefficients were adjusted using the sequential Bonferroni
method (Rice 1989).
Trait allometry
The allometric slopes were estimated by the ordinary leastsquare (OLS) regression and the major axis (MA) regression
methods (Sokal and Rohlf 1995), regressing the log10-transformed value of the traits including forewing, hind wing, antenna, clasper, and hind wing tail on the log10-transformed
value of the cube root of body weight as an indicator of bodysize. Those 99% confidential intervals were estimated by bootstrap resampling procedure (10 000 random resamplings withreplacement).
Measurement of sexual selection
Sexual selection was estimated using the fitness-regression
approach developed by Lande and Arnold (1983) (see also573

Mitchell-Olds and Shaw 1987; Brodie et al. 1995). After maleswith damaged traits were excluded, 274 males out of 309 wereused in the following analysis. Male mating success was used
as a fitness component in the selection analysis. We assigned
absolute fitness values of 1 to mating males and 0 to nonmating males. Individual measures of the fitness component weretransformed to relative values scaled to mean 1, dividing eachindividual’s value by the overall mean.
The values of all traits were log transformed and standardized to a mean 0, variance 1, and thus the selection differentials and gradients described below represent standardized
values. Each unsigned FA, that is, an absolute value of R  L(|FA|), was transformed using the Box–Cox method before
standardization (Swaddle et al. 1994).
Linear directional selection differentials (s#), describing
changes in trait means, and nonlinear selection differentials(c#), describing changes in trait variance, were calculated as
the covariance of trait values with relative fitness components.
Significance testing of selection differentials was made with
Spearman’s rank correlation tests.
Selection gradients estimate the direct effects of a particular
trait on fitness by controlling for other traits. We estimatedlinear selection gradients (b#) for all 11 traits. We also estimated quadratic selection gradients (c#) for all traits except
their FAs because it is difficult to interpret the nonlinear effects of unsigned FA on fitness components. The dependent
variable (i.e., fitness component) is binary, and thus, we usedlogistic regression analysis for significance tests for single
partial regression coefficients (Mitchell-Olds and Shaw 1987;
Janzen and Stern 1998).
To test the temporal change in selection regimes throughout the mating period, we included the interaction term ‘‘traitby date’’ in the logistic regression models. No significant interactions were detected for any trait in these tests (P . 0.5 inboth linear and quadratic models, after sequential Bonferroni
correction), implying no significant temporal effects on sexual selection. All data from the entire mating period were,
therefore, pooled for the sexual selection analysis.
All statistical analyses were performed using SPSS 11.0 (SPSS
Inc. 2001), statistical program R 2.2.1 (R Development CoreTeam 2005), and JMP 5.1 (SAS Institute 2004).
RESULTS
Phenotypic correlations and allometry
The mean values of the 6 trait sizes (body weight and antenna,
forewing and hind wing, hind wing tail, and clasper lengths)
as well as 5 unsigned FAs and 5 relative unsigned FAs of males
approaching to females are summarized in Table 1. Varianceof relative signed FAs among traits was significantly different(Bartlett’s test: P , 0.0001).
The allometric slopes of forewing and clasper length on
the cube root of body weight were lower than 1.0 and theirconfidence intervals did not include 1.0 in either regression
method (coefficient and 99% confidence interval of forewing—OLS: 0.71, 0.65–0.76, MA: 0.78, 0.72–0.85; clasper—OLS: 0.41, 0.34–0.47, MA: 0.49, 0.42–0.57), although thoseof other traits were inconsistent (antenna—OLS: 0.64, 0.55–
0.74, MA: 0.86, 0.73–1.02; hind wing—OLS: 0.90, 0.83–0.96,
MA: 1.02, 0.94–1.11; hind wing tail—OLS: 1.10, 1.00–1.20,
MA: 1.41, 1.26–1.57).
The 6 trait sizes were highly positively correlated to eachother (Table 2). Positive correlations were also found between
hind wing |FA| and antenna |FA|, antenna |FA| and clasper|FA|, and forewing |FA| and clasper |FA|. Negative correlations
between a trait size and its |FA| were found in the antenna andthe clasper.

Behavioral Ecology

574

Table 1
Summary of morphological traits of all males coming to females
TraitsN

Mean 6 standard
deviation

Size
Body weight (mg)Antenna (mm)Forewing (mm)Hind wing (mm)Tail (mm)Clasper (mm)

309
298
309
287
285
309

86.92 6 22.02
13.52 6 1.04
28.43 6 1.98
31.59 6 2.82
13.65 6 1.57
4.32 6 0.23

|FA| (mm)Antenna
Forewing
Hind wing
Tail
Clasper

298
309
283
285
309

0.22 6 0.31
0.23 6 0.36
0.36 6 0.42
0.34 6 0.29
0.02 6 0.05

Relative |FA| (%)
Antenna
Forewing
Hind wing
Tail
Clasper

298
309
283
285
309

1.68 6 2.67
0.83 6 1.34
1.17 6 1.50
2.58 6 2.29
0.55 6 1.19

For detailed explanations of traits, see Figure 1. |FA|: the unsigned
mean of 3 measures of the left minus the right value of each bilateral
trait. Relative |FA|: the unsigned FA divided by size of each trait.

Sexual selection regimes
Linear selection estimates, differentials and gradients, areshown in Table 3. The numbers of mating and nonmating
males were 159 and 115, respectively, after males with damaged traits were excluded. The directional selection differential was positive for all trait sizes, whereas that for |FA|, except
tail |FA|, was always negative.
The coefficient of determination of the multivariate linear
model was highly significant (Table 3). Negative selection gradients were detected for 3 FAs: antenna |FA|, hind wing |FA|,and clasper |FA|. However, no significant directional selection
gradients were found for any of the 6 trait sizes, despite significant estimates for these selection differentials. This lackof effect of size may be due to problems with multicollinearity,especially because phenotypic correlations across traits werehigh (Table 2). We thus adopted a stepwise regression approachwith backward elimination using likelihood ratio in order toreduce the influence of the multicollinearity (Mitchell-Olds
and Shaw 1987). After choosing variables by the stepwise
method, significant directional selection gradients weredetected in 4 traits: positive selection on hind wing size
and negative selection on antenna |FA|, hind wing |FA|, andclasper |FA|.
The nonlinear selection differential was negative but notsignificant for any trait size (c# ranged from 0.087 to
0.177, 0.160 , P , 0.786). The coefficient of determinationof the multivariate model including quadratic and correlational terms was also not significant (linear regression model:
R2 ¼ 0.106, F27,246 ¼ 1.085, P ¼ 0.36; logistic regression model:v2 ¼ 31.23, LR ¼ 31.22, P ¼ 0.26).

DISCUSSION
The selection gradient analysis revealed that selection for successful mating favored E. westwoodii males with more symmetrical claspers and more symmetrical antennae (Table 3). The
magnitude of FA in male traits has often been shown to beimportant in precopulatory sexual selection in some insects(e.g., Thornhill 1992a, 1992b; McLachlan and Cant 1995;
Møller and Zamora-Muñoz 1997), although the relationshipbetween selection and morphological asymmetry is often controversial in animal taxa (e.g., Markow 1995; Clarke 1997;
Palmer 1999; Tomkins and Simmons 2003). As well as in E.
westwoodii, males of a cerambycid beetle with symmetric antennae enjoy a mating advantage (Møller and Zamora-Muñoz1997). However, the causal relationship between antennal
symmetry and sexual selection remains unknown in the both
species.
There have been many studies on the relationship between
indirect benefits and FA (Swaddle 2003). Such indirect mechanism could operate in mate choice or intrasexual competition if the asymmetry is related to some unidentified propertyof an individual. In these cases, FA in secondary sexual traits
should reliably signal male phenotypic condition (Møller 1990;
Møller and Pomiankowski 1993; Møller 1997). For example,
FAs are negatively correlated with male longevity in ladybird
beetles (Ueno 1994), forest tent caterpillar moths (Naugler
and Leech 1994), sphragis-bearing butterflies (Tsubaki andMatsumoto 1998), and water boatmen (Nosil and Reimchen2001), whereas no such relationships have been detected inother species (Hunt and Simmons 1997; Leung and Forbes
1997; Windig 1998; Woods et al. 2002).
If males with more elaborate secondary sexual traits are ofbetter phenotypic condition, and the degree of FA signals thisphenotypic condition, then FA should be negatively related toTable 2
Phenotypic correlations between traits (Spearman’s rank correlation coefficient: rs)
Forewing Hind wing Antenna Clasper
Body Weight
0.862**
Forewing
Hind wing
Antenna
Clasper
Tail
Forewing |FA|
Hind wing |FA|
Antenna |FA|
Clasper |FA|0.872**
0.963**

0.721**
0.793**
0.810**For an explanation of |FA|, see Table 1.
P , 0.05.
**
P , 0.05 after the sequential Bonferroni correction.
*

0.653**
0.742**
0.745**
0.653**

Tail

Forewing |FA| Hind wing |FA| Antenna |FA| Clasper |FA| Tail |FA|

0.817**
0.887**
0.938**
0.776**
0.674**

0.025
0.089
0.082
0.018
0.0840.104

0.0900.160*0.171*
0.185*
0.101
0.1540.067

0.116*
0.128*
0.136*
0.186**
0.077
0.0940.121*
0.211**

0.143
0.135
0.118
0.168*
0.190**0.1210.219**
0.175*
0.353**

0.042
0.071
0.0900.011
0.073
0.0970.070
0.113
0.0750.075

Koshio et al. • Sexual selection in a moth

575

Table 3Standardized directional selection differentials (s#) and gradients (b#) using male mating success as a fitness component

Body weight
Antenna
Forewing
Hind wing
Tail
Clasper
Antenna |FA|
Forewing |FA|
Hind wing |FA|
Tail |FA|
Clasper |FA|
Linear regression model
Logistic regression modelbs#Pb# 6 SE

0.151
0.143
0.163
0.177
0.170
0.160
0.294
0.159
0.212
0.0700.394

0.004
0.006
0.002
0.001
0.003
0.004
,0.001
0.002,0.001
0.242,0.0010.047 6 0.097
0.694
0.118 6 0.078
0.121
0.034 6 0.190
0.780
0.137 6 0.243
0.525
0.042 6 0.130
0.717
0.018 6 0.070
0.704
0.171 6 0.048
0.002
0.077 6 0.045
0.135
0.116 6 0.047
0.029
0.028 6 0.045
0.832
0.319 6 0.047,0.001
2
R ¼ 0.322, F11,262 ¼ 11.318, P , 0.001
v2 ¼ 103.37, LR ¼ 269.38, P , 0.001

Pb# 6 SEaPa

0.127 6 0.074

0.089

0.210 6 0.073

0.008

0.171 6 0.046

0.002

0.117 6 0.045

0.027

0.335 6 0.045,0.001R 2 ¼ 0.311, F5,268 ¼ 24.25, P , 0.001av2 ¼ 100.39, LR ¼ 272.35, P , 0.001a

Significance tests of single partial regression coefficients are made using logistic regression analysis. For an explanation of |FA|, see Table 1.
SE, standard error.
aAfter stepwise regression with backward elimination using likelihood ratio.
b
LR represents 2 log likelihood ratio.

trait size in secondary sexual traits, whereas it should be U
shaped or flat in other traits (Møller and Pomiankowski 1993;
Blanckenhorn et al. 1998; Møller and Cuervo 2003). In
E. westwoodii, correlations between each trait size and themagnitude of its FA were always negative but were statisticallysignificant only in secondary sexual traits, the antenna and theclasper (Table 2), the FAs of which were under directional
selection (Table 3).
If the FA reflects developmental instability and thus individual phenotypic condition, an organism-wide indication of FAs
and among-trait correlations in FAs within the same individual
may be expected (Polak et al. 2003). Nevertheless, there islittle evidence to support this prediction, and recently several
explanations for the lack of among-trait correlations havebeen developed (Swaddle 2003). In E. westwoodii, correlations
among FAs were always positive, except for that between tailand antenna, and some correlations were significant even after the sequential Bonferroni correction (Table 2).
Concerning the role of FA as an indicator in sexual selection, Blanckenhorn et al. (1998) addressed other assumptions
and predictions; FA should be heritable; FA of male secondary
sexual traits is greater than that of homologous female traits;
FA of sexually selected traits is greater than that of less costly
nonsexual traits. We have no available data on the former two
in E. westwoodii. For the last issue, we could not detect any such
trends in E. westwoodii (Table 1); although the relative FAs
among male traits were significantly different, those of malesecondary sexual traits, antennae and claspers, were not consistently greater than those of the forewings and hind wings.
Blanckenhorn et al. (1998) also predicted that FA as an indicator of male genetic quality should be related to fitness
components other than mating success, such as mate fecundity or offspring survivorship. In E. westwoodii, however, FA ofmale morphological traits did not affect either female fecundity or the hatching rates of their eggs (Koshio C, Muraji
M, Kudo S, unpublished data).
In summary, the negative correlations between size andFA and the positive correlations between FAs suggest that thedegrees of both genital clasper and antenna FA may be indicators of male phenotypic condition in E. westwoodii. However,some other hypotheses have been proposed to interpret thenegative correlations between size and FA. One interpretationis that individuals developing more exaggerated traits areconstrained to more symmetrical development (Evans 1993;

Evans and Hatchwell 1993). This hypothesis predicts thatthe cost of a given asymmetry will increase with increasing
ornament size and that the rate of increase in cost as ornament size increases should be greater when asymmetrical thanwhen symmetrical (Evans 1993). There are no available dataon these points in E. westwoodii. Polak and Starmer (2005)
proposed an alternative hypothesis, the environmental heterogeneity hypothesis, to explain the negative correlations
between size and FA in natural populations. This hypothesis
states that negative size–FA correlations arise because of environmental heterogeneity instead of heterogeneity of male
genetic quality. Further evidence, therefore, is needed to
explicitly test the indicator hypothesis.
Recent studies have shown that asymmetry can directly influence mating success (see Swaddle 2003). In the shore crab
Carcinus meanas, males with more symmetrical fifth pereiopods are advantaged in male–male antagonistic interactions,probably due to their greater physical stability during pushingcontests (Sneddon and Swaddle 1999). The mating advantageof periodical cicada males with symmetrical forelegs is also
explained by the functional hypothesis; more symmetricalforelegs might advance mobility or maneuverability during
male–male competition (Cooley 2004). In the yellow dung fly,
Scatophaga stercoraria, males with more symmetrical wings
and hind tibia enjoy greater mating success, probably due tothe greater competitive ability of those males in the male–
male competition (Liggett et al. 1993). In another dung fly
Sepsis cynipsea, it has been suggested that males with symmetrical fore tibia have a functional advantage in coping withprecopulatory struggling by females (Allen and Simmons
1996; Blanckenhorn et al. 1998). The effect of asymmetry on
mating success, however, varies among populations in dungflies (Blanckenhorn et al. 2003, 2004).
In E. westwoodii, the male clasper has an essential function
during mating behavior: grasping the female abdominal tip(Koshio and Hidaka 1995). It is, therefore, more likely thatsymmetrical claspers have a functional advantage. Duringa copulatory attempt, males try to grasp a female’s abdominal
tip with their claspers but often fail to do so because of female
reluctance to mate (Koshio and Hidaka 1995). Furthermore,
female morphology may be associated with a male’s failure; thesurfaces of the seventh and eighth segments of the female
abdomen are smooth and strongly sclerotized, and thus, it
seems to be difficult for males to grasp them during copulatory

Behavioral Ecology

576

attempts. A male’s ability to grasp a female’s abdomen with his
claspers is essential for successful mating, and it is most likelythat males with more symmetrical claspers are better able tocontrol females. Smooth and sclerotized female abdominal tips
are also observed in the related species with forceps-like male
genital claspers (Elwes 1890; Owada 1992; Owada et al. 1999).
In a few animal taxa, directional asymmetry is also detected
in grasping apparatus, which are usually used to manipulate
asymmetrical objects (e.g., Smith and Palmer 1994). In waterscavenger Hydrophilus acuminatus, for example, larvae haveasymmetric mandibles, which are effective in holding and manipulating right-handed snails (Inoda et al. 2003). However,this is not the case in genital claspers of E. westwoodii; female abdominal tips are not only smooth and sclerotized but
also symmetrical (Koshio C, Muraji M, Kudo S, unpublisheddata).
Female reluctance and their smooth and sclerotized abdomens are likely to have evolved as a response to male copulatory attempts using their forceps-like claspers. For such female
resistance, 2 nonmutually exclusive explanations have been
proposed, a mating-rate reduction process and a male screening process (Chapman et al. 2003; Cordero and Eberhard
2003; Arnqvist and Rowe 2005), both of which may work simultaneously or sequentially (Cordero and Eberhard 2005).
The former especially is widely regarded as sexual conflict,broadly defined as differences in the evolutionary interests
between males and females (Parker 1979; Chapman et al.
2003), or the chase-away process (Holland and Rice 1998).
In water striders, in which multiple matings occur but are
actually costly for females, males have a grasping apparatus
to control reluctant females, and the length of this apparatus
is important for their mating success (Arnqvist 1989). Females
show morphological counteradaptations to these male devices(Arnqvist 1997; Arnqvist and Rowe 2002). Many theoreticalmodels, however, have been developed to understand theevolution of sexual conflict over mating rate, assuming potentially polyandrous females (Gavrilets et al. 2001; Gavrilets andHayashi 2006; Lessells 2005; Rowe et al. 2005). The mating
system of E. westwoodii is monogamous (Koshio and Hidaka
1995), and thus, it is unlikely that sexual conflict over mating
rate is the driving force to evolve male genital claspers andfemale sclerotized abdomens.
On the other hand, under such a strict monandrous system,
females should choose higher quality males. Therefore, female reluctance and smooth and sclerotized abdominal tips
in E. westwoodii may have been under indirect selection forgenetic benefits by screening males of different qualities(Eberhard 2002). If FAs of male traits are reliable indicatorsof genetic quality in this species, reluctant females could obtain dual indirect benefits from sons with not only good ability
to control females but also good genetic qualities.
Different modes of selection have implications for trait allometry, particularly when dealing with sexually selected traits(see Hosken and Stockley 2004). Positive allometry, the tendency for large individuals to have relatively larger morphological traits, is observed more frequently in secondary sexual
traits than in other traits because of directional sexual selection (Petrie 1992; Simmons and Tomkins 1996; see also Hosken
and Stockley 2004). In male genitalia, in contrast, negative
allometry probably due to stabilizing selection is common
in arthropods (Eberhard et al. 1998; Tatsuta et al. 2001;
Bernstein S and Bernstein R 2002; Ohno et al. 2003). In E.
westwoodii, the allometric slopes of forewing and clasper length
were below 1.0 using both OLS and MA regressions and that ofhind wing tail was higher than 1.0 using MA regression, although neither directional nor quadratic selection was detected in these traits (Table 3). The allometric slope of genital
clasper, furthermore, was sufficiently lower than those of othertraits, suggesting developmental constraints preventing theelongation of male genital claspers in this species.
Here we analyzed sexual selection on males in the mating
event after starting a copulatory attempt. Our results indicatedthat males with symmetrical claspers were advantageous inthis episode. However, selection in other episodes, such as thelocation and detection of females by males, remains unknown.
Deinert et al. (1994) showed the opposing selective force inthe 2 distinct selective mating sequence events in Heliconius
hewitsoni. To clarify the whole picture of sexual selection onmale morphology in E. westwoodii, further study on the selective
forces in different mating episodes will be required.
We are grateful to Carlos Cordero and Anthony R. Chittenden for
their valuable comments on earlier drafts of the manuscript. Göran
Arnqvist and 2 anonymous reviewers provided constructive comments,
which greatly improved the manuscript. We also thank Yoshitaka
Kamimura, Kenji Ohara, Mamoru Owada, and Kiyoshi Horie for theiruseful suggestions.

REFERENCES
Allen GR, Simmons LW. 1996. Coercive mating, fluctuating asymmetry
and male mating success in the dung fly Sepsis cynipsea. Anim Behav.
52:737–741.
Andersson M. 1994. Sexual selection. Princeton (NJ): Princeton University Press.
Arnqvist G. 1989. Sexual selection in a water strider: the function,mechanism of selection and heritability of a male grasping apparatus. Oikos. 56:344–350.
Arnqvist G. 1997. The evolution of water strider mating systems:
causes and consequences of sexual conflicts. In: Choe JC, Crespi
BJ, editors. The evolution of mating systems in insects and arachnids. Cambridge: Cambridge University Press. p. 146–163.
Arnqvist G, Rowe L. 2002. Correlated evolution of male and female
morphologies in water striders. Evolution. 56:936–947.
Arnqvist G, Rowe L. 2005. Sexual conflict. Princeton (NJ): Princeton
University Press.
Balmford A, Jones IL, Thomas AIR. 1993. On avian asymmetry: evidence of natural selection for symmetrical tails and wings in birds.
Proc R Soc Lond B Biol Sci. 252:245–251.
Bernstein S, Bernstein R. 2002. Allometry of male genitalia in a speciesof soldier beetle: support for the one-size-fits-all hypothesis. Evolution. 56:1707–1710.
Blanckenhorn WU, Kraushaar U, Reim C. 2003. Sexual selection onmorphological and physiological traits and fluctuating asymmetry
in the yellow dung fly. J Evol Biol. 16:903–913.
Blanckenhorn WU, Kraushaar U, Teuschl Y, Reim C. 2004. Sexual
selection on morphological and physiological traits and fluctuating
asymmetry in the black scavenger fly Sepsis cynipsea. J Evol Biol.
17:629–641.
Blanckenhorn WU, Reusch T, Mühlhäuser C. 1998. Fluctuating
asymmetry, body size and sexual selection in the dung fly Sepsis
cynipsea—testing the good genes assumptions and predictions.
J Evol Biol. 11:735–753.
Brodie ED III, Moore AJ, Janzen FJ. 1995. Visualizing and quantifying
natural selection. Trends Ecol Evol. 10:313–318.
Chapman T, Arnqvist G, Bangham J, Rowe L. 2003. Sexual conflict.
Trends Ecol Evol. 18:41–47.
Clarke GM. 1997. The genetic basis of developmental stability. III.
Haplo-diploidy: are males more unstable than females? Evolution.
51:2021–2028.
Cooley JR. 2004. Asymmetry and mating success in a periodical cicada,
Magicicada septendecim (Hemiptera: Cicadidae). Ethology. 110:745–
759.
Cordero C, Eberhard WG. 2003. Female choice of sexually antagonistic male adaptations: a critical review of some current research.
J Evol Biol. 16:1–6.
Cordero C, Eberhard WG. 2005. Interaction between sexually antagonistic selection and mate choice in the evolution of female responses to male traits. Evol Ecol. 19:111–122.
Cox CR, Le Boeuf BJ. 1977. Female incitation of male competition:a mechanism in sexual selection. Am Nat. 111:317–335.

Koshio et al. • Sexual selection in a moth

Deinert EI, Longino JT, Gilbert LE. 1994. Mate competition in butterflies. Nature. 370:23–24.
Eberhard WG. 1996. Female control: sexual selection by cryptic female choice. Princeton (NJ): Princeton University Press.
Eberhard WG. 2002. The function of female resistance behavior:
intromission by male coercion vs. female cooperation in sepsid flies(Diptera: Sepsidae). Rev Biol Trop. 50:485–505.
Eberhard WG, Huber BA, Rodriguez SRL, Briceño RD, Salas I,
Rodriguez V. 1998. One size fits all? Relationships between the sizeand degree of variation in genitalia and other body parts in twenty
species of insects and spiders. Evolution. 52:415–431.
Elwes HJ. 1890. On some new moths from India. Proc Zool Soc Lond.
890:378–401.
Evans MR. 1993. Fluctuating asymmetry and long tails: the mechanicaleffects of asymmetry may act to enforce honest advertisement. Proc
R Soc Lond B Biol Sci. 253:205–209.
Evans MR, Hatchwell BJ. 1993. New slants on ornament asymmetry.
Proc R Soc Lond B Biol Sci. 251:171–177.
Evans MR, Martins TLF, Haley M. 1994. The asymmetrical cost of tailelongation in red-billed streamertails. Proc R Soc Lond B Biol Sci.
256:97–103.
Gavrilets S, Arnqvist G, Friberg U. 2001. The evolution of female
mate choice by sexual conflict. Proc R Soc Lond B Biol Sci. 268:
531–539.
Gavrilets S, Hayashi T. 2006. The dynamics of two- and three-way
sexual conflicts over mating. Philos Trans R Soc B. 361:345–354.
Holland B, Rice WR. 1998. Chase-away sexual selection: antagonistic
seduction versus resistance. Evolution. 52:1–7.
Hosken DJ, Stockley P. 2004. Sexual selection and genital evolution.
Trends Ecol Evol. 19:87–93.
Hunt J, Simmons LW. 1997. Patterns of fluctuating asymmetry inbeetle horns: an experimental examination of the honest signalling
hypothesis. Behav Ecol Sociobiol. 41:109–114.
Inoda T, Hirata Y, Kamimura S. 2003. Asymmetric mandibles of waterscavenger larvae improve feeding effectiveness on right-handedsnails. Am Nat. 162:811–814.
Janzen FJ, Stern HS. 1998. Logistic regression for empirical studies ofmultivariate selection. Evolution. 52:1564–1571.
Koshio C. 1996. Reproductive behaviour of the white tailed zygaenid
moth Elcysma westwoodii (Lepidoptera Zygaenidae). II. Female mating strategy. J Ethol. 14:21–25.
Koshio C, Hidaka T. 1995. Reproductive behaviour of the white tailed
zygaenid moth Elcysma westwoodii (Lepidoptera Zygaenidae). I. Mating sequence. J Ethol. 13:159–163.
Lande R, Arnold SJ. 1983. The measurement of selection on correlated characters. Evolution. 37:1210–1226.
Lessells CM. 2005. Why are males bad for females? Models for theevolution of damaging male mating behavior. Am Nat. 165:S46–S63.
Leung B, Forbes MR. 1997. Fluctuating asymmetry in relation toindices of quality and fitness in the damselfly, Enallagma ebrium(Hagen). Oecologia. 110:472–477.
Liggett AC, Harvey IF, Manning JT. 1993. Fluctuating asymmetryin Scatophaga stercoraria L.: successful males are more symmetrical.
Anim Behav. 45:1041–1043.
Markow TA. 1995. Evolutionary ecology and developmental instability.
Annu Rev Entomol. 40:105–120.
McLachlan A, Cant M. 1995. Small males are more symmetrical: mating success in the midge Chironomus plumosus L. (Diptera: Chironomidae). Anim Behav. 50:841–846.
Mitchell-Olds T, Shaw RG. 1987. Regression analysis of natural selection: statistical inference and biological interpretation. Evolution.
41:1149–1161.
Møller AP. 1990. Fluctuating asymmetry in male sexual ornaments
may reliably reveal male quality. Anim Behav. 40:1185–1187.
Møller AP. 1997. Developmental stability and fitness: a review. Am Nat.
149:916–932.
Møller AP, Cuervo JJ. 2003. Asymmetry, size, and sexual selection:
factors affecting heterogeneity in relationships between asymmetry
and sexual selection. In: Polak M, editor. Developmental instability:
causes and consequences. New York: Oxford University Press.
p. 262–275.
Møller AP, Pomiankowski A. 1993. Fluctuating asymmetry and sexual
selection. Genetica. 89:267–279.
Møller AP, Swaddle JP. 1997. Asymmetry, developmental stability, andevolution. Oxford: Oxford University Press.

577

Møller AP, Zamora-Muñoz C. 1997. Antennal asymmetry and sexual
selection in a cerambycid beetle. Anim Behav. 54:1509–1515.
Naugler CT, Leech SM. 1994. Fluctuating asymmetry and survival ability in the forest tent caterpillar moth Malacosoma disstria: implications for pest management. Entomol Exp Appl. 70:295–298.
Nosil P, Reimchen TE. 2001. Tarsal asymmetry, nutritional condition, andsurvival in water boatmen (Callicorixa vulnerata). Evolution. 55:712–720.
Ohno S, Hoshizaki S, Ishikawa Y, Tatsuki S, Akimoto S. 2003. Allometry of male genitalia in a lepidopteran species, Ostrinia latipennis(Lepidoptera: Crambidae). Appl Entomol Zool. 38:313–319.
Owada M. 1992. The chalcosiine moths of Agalope (Lepidoptera, Zygaenidae) from Taiwan, with notes on allied genera. Jpn J Entomol.
60:85–95.
Owada M, Horie K, Xue D. 1999. Three new chalcosiine moths of thegenus Achelura (Lepidoptera, Zygaenidae) from the northern IndoChinese Peninsula and southwestern China. Tinea. 16:20–28.
Palmer AR. 1994. Fluctuating asymmetry analyses: a primer. In: Markow
TA, editor. Developmental instability: its origins and evolutionary implications. Dordrecht (The Netherlands): Kluwer. p. 335–364.
Palmer AR. 1999. Detecting publication bias in meta-analyses: a casestudy of fluctuating asymmetry and sexual selection. Am Nat. 154:
220–233.
Palmer AR, Strobeck C. 1986. Fluctuating asymmetry: measurement,
analysis, patterns. Ann Rev Ecol Syst. 17:391–421.
Palmer AR, Strobeck C. 1992. Fluctuating asymmetry as a measure ofdevelopmental stability: implications of non-normal distributions
and power of statistical tests. Acta Zool Fenn. 191:57–72.
Parker GA. 1979. Sexual selection and sexual conflict. In: Blum MS,
Blum NA, editors. Sexual selection and reproductive competition in
insects. New York: Academic Press. p. 123–166.
Petrie M. 1992. Are all secondary sexual display structure positively
allometric and, if so, why? Anim Behav. 43:173–175.
Polak M, Møller AP, Gangestad SW, Kroeger DE, Manning JT,
Thornhill R. 2003. Does an individual asymmetry parameter exist?
A meta-analysis. In: Polak M, editor. Developmental instability: causes
and consequences. New York: Oxford University Press. p. 81–96.
Polak M, Starmer WT. 2005. Environmental origins of sexually selected variation and a critique of the fluctuating asymmetry-sexual
selection hypothesis. Evolution. 59:577–585.
R Development Core Team. 2005. R: a language and environment for
statistical computing [Internet]. Vienna (Austria): R Foundationfor Statistical Computing. [cited 2005 December 20]; Available
from: http://www.R-project.org.
Rice WR. 1989. Analyzing tables of statistical tests. Evolution. 43:223–225.
Rowe L, Cameron E, Day T. 2005. Escalation, retreat, and female indifference as alternative outcomes of sexually antagonistic coevolution. Am Nat. 165:S5–S18.
SAS Institute. 2004. JMP version 5.1. Cary (NC): SAS Institute Inc.
Simmons LW, Tomkins JL. 1996. Sexual selection and the allometry of
earwig forceps. Evol Ecol. 10:97–104.
Smith LD, Palmer AR. 1994. Effects of manipulation diet on size andperformance of brachyuran crab claws. Science. 264:710–712.
Sneddon LU, Swaddle JP. 1999. Asymmetry and fighting performancein the shore crab Carcinus maenas. Anim Behav. 58:431–435.
Sokal RR, Rohlf FJ. 1995. Biometry. 3rd ed. New York: W. H. Freeman
and Company.
SPSS Inc. 2001. SPSS for Windows 11.0. Chicago (IL): SPSS Inc.
Swaddle JP. 2003. Fluctuating asymmetry, animal behavior, and evolution. Adv Study Behav. 32:169–205.
Swaddle JP, Witter MS, Cuthill IC. 1994. The analysis of fluctuating
asymmetry. Anim Behav. 48:986–989.
Tatsuta H, Mizota K, Akimoto S. 2001. Allometric pattern in heads
and genitalia in the stag beetle, Lucanus maculifemoratus (Coleoptera:
Lucanidae). Ann Entomol Soc Am. 94:462–466.
Thomas ALR. 1993. The aerodynamic costs of asymmetry in the wings
and tail of birds: asymmetric birds can’t fly round tight corners.
Proc R Soc Lond B Biol Sci. 254:181–189.
Thornhill R. 1992a. Fluctuating asymmetry, interspecific aggression
and male mating tactics in two species of Japanese scorpionflies.
Behav Ecol Sociobiol. 30:357–363.
Thornhill R. 1992b. Female preference for the pheromone of maleswith low fluctuating asymmetry in the Japanese scorpionfly (Panorpa
japonica: Mecoptera). Behav Ecol. 3:277–283.
Thornhill R, Alcock J. 1983. The evolution of insect mating systems.
Cambridge: Harvard University Press.

578

Tomkins JL, Simmons LW. 2003. Fluctuating asymmetry and sexual
selection: paradigm shifts, publication bias, and observer expectation. In: Polak M, editor. Developmental instability: causes and consequences. New York: Oxford University Press. p. 231–261.
Tsubaki Y, Matsumoto K. 1998. Fluctuating asymmetry and malemating success in a sphragis-bearing butterfly Luehdorfia japonica(Lepidoptera: Papilionidae). J Insect Behav. 11:571–582.
Ueno H. 1994. Fluctuating asymmetry in relation to two fitness components, adult longevity and male mating success in a ladybird bee-

Behavioral Ecologytle, Harmonia axyridis (Coleoptera: Coccinellidae). Ecol Entomol.
19:87–88.
Van Valen L. 1962. A study of fluctuating asymmetry. Evolution. 16:
125–142.
Windig JJ. 1998. Evolutionary genetics of fluctuating asymmetry in thepeacock butterfly (Inachis io). Heredity. 80:382–392.
Woods RE, Sgrò CM, Hercus MJ, Hoffmann AA. 2002. Fluctuating asymmetry, fecundity and development time in Drosophila: is there an association under optimal and stress conditions? J Evol Biol. 15:146–157.