Female morphs of a colour polymorphic damselfly differ in developmental instability and fecundity

© Koninklijke Brill NV, Leiden, 2009 DOI 10.1163/157075609X417080

Animal Biology 59 (2009) 41–54 www.brill.nl/ab

Female morphs of a colour polymorphic damselfl y diff er in developmental instability and fecundity

Jessica Bots 1 * , Stefan Van Dongen 1 , Tim Adriaens 2 , Henri J. Dumont 3 Robby Stoks 4 and Hans Van Gossum 1

1 Evolutionary Ecology Group, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

2 Research Institute for Nature and Forest, Kliniekstraat 25, B-1070 Brussels, Belgium 3 Institute of Animal Ecology, Ghent University, Ledeganckstraat 35, B-9000 Ghent, Belgium

4 Laboratory of Aquatic Ecology, University of Leuven (KULeuven) Debériotstraat 32, B-3000 Leuven, Belgium

Abstract Sex-limited colour polymorphism occurs in several animal taxa and is usually explained in the context of sexual selection. Specifi cally, for polymorphism restricted to the female sex, multiple phenotypes may have evolved in response to male harassment. Such male harassment is generally considered to entail diff erential costs to female morphs, which may ultimately result in fi tness diff erences. However, contrary to this pre-diction, most previous studies do not support that female morphs (andromorphs and heteromorphs) diff er in measures of quality and (or) fi tness components. In this study, we evaluate quality and fi tness diff erences between mated female morphs of the damselfl y Enallagma cyathigerum . We suggest that many earlier studies may have failed to observe morph diff erences in quality or fi tness because selection by male harassment was weak. Here, we selected a study population for which our expectation was that levels of per female capita male harassment were high. Nevertheless, also in this population mated female morphs did not diff er in body size or condition (body mass/body length). However, mated female morphs did diff er in levels of developmental instability: heteromorphs consistently showed a higher level of fl uctuating asymmetry than andromorphs. Also, mated female morphs diff ered in fecundity: andromorphs had a lower clutch size than heteromorphs. In addition, larger females contained more eggs, but the slope of this relationship was steeper in heteromorphs. In conclusion, mated female morphs of the damselfl y E. cyat-higerum at our study site clearly diff ered in one quality estimate (developmental instability) and in our measure of fi tness (fecundity). © Koninklijke Brill NV, Leiden, 2009.

Keywords Damselfl y; fi tness; sexual harassment; female-limited polymorphism ; quality

*) Corresponding author; e-mail: [email protected]

42 J. Bots et al. / Animal Biology 59 (2009) 41–54

Introduction

Sex-limited colour polymorphism is observed in a variety of vertebrate and inverte-brate species, with examples in insects (Nielsen and Watt, 2000 ), birds (Galeotti et al., 2003 ), fi sh (Olendorf et al., 2006 ) and lizards (Vercken et al., 2007 ). In many of these examples, sexual selection is considered the driving force for coexistence of multiple male or female morphs in natural populations (Gross, 1996 ; Gray and Mckinnon, 2007). Specifi cally for species showing multiple female morphs, female-limited poly-morphism may have evolved in response to costly intersexual interactions (Robertson, 1985 ; Cook et al., 1994 ; Bergsten et al., 2001 ). Laboratory cross experiments with damselfl ies (Insecta: Odonata) indicate a genetic basis for female-limited polymor-phism (Sánchez-Guillén et al., 2005 ) and selection appears to contribute to the coex-istence of multiple female phenotypes (e.g. Wong et al., 2003 ; Van Gossum et al., 2005 ).

Males of most polymorphic damselfl y species practise scramble mate competition (Corbet, 1999 ). Males will thus maximise their reproductive success by rapidly locat-ing and mating with numerous females. Females, however, only need one or few mat-ings to fertilize their lifetime number of eggs (Fincke, 1987 ; see also Arnqvist and Nilsson 2000 for a review on insects in general). Consequently, females will receive more male attention than is desired, which may waste female time and energy budgets and may even lead to injuries (Robertson, 1985 ; Sirot and Brockman, 2001). To avoid these costs of excessive male harassment, females will be selected to develop counter adaptations and female-limited polymorphism may be such a trait. Th e idea is that males may easily recognise females when only one type is present, but become dis-tracted when faced with multiple female morphs (e.g. Fincke, 2004 ). Similar to preda-tors forming a search image for the most common prey type (e.g. Murdoch, 1969 ), frequency-dependent mate selection may act on female morphs, with males harassing and preferring to mate with the most common female morphs in the population (e.g. Miller and Fincke, 1999 ; Van Gossum et al., 1999 ; 2001 ). Alternatively (or in addition), mate-searching males may be challenged when part of their potential female partners resemble the conspecifi c male’s phenotype and behaviour. Such a female morph is considered a functional male-mimic that gains an advantage over the other female morph as it receives less male attention (e.g. Robertson, 1985 ; Sherratt, 2001 ). Th erefore, female morphs are clearly expected to suff er diff erent amounts of costly male harassment. Consequently, even though overall fi tness may be equal on the long term, diff erences between female morphs in components of fi tness should be evident.

However, contrary to this prediction, most previous work in natural populations does not support female morphs to diff er in fi tness. Indeed, the absence of fi tness dif-ferences has been indicated by the study of a variety of fi tness components and also less directly by examining quality. High quality can be defi ned as the probability of high fi tness. Under stressful conditions, this probability should be lower, but the diff erence between high and low quality individuals should become more pronounced (Leung and Forbes, 1997a ). More in detail, female morphs did not diff er in immune function or resistance to parasite infection (Joop et al., 2006 ; Robb et al., 2006 ), body size or

J. Bots et al. / Animal Biology 59 (2009) 41–54 43

mass (e.g. Th ompson, 1989 ; Lajeunesse and Forbes, 2003 ; but see Cordero, 1992 ; Cordero et al., 1998 ), developmental instability (Lajeunesse and Forbes, 2003 ), life-time mating frequency (Cordero and Egido Pérez, 1998 ; Sirot et al., 2003 ) or survival (Cordero, 1992 ; Fincke, 1994 ; Cordero et al., 1998 ). It is interesting though that two studies (Banham, 1990 ; Svensson and Abbott, 2005 ) showed that female morphs do diff er in fecundity while in another study such diff erences in fecundity were not found (Joop et al., 2007 ). Hence, the question arises why diff erences in fi tness between female morphs are rarely encountered. Possibly, the selective potential of male harassment may diff er between species or populations (cf. Plath et al., 2003 ). For example, when encounters between individuals are limited (low population density) and (or) sexual interactions are scarce (relatively low proportion of males), male harassment will be less intense and may not negatively aff ect female fi tness (Head and Brooks, 2006 ; Kokko and Rankin, 2006 ). In such situation, fi tness diff erences between female morphs may be absent. Th e aim of the present study is to evaluate quality and fi tness diff erences between mated female morphs in a population with high potential for selection by male harassment (high population density and strongly male-biased sex ratio).

Material and Methods

Study species

Our study was conducted with the damselfl y Enallagma cyathigerum (Charpentier 1840), a species showing one male and two discrete female colour morphs. Male body colouration is conspicuous blue (especially abdominal segments eight and nine) with black markings. Th e body colour of heteromorphs is brown to greenish brown, while andromorphs are blue and closely resemble the conspecifi c male’s colouration but both female morphs diff er in the amount of abdominal black patterning from the male (see Askew, 2004 for illustrations).

E. cyathigerum occurs at a wide variety of European wetlands, but is especially abun-dant at oligotrophic acidic pools with sandy soils (Mostert and Dingemans, 2002; Askew 2004 ). Males and females reproduce along the shoreline of these waters. Female damselfl ies lay eggs in successive clutches separated by periods during which new fol-licles are ripened (Corbet, 1999 ). When a new set of eggs is mature, females will engage in mating. After copulation, eggs will be oviposited in submerged vegetation (Doerksen, 1980 ).

Sampling details

Andromorph and heteromorph females were collected on 23 august 1999 at an acidic fen (Groot Loodven; 51°21’N, 04°34’E) in the military area ‘Groot Schietveld’ situ-ated in the north of Belgium. In comparison with other populations of the study spe-cies, this study site shows a high density and strongly male-biased sex ratio (Bots et al., 2007 ). We thus a priori expect the per-capita male harassment and the potential to detect diff erences between female morphs at this population to be high.


Sampling was limited to adult mating individuals to avoid catching females that carried undeveloped clutches (andromorph: N = 35; heteromorph: N = 28). In addi-tion, we restricted sampling from 10 am to 1 pm. Th ese methodological considerations are crucial since including unmated females who may have incomplete or undeveloped clutches will bias our measures for clutch size, which would then not provide suitable fi tness estimates. Following capture, females were stored in individual vials fi lled with 70% ethanol until measurement of quality and fi tness.

Quality and fi tness measurements

We evaluated whether mated andromorphs and heteromorphs diff ered in quality (body mass, body size, developmental instability and condition) and (or) in fi tness (fecun-dity). At fi rst, total body mass was determined using an electronic balance (accuracy: 0.1mg). Just before the measurement, each individual was taken out its vial and dried on fi lter paper for 20 seconds such that all individuals achieved the same degree of desiccation. For body size we obtained several widely used measures (e.g. Johansson, 2003 ): total body length (from the proximal side of the head to the distal end of the abdomen), abdomen length and head width; which were all determined using digital callipers (accuracy: 0.01mm). Condition was calculated by dividing body mass with total body length.

Here after, fore and hind wings of each individual were cut off near implantation on the thorax with micro-scissors for the study of developmental instability. Developmental stability is one of the components of developmental homeostasis through which organisms reduce phenotypic variation resulting from developmental perturbations. Fluctuating asymmetry (FA) is the most frequently used measure of developmental stability. FA describes small random deviations from perfect bilateral symmetry, which are random with respect to side and show a normal (e.g. Palmer, 1994 ) or a leptokurtic distribution (Leung and Forbes, 1997b ; Gangestad and Th ornhill, 1999 ; Hardersen and Frampton, 2003 ). Levels of FA have been indicated to relate to several environmental and genetic stress factors (e.g. Palmer and Strobeck, 1986 ; Leary and Allendorf, 1989 ). Particularly in damselfl ies, levels of FA have been shown related to pollution stress (e.g. Hardersen and Frampton, 1999 ), but the relationship with fi tness is less straightforward as results are heterogeneous. While some studies could not detect any relationship between FA and fi tness (Leung and Forbes, 1997a ; Carchini et al., 2000 ; 2001 ), other studies did show that more sym-metric individuals have a higher short-term or lifetime mating success (Harvey and Walsh, 1993 ; Córdoba-Aguilar, 1995 ; Beck and Pruett-Jones, 2002 ; Szállassy et al., 2003 ; De Block and Stoks, 2007 ). Here, we examine FA between left and right wings based on one metric and three meristic characters. Wings were digitalised using a camera (type IAI Protec Shutter WB) mounted on a stereoscopic microscope and measured with image analysing software (Bioscan Optimas). As metric character, we measured the length of the forewing (from the intersection of the fi rst ante nodal branch with the costa to the distal end of the wing top). Meristic characters were determined on both fore and hind wing. Th ese included the number of post


nodal wing cells (post fore, post hind), the number of cells from the branching of the radius branch 3 and the radius branch 4 (r3r4 fore, r3r4 hind), the number of single cells at the joint of radius 2 and radius 3 (inte fore, inte hind) (see fi gure 1 in Harderson and Frampton, 1999). Because levels of FA are usually small (1% or less), metric and meristic characters were determined twice to separate measurement error (ME) from real FA.

We determined fecundity by counting the number of eggs in a single clutch. Th e abdomen of the collected females was dissected and only fully developed eggs were counted under a stereomicroscope. Developed eggs could be distinguished from unde-veloped eggs based on size and colouration (see also Forbes and Baker, 1991 ). All clutches were counted twice to determine repeatability. In addition, earlier work indi-cated clutch size to be positively correlated with body size in damselfl ies (e.g. Cordero, 1991 ). To our knowledge, the relationship between fecundity and abdomen length has never been evaluated separately for andromorphs and heteromorphs. Here, we tested whether this relationship is identical in both morphs.

Figure 1. Fecundity versus abdomen length for andro- and heteromorphs. Because several individuals had unexpected low numbers of eggs we ran a model (see methods and results) to assign every individual to either the group with full clutches or the group with partial clutches. As partial clutches are assumed not to refl ect fi tness, results are only presented for full clutches.

Abdomen length (mm)

Num

ber

of e

ggs

AndromorphHeteromorph

22 26 2824

300

400

500

600

700


Statistical analyses

Clutch size varied considerably among mated females, with some females carrying very low numbers of eggs. Th is suggests that despite our sampling precautions, some females had already oviposited some of their eggs prior to collection. Since it was not a priori possible to assign clutches unambiguously to be either full or partial, this uncertainty was incorporated in our statistical model. Diff erences in clutch size between mated female morphs and associations with abdomen length (or body mass) were determined using a generalized linear model with log link function and Poisson error structure (replacing the Poisson by a Negative Binomial distribution to account for overdisper-sion did not change any of the conclusions). Female morph was added as a factor, and abdomen length (or mass) as continuous covariate. Th e two-way interaction was also tested. To incorporate diff erences in number of eggs between full and partial clutches, a mixture approach was applied, such that for both full and partial clutches the param-eters of the above ANCOVA-like model were obtained. Th e advantage of a mixture type of approach is that it does not require an a priori classifi cation into full or partial clutches. For each clutch, a probability was estimated indicating the likelihood it should be considered a full or a partial clutch. Th e estimation of this probability is based on the observed clutch size and conditional on the levels of the explanatory vari-ables. Formally, the generalised linear model can be described as follows:

log ( y i ) = p i β F + ( 1 − p i ) β P + ε i

p i ~ B e r n o u l l i ( P )

Where y i represent the clutch size, β F and β P the linear predictors (ANCOVA type of model) for the full and partial clutches respectively. Th e residual error term ε i is assumed to follow a Poisson distribution. Th e indicator variable p i can take two values since it is assumed to follow a Bernoulli distribution. If it equals zero, the clutch size of individ-ual i is assigned to the group of full clutches, while if p i equals one the clutch is assumed to be partial. For each individual, the parameter p i will have a Bernoulli distribution with probability equal to the likelihood that the clutch was partial. Th e parameter P estimates the proportion of full clutches in the dataset. Th e model was fi tted in a Bayesian framework in OPENBUGS ( http://mathstat.helsinki.fi /openbugs/Home.html ) assuming fl at non-informative priors for all parameters. For all β ’s a normal distribution with zero mean and variance equal to 10 6 was used, while for the param-eter P a uniform between 0 and 1 prior was chosen. All estimates are based on means and 95% credibility intervals of posterior distributions, obtained after 5000 iterations of 5 independent Monte Carlo Markov Chains. Th e advantage of the statistical model is that it incorporates all information for a particular individual (morph, body length, egg number) and on the basis of this joint information individuals are assigned to the full or partial clutch category with a certain probability. Following, we will only report on results for full clutches, since data on partial clutches are considered not to refl ect fi tness.

Prior to testing for diff erences in FA between mated female morphs, degrees of ME and tests for directional asymmetry (DA) for the metric traits were obtained from


mixed regression models (Van Dongen et al., 1999 ). For meristic traits, measurement error was negligible (out of 286 counts of cell numbers between 2 and 20, only one count diff ered from the fi rst: 18 cells were counted instead of 17), and levels of DA were obtained from paired t-tests. No DA was detected for any trait (all p > 0.2) and for wing length the between sides variation (0.013) was greater than ME (0.001). Since the kurtosis of the distributions of the signed asymmetries (left - right) was positive, we found no indications for anti symmetry. Abdomen length, condition, total body length, head width and levels of unsigned asymmetries (i.e., absolute value of the signed FA) of all traits separately and averaged across traits (after standardisation) were compared between the two female morphs using t-tests (Leung et al., 2000 ).

Results

We could not detect any diff erences between mated andro- and gynomorphs in condi-tion, total body length, abdomen length or head width ( table 1 ).

Repeatability of egg counts was high (99.9%) and all clutches could be assigned to either full (N = 41) or partial clutches (N = 22) with 100% confi dence applying the mixture log-linear model described above. Comparing fecundity, mated female morphs diff ered signifi cantly in full clutch size (diff erences between andro- and heteromorphs: 46.81; 95% CI = [33.51 - 60.39]), with heteromorphs carrying a higher number of eggs (andromorph: 449 ± 20, N = 19; heteromorph: 482 ± 25, N = 21).

Table 1.A comparison of quality measures between andromorphs and heteromorphs. Signifi cant diff erences are indicated in bold. Note that after Bonferroni correction female morphs do not diff er in any single trait FA measure (all P > 0.05). However, female morphs do diff er in the average FA of all traits. Average trait values for andro- and heteromorphs are given as mean ± se

d.f. T value P Andromorph HeteromorphFA measures: R3r4 fore 43.8 -0.81 0.424 0.57 ± 0.10 0.93 ± 0.19R3r4 hind 42 -1.64 0.108 0.62 ± 0.10 0.79 ± 0.18Inte fore 43.5 -0.56 0.578 0.43 ± 0.09 0.54 ± 0.17Inte hind 61 -1.31 0.194 0.23 ± 0.07 0.39 ± 0.11Post fore 61 -0.09 0.924 0.49 ± 0.10 0.50 ± 0.11Post hind 61 -2.15 0.036 0.23 ± 0.07 0.50 ± 0.11Wing length 56 -0.11 0.912 0.09 ± 0.01 0.09 ± 0.01Average FA 55 -2.15 0.035 -0.13 ± 0.07 0.12 ± 0.09Other quality

measures:

Abdomen length 61 -0.90 0.370 25.2 ± 0.17 25.5 ± 0.18Body length 61 -1.08 0.286 31.8 ± 0.23 32.2 ± 0.23Head width 61 0.45 0.657 4.0 ± 0.03 3.9 ± 0.03Condition 61 1.08 0.283 15x10 -4 ± 0.3x10 -4 14x10 -4 ± 0.3x10 -4


We tested for a log-linear relationship between fecundity and body size. Abdomen length explained part of the variation in clutch size. Results for body mass are similar to abdomen length. We only report on the relation between fecundity and abdomen length, which predicted clutch size in full clutches diff erently for mated andro- and heteromorphs ( fi g. 1 ). Egg number for both hetero- (slope = 0.131; 95% CI = [0.098 – 0.164]) and andromorphs (slope = 0.035; 95% CI = [0.019 – 0.050]) augmented with increasing abdomen length, however, the slope of this relation was steeper for hetero-morphs (diff erence in slope = - 0.096; 95% CI = [-1.132 – -0.059]). Th e coeffi cient of determination of the regression line was higher in heteromorphs (32%) compared to andromorphs (5%), which indicates that the association between body size and fecun-dity is not very strong for andromorphs. Th e analysis was repeated without three potential outliers (see fi g. 1 : two very small andromorphs and one very large hetero-morph). Results were very similar to those presented above, which indicates that the relationship between clutch size and abdomen length for andro- and heteromorphs is not shaped by these outliers.

Mated heteromorphs were consistently more asymmetric than mated andromorphs. However, according to the individual t-tests followed by Bonferroni correction, none of the individual traits showed a signifi cant diff erence ( table 1 ). Nevertheless, the aver-age FA of all traits for heteromorphs was signifi cantly higher than for andromorphs

Figure 2. Levels of FA for a variety of wing measures for both female morphs. Graphic interpretation and averaged trait analysis (see results) indicate heteromorphs to consistently show higher levels of fl uctuating asymmetry compared to andromorphs.

Trait

r3r4 forer3r4 hind

inte foreinte hind

post forepost hind

wing length

Flu

ctua

ctin

g A

sym

met

ry In

dex

0.0

0.2

0.4

0.6

0.8

1.0

1.2

AndromorphHeteromorph


( table 1 ). Indeed, fi gure 2 suggests heteromorphs to be consistently more asymmetric than andromorphs ( fi g. 2 ).

Discussion

We showed that mated females of the two morphs of the damselfl y E. cyathigerum at our study site do not vary in body size or condition, but do diff er in developmental instability and fecundity patterns. Many earlier studies failed to detect diff erences in quality and fi tness components between female morphs (see introduction). Diff erences in fi tness are expected, if male harassment is costly and levels of male harassment diff er between female morphs. In support, male harassment has been shown to drastically reduce female fi tness (lowered foraging effi ciency, survival and fecundity) in a variety of invertebrates including water striders, isopods and dung fl ies (e.g. Rowe et al., 1994 ; Jormalainen et al., 2001 ; Muhlhauser and Blanckenhorn, 2002 ). Experimental support for the fact that male harassment reduces female fi tness diff erently for andro- and heteromorphs is, however, currently limited. Th us far, only an experimental study has linked male harassment directly to female morph fi tness and indicated costs of sexual interactions to andromorphs but not to heteromorphs (Sirot and Brockmann, 2001 ). In the fi eld, evidence for diff erential male harassment comes from the observa-tion that males attempt to mate more often than females desire and that female morphs diff er in the propensity to accept matings (Gosden and Svensson, 2007 ). Less directly, Svensson et al. ( 2005 ) applied a population genetic model in combination with estimates of fecundity and morph frequencies to support that frequency-and density-dependent male harassment may drive the maintenance of female-limited polymorphism.

Similarly to studies on Ischnurid damselfl ies, we here also detected diff erences in fecundity between mated female morphs for E. cyathigerum . For Ischnura elegans , a species that shows one andromorph female and two forms of heteromorph females, clutch size was lower in andromorphs compared to one type of heteromorph (called infuscans), but showed equal clutch size in comparison to the other less abundant heteromorph (called rufescens-obsoleta) (Svensson and Abbott, 2005 ). Banham ( 1990 ) also found that andromorphs carry the smallest amount of eggs, but this result was only true for a population sample (containing individuals independent of their mating status) and not for a sample of copulating individuals. For the damselfl y I. ramburi , diff erences in fecundity have been detected under experimental condiions. Clutch size of andromorphs was lowered when they were kept in insectaries at a male-biased sex ratio compared to when housed with equal numbers of males and females. No such diff erence between treatments was detected for heteromorphs (Sirot and Brockman, 2001). Here, we also observed a lower clutch size in mated andromorphs compared to mated heteromorphs for E. cyathigerum . It may be early to evaluate for generalities, but if any of the female morphs shows lower fecundity this rather consistently appears to be the andromorph. However, if selection is acting to maintain both female morphs in a natural population, andro- and heteromorphs should ultimately have an equal overall


fi tness. Perhaps, female morphs gain an equal lifetime fecundity using diff erent repro-ductive strategies. For example, andromorphs may lay smaller clutches but oviposit more frequently (i.e. shorter inter-clutch interval) than heteromorphs. Additionally, also other fi tness components, such as survival, are likely to contribute to overall fi tness, which might be even more important than fecundity in shaping lifetime reproductive success (Banks and Th ompson, 1987 ).

Female morph fecundity does not only diff er between mated morphs of E. cyat-higerum , but also appears to depend diff erentially on body size. Egg number aug-mented with increasing body size and mass, but this pattern was more pronounced in heteromorphs than in andromorphs. Selection for larger clutch size may thus be expected to lead to disproportional larger heteromorphs than andromorphs ( cf. Finke et al., 1997). However, we, amongst other studies, could not detect any morph diff erences in body size (e.g. Th ompson, 1989 ; Lajeunesse and Forbes, 2003 ). On the contrary, diff erences in body length have been found for I. graellsii (Cordero, 1992 ) and I. elegans (Cordero et al., 1998 ). However, in both species andromorphs were larger than heteromorphs. Th is suggests that the relationship between fecundity and body size for andro- and heteromorphs may diff er between species.

Whereas mated andromorphs in our study had lower fecundity compared to mated heteromorphs, mated heteromorphs were on average more asymmetric than mated andromorphs. It is relevant that traits such as body size and wing morphology remain fi xed after metamorphosis to adulthood, while condition and fecundity may be directly aff ected by male harassment just prior to collection. Th e level of developmental insta-bility of such morphological traits may be aff ected by male harassment on maternal morphs (i.e. lower quality off spring), stressful conditions during larval development or developmental costs for producing a particular phenotype (cf . Talloen et al., 2004 ). For birds, it has been reported that wing asymmetry negatively aff ects fl ight performance (Swaddle, 1997 ). If levels of FA also infl uence fl ight abilities in damselfl ies, this may ultimately aff ect heteromorph fi tness, for instance, because they might be less capable to escape predators or harassing males. However, when discussing the consequences of our result, it is important to note that we restricted this study to mated individuals. For males of several Odonate species, it has been reported that mated individuals have a lower level of FA than unmated ones (e.g. Harvey and Walsh, 1993 ; Córdoba-Aguilar, 1995 , but see Hardersen, 2000 ). Although, no data on the eff ect of FA onto mating success are present for females, we cannot exclude this possibility. In addition, andro- and heteromorphs are expected to mate with an unequal probability (cf. Gosden and Svensson, 2007 ), which might have biased our FA result. Th erefore, it may be worth-while to evaluate the generality of our result in a future study using a random sample of the entire population. Interestingly, a previous study with the damselfl y Nehalennia irene using such a sample failed to fi nd diff erences between morphs (Lajeunesse and Forbes, 2003 ).

In conclusion, we show that mated andromorphs and heteromorphs diff er in one quality measure (developmental instability) and one component of fi tness (fecundity) in a population where the potential for selection by male harassment is high. However,


it remains to be assured whether the observed fi tness diff erence between mated female morphs is indeed caused by male harassment. Future work will need to quantify the cost of male sexual behaviour towards female morphs and its subsequent negative eff ect to female fecundity in natural populations of E. cyathigerum .

Acknowledgements

We would like to thank two anonymous referees for their thoughtful comments that improved our manuscript. Th e “Institute for the Promotion of Innovation through Science and Technology in Flanders” (IWT-Vlaanderen) supported this work (research grant to JB). HVG is a postdoctoral fellow with the Fund for Scientifi c Research-Flanders (FWO). All research conducted was conform local ethical guidelines and approved by AMINAL Flanders.

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Female morphs of a colour polymorphic damselfly differ in developmental instability and fecundity

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