Phenotypic variation and fluctuating asymmetry in sexually dimorphic feather ornaments in relation to sex and mating system

Secondary sexual characters have been hypothesized to demonstrate increased phenotypic variation between and within individuals as compared to ordinary morphological traits. We tested whether this was the case by studying phenotypic variation, expressed as the coefficient of variation (CV), and developmental instability, measured as fluctuating asymmetry (FA), in ornamental and non-ornamental traits of 70 bird species with feather ornamentation while controlling for similarity among species due to common descent. Secondary sexual characters differed from ordinary morphological traits by showing large phenotypic CV and FA. This difference can be explained by the different mode of selection operating on each kind of trait: a history of intense directional (ornaments) and stabilizing selection (non-ornaments). Phenotypic variation is reduced in the sex with more intense sexual selection (males), but does not differ among species with different mating systems. The strength of stabilizing selection arising from natural selection is associated with decreased CV (wing CV is smaller than tarsus or tail CVs). We found evidence of FA being reduced in ornamental feathers strongly affected by aerodynamics (tail feathers) compared to other ornaments, but only in females. In conclusion, CV and FA were not related, suggesting mat phenotypic plasticity and developmental instability are independent components of phenotypic variation.     

The allometric pattern of sexually size dimorphic feather ornaments and factors affecting allometry

The static allometry of secondary sexual characters is currently subject to debate. While some studies suggest an almost universal positive allometry for such traits, but isometry or negative allometry for nonornamental traits, other studies maintain that any kind of allometric pattern is possible. Therefore, we investigated the allometry of sexually size dimorphic feather ornaments in 67 species of birds. We also studied the allometry of female feathers homologous to male ornaments (female ornaments in the following) and ordinary nonsexual traits. Allometries were estimated as reduced major axis slopes of trait length on tarsus length. Ornamental feathers showed positive allometric slopes in both sexes, although that was not a peculiarity for ornamental feathers, because nonsexual tail feathers also showed positive allometry. Migration distance (in males) and relative size of the tail ornament (in females) tended to be negatively related to the allometric slope of tail feather ornaments, although these results were not conclusive. Finally, we found an association between mating system and allometry of tail feather ornaments, with species with more intense sexual selection showing a smaller degree of allometry of tail ornaments. This study is consistent with theoretical models that predict no specific kind of allometric pattern for sexual and nonsexual characters.     

Allometry of the baculum and sexual size dimorphism in American martens and fishers (Mammalia: Mustelidae)

Genitalia are among the most variable of morphological traits, and recent research suggests that this variability may be the result of sexual selection. For example, large bacula may undergo post‐copulatory selection by females as a signal of male size and age. This should lead to positive allometry in baculum size. In addition to hyperallometry, sexually selected traits that undergo strong directional selection should exhibit high phenotypic variation. Nonetheless, in species in which pre‐copulatory selection predominates over post‐copulatory selection (such as those with male‐biased sexual size dimorphism), baculum allometry may be isometric or exhibit negative allometry. We tested this hypothesis using data collected from two highly dimorphic species of the Mustelidae, the American marten (Martes americana) and the fisher (Martes pennanti). Allometric relationships were weak, with only 4.5–10.1% of the variation in baculum length explained by body length. Because of this weak relationship, there was a large discrepancy in slope estimates derived from ordinary least squares and reduced major axis regression models. We conclude that stabilizing selection rather than sexual selection is the evolutionary force shaping variation in baculum length because allometric slopes were less than one (using the ordinary least squares regression model), a very low proportion of variance in baculum length was explained by body length, and there was low phenotypic variability in baculum length relative to other traits. We hypothesize that this pattern occurs because post‐copulatory selection plays a smaller role than pre‐copulatory selection (manifested as male‐biased sexual size dimorphism). We suggest a broader analysis of baculum allometry and sexual size dimorphism in the Mustelidae, and other taxonomic groups, coupled with a comparative analysis and with phylogenetic contrasts to test our hypothesis. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 955–963.     

Evolutionary rates of secondary sexual and non-sexual characters among birds

The rate of evolutionary morphological change in secondary sexual characters among species has traditionally been assumed to exceed that for non-sexual characters, giving rise to a larger degree of divergence. We used a large data set of independent evolutionary events of exaggerated secondary sexual feather characters across all birds to test whether that was the case. Comparative analyses revealed that secondary sexual tail feather characters diverged more than wing feathers in females, and we also found that secondary sexual head feather characters diverged more than tarsi in males, when only including intra-order comparisons in the analyses. These results are in the predicted direction, with secondary sexual characters diverging more than ordinary morphological traits, partially supporting the general impression that secondary sexual characters are more variable among species than ordinary morphological characters. However, the degree of divergence among secondary sexual characters was generally not much larger than that among ordinary characters. Some non-significant differences in divergence between secondary sexual characters and ordinary characters could be explained by the cost-reducing function of ordinary morphological traits. There was no evidence of significant differences in divergence between sexes for secondary sexual characters, maybe because of genetic correlations in morphology between the sexes. However, male tarsi diverged more than female tarsi, and sexual selection might play a role in this difference in divergence. This revised version was published online in July 2006 with corrections to the Cover Date.     

A PATH-ANALYTIC MODEL FOR THE MEASUREMENT OF SELECTION ON MORPHOLOGY

This paper describes a path model for the analysis of phenotypic selection upon continuous morphological characters. The path-analysis model assumes that selection occurs on unmeasured general size and shape allometry factors that summarize linear relations among sets of ontogenetically, phylogenetically, or functionally related traits. An unmeasured factor for general size is considered the only aspect of morphometric covariance matrices for which there is an a priori biological explanation. Consequently, selection coefficients are derived for each measured character by holding constant only a general size factor, rather than by using multiple regression to adjust for the full covariance matrix. Fitness is treated as an unmeasured factor with loadings, representing directional selection coefficients, computed as the covariances of the size-adjusted characters with the measured fitness indicator. The magnitudes and signs of the selection coefficients, combined with biological insight, may suggest hypotheses of selection on one or more shape allometry factors. Hypotheses of selection on general size and shape allometry factors are evaluated through cycles of measurement, analysis, and experimentation, designed to refine the path diagram depicting the covariances among the measured characters, the measured indicator of fitness, and unmeasured factors for morphology and fitness. The path-analysis and multiple-regression models were applied to data from remeasurement of Lande and Arnold's (1983) pentatomid bugs and to Bumpus's (1899) data on house sparrows. The path analysis suggested the hypothesis that variation in bug survivorship was an expression of directional selection on wing loading. Bumpus's data are consistent with a hypothesis of stabilizing selection on general size in females and directional selection for small wing size relative to body size in males.     

Bat genitalia: allometry, variation and good genes

Male genitalia are typically highly variable across species, for which sexual selection is thought to be responsible. Sexually selected traits characteristically show positive allometry and high phenotypic variation, although genitalia seem to be typified by negative allometry due to stabilizing selection. Additionally, while sexual selection appears to be the primary force responsible for genital evolution, the precise mechanism is unclear, but good-genes selection could be involved. If so, male genital variation should correlate with some male quality measure(s). We investigated the allometry of male Nyctalus noctula genitalia and investigated associations between genital size and three phenotypic measures of male quality (body size, relative body mass, and fluctuating asymmetry (FA)). We found that the penis exhibited positive allometry and high phenotypic variation, and was positively associated with male body size and relative body mass, but not with FA. This pattern is more typical of sexually selected display traits, contrasting with general patterns of genital allometry. The baculum was negatively allometric and was not associated with any quality measure. Our results suggest that the N. noctula penis is under directional sexual selection and is a reliable indicator of male phenotypic quality.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 497–507.     

Genital and body allometry in two species of noctuid moths (Lepidoptera: Noctuidae)

One‐size‐fits‐all and related hypotheses predict that static allometry slopes for male genitalia will be consistently lower than 1.0 and lower than the slopes for most other body parts (somatic traits). We examined the allometry of genitalic and somatic morphological traits in males and females of two species of noctuid moths, Spodoptera exigua (Hübner, [1808]) and Helicoverpa armigera (Hübner, [1808]). The relationship between genitalic traits and body size was generally strongly negative‐allometric in males but with no significant differences from 1.00 in females of the two species examined. However, in females, the slope of genital traits was also lower than the slopes for somatic traits. The relationship between somatic traits and the body size indicator was approximately isometric in most cases in males, except in four traits in S. exigua, in which the slopes showed slight negative allometry, and the hind tibia in H. armigera, in which the slope had positive allometry. However, in females, some somatic traits showed isometric and some other showed negative allometry in both species. The coefficients of variation (CV) for all structures in the males were low, not exceeding 10%. Genitalic traits showed significantly lower CV than somatic traits in males. In females, somatic traits showed lower CV than genitalic traits but with no significant difference in the H. armigera. Our observations of strongly negative allometry for genitalic traits in males are consistent with stabilizing selection on genital size and we suggest that male performance in interactions with females is the source of selection on male genital allometry. The difference in the degree of phenotypic variation between genitalic and somatic traits in the two studied species is attributed to the different developmental‐genetic architectures of these traits. Female genitalia showed a similar trend to the males, although the difference between genital and somatic traits was not significant in females. This finding suggests that selection is acting differently on male and female genitalia. Positive allometry of hind tibia in H. armigera may be a result of secondary sexual function.     

Developmental evolution of sexual ornamentation: model and a test of feather growth and pigmentation

A tremendous diversity of avian color displays has stimulatednumerous studies of natural and sexual selection. Yet, the developmentalmechanisms that produce such diversification, and thus the proximatetargets of selection pressures, are rarely addressed and poorlyunderstood. In particular, because feathers are colored duringgrowth, the dynamics of feather growth play a deterministicrole in the variation in ornamentation. No study to date, however,has addressed the contribution of feather growth to the expressionof carotenoid-based ornamentation. Here, we examine the developmentalbasis of variation in ornamental feather shapes in male housefinches (Carpodacus mexicanus)—a species in which carotenoiddisplays are under strong natural and sexual selection. First,we use geometric morphometrics to partition the observed shapevariation in fully grown feathers among populations, ages, degreesof elaboration, ornamental body parts, and individuals. Second,we use a biologically informed mathematical model of feathergrowth to predict variation in shape of ornamental feathersdue to simulated growth rate, angle of helical growth of featherbarbs, initial number of barb ridges, rate of addition of newbarbs, barb diameter, and ramus-expansion angle. We find closeconcordance between among-individual variation in feather shapeand hue of entire ornament, and show that this concordance canbe attributed to a shared mechanism—growth rate of featherbarbs. Predicted differences in feather shape due to rate ofaddition of barbs and helical angle of feather growth explainedobserved variation in ornamental area both among individualsand between populations, whereas differences in helical angleof growth and the number of barbs in the feather follicle explaineddifferences in feather shape between ornamental parts and amongmales of different ages. The findings of a close associationof feather growth dynamics and overall ornamentation identifythe proximate targets of selection for elaboration of sexualdisplays. Moreover, the close association of feather growthand pigmentation not only can reinforce condition-dependencein color displays, but can also enable phenotypic and geneticaccommodation of novel pigments into plumage displays providinga mechanism for the observed concordance of within-populationdevelopmental processes and between-population diversificationof color displays.     

Male Genital Allometry in Scathophagidae (Diptera)

Male genital structures are extremely divergent across species and sexual selection is largely responsible. Many sexually selected traits show positive allometry and have high phenotypic coefficients of variation (CV). Sexually-selected genital traits that come into contact with females during copula may be an exception to this general pattern. We compared the within species size allometry of the genital claspers, mandibular palps, and testes in a comparative study across the Scathophagidae. We additionally compared the levels of phenotypic variation in these traits and in hind tibia length. Within species, claspers typically displayed negative allometry and had low CV, indicative of stabilizing selection. In contrast, testis size was more like sexually selected display traits, typically being positively allometric and having very large CV. Palps tended to be positively allometric or isometric, and intermediate in levels of phenotypic variation, much like leg length. In spite of intraspecific stabilizing selection on the genital claspers, there has been major divergence of these characters across species. Co-ordinating editor. F. Stuefer     

Allometry and sexually dimorphic traits in male anurans

Allometry of secondary sexual traits has been the subject of recent debate, and the generality of positive allometry and its association with sexual selection have been recently questioned. Whereas some studies suggest an almost universal positive allometry for traits under sexual selection and isometry or a negative allometry for traits not under such pressure, other studies argue that this pattern results from the study of exaggerated (ornamental) traits. To answer the call for an examination of the allometry of less-exaggerated sexually selected traits, we have examined morphological data from 14 sexually dimorphic traits and six monomorphic traits from three anuran species. Although we found evidence of positive allometry in male secondary sexual traits of several species and populations, not all nonsexual traits were isometric or exhibited negative allometry. Furthermore, our results indicate that larger traits in the populations that we studied were not associated with greater allometric slopes. Therefore, our study is in line with the contention suggesting no specific kind of allometric pattern for sexual and nonsexual characters, and we can only advocate for further investigation of trait allometry and sexual selection to understand the complexity underlying the evolution of allometry in sexual traits.     

Population differences in density and resource allocation of ornamental tail feathers in the barn swallow

Many organisms show well‐defined latitudinal clines in morphology, which appear to be caused by spatially varying natural selection, resulting in different optimal phenotypes in each location. Such spatial variability raises an interesting question, with different prospects for the action of sexual selection on characters that have a dual purpose, such as locomotion and sexual attraction. The outermost tail feathers of barn swallows (Hirundo rustica) represent one such character, and their evolution has been a classic model subject to intense debate. In the present study, we examined individuals from four European populations to analyze geographical variation in the length and mass of tail feathers in relation to body size and wing size. Tail feather length differed between sexes and populations, and such variation was a result of the effects of natural selection, acting through differences in body size and wing size, as well as the effects of sexual selection that favours longer tails. The extra enlargement of the tail promoted by sexual selection (i.e. beyond the natural selection optimum) could be achieved by increasing investment in ornaments, and by modifying feather structure to produce longer feathers of lower density. These two separate processes accounting for the production of longer and more costly tail feathers and less dense feathers, respectively, are consistent with the hypothesis that both Zahavian and Fisherian mechanisms may be involved in the evolution of the long tails of male barn swallows. We hypothesize that the strength of sexual selection increases with latitude because of the need for rapid mating as a result of the short duration of the breeding season at high latitudes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 925–936.     

Sexual selection and allometry: a critical reappraisal of the evidence and ideas

One of the most pervasive ideas in the sexual selection literature is the belief that sexually selected traits almost universally exhibit positive static allometries (i.e., within a sample of conspecific adults, larger individuals have disproportionally larger traits). In this review, I show that this idea is contradicted by empirical evidence and theory. Although positive allometry is a typical attribute of some sexual traits in certain groups, the preponderance of positively allometric sexual traits in the empirical literature apparently results from a sampling bias reflecting a fascination with unusually exaggerated (bizarre) traits. I review empirical examples from a broad range of taxa illustrating the diversity of allometric patterns exhibited by signal, weapon, clasping and genital traits, as well as nonsexual traits. This evidence suggests that positive allometry may be the exception rather than the rule in sexual traits, that directional sexual selection does not necessarily lead to the evolution of positive allometry and, conversely, that positive allometry is not necessarily a consequence of sexual selection, and that many sexual traits exhibit sex differences in allometric intercept rather than slope. Such diversity in the allometries of secondary sexual traits is to be expected, given that optimal allometry should reflect resource allocation trade-offs, and patterns of sexual and viability selection on both trait size and body size. An unbiased empirical assessment of the relation between sexual selection and allometry is an essential step towards an understanding of this diversity.     

Feather melanin and microstructure variation in dark‐eyed junco Junco hyemalis across an elevational gradient in the Selkirk Mountains

Variation in feather melanism and microstructure can arise through sexual selection and ecological functional drivers. Melanin‐based plumage traits are associated with sexual dichromatism and the intensity of sexual selection in many avian species, but also have several ecological benefits such as protection against ultra‐violet (UV) radiation, camouflage, and feather strength. Additionally, feather microstructure influences thermoregulation. Plumage variation across species is well documented; however, the relative role of sexual selection and ecological drivers in intra‐specific and within‐population variation is less established. We investigated UV reflectance, melanism, and feather microstructure in a population of Oregon dark‐eyed juncos Junco hyemalis oreganus between high (1900–2200 m a.s.l.) and low (450–800 m a.s.l.) elevations in the Selkirk Mountains to evaluate potential sexual selection and ecological drivers of variation. We found no difference in UV reflectance or lightness (melanism) of head feathers between elevations, but individuals at high elevation had lighter (less melanism) and less brown (less pheomelanin) body contour feathers than at low elevations. High elevation individuals also had longer contour feathers with more pronounced plumulaceous regions. Sexual dichromatism did not vary between elevations, leading us to reject sexual selection in favour of ecological functional drivers of plumage variation in this system. To our knowledge, this is the first study to identify within‐population differences in feather melanism and microstructure between different elevations.     

Baculum variation and allometry in the muskrat (Ondatra zibethicus): a case for sexual selection

Sexual selection is a powerful force that influences the evolution of a variety of traits associated with female mate choice and male–male competition. Although other factors have been implicated, sexual selection may be particularly important in the evolution of the genitalia. Traits under sexual selection typically have high phenotypic variance and positive allometry relative to non-sexual traits. Here, we test the hypothesis that the baculum (os penis) of the muskrat (Ondatra zibethicus) is under sexual selection by examining phenotypic variance and allometry relative to non-sexual traits. Muskrats were sampled from Ontario, Canada, and a variety of traits measured. Measurements included baculum length and width, and three non-sexual traits (skull length, skull width, hind foot length). We used coefficient of variation (CV) and allometric slopes calculated using reduced major axis regression to test our hypotheses. Baculum traits had significantly higher CV’s relative to non-sexual traits. Baculum traits also showed positive allometry, whereas all non-sexual traits had negative allometric relationships. In addition, baculum width had higher CV’s and steeper allometric slopes than baculum length, indicating that, in muskrat, baculum width may be more influenced by sexual selection than baculum length. Positive allometry of the baculum is consistent with other examples of mammalian genitalia, but contrasts with negative allometry found in many insects. Other examples of positive allometry and high phenotypic variance of the baculum have suggested that females may use the baculum as an indicator of male quality. “Good genes” indicator traits may be particularly important in species that mate in an environmental context that prohibits female assessment of male quality. Muskrats mate aquatically, and thus females may be unable to properly assess males prior to copulation.     

The scaling of the size and stiffness of primary flight feathers

Birds encompass a large range of body sizes, yet the importance of body size on feather morphology and mechanical properties has not been characterized. In this study, I examined the scaling relationships of primary flight feathers within a phylogenetically diverse sample of avian species varying in body size by nearly three orders of magnitude. I measured the scaling relationships between body mass and feather linear dimensions as well as feather flexural stiffness. The resnlts of an independent contrasts analysis to test the effects of phylogenetic history on the characters measured had no effect on the scaling relationships observed. There was slight, but not significant, positive allometry in the scaling of shaft diameter with respect to feather length across a range of body masses. The scaling of feather length and diameter against body mass was not significantly different from isometry. Flexural stiffness, however, exhibited strong negative allometry. Therefore, larger birds have relatively more flexible feathers than smaller birds. The more flexible primary feathers of large birds may reduce stresses on the wing skeleton during take-off and landing and also make these feathers less susceptible to mechanical failure. Conversely, the greater flexibility of these feathers may also reduce their capacity to generate aerodynamic lift.     

No evidence of current sexual selection on sexually dimorphic traits in a bird with high variance in mating success

Sexual dimorphism, particularly in ornamental traits, is likely to have arisen by sexual selection. Most empirical and theoretical studies of sexual dimorphism assume that ongoing sexual selection also maintains the dimorphism. Over four seasons, I measured the sexual selection acting on three sexually dimorphic attributes (epaulet size, body size, and the blackness of the body plumage) of male red-winged blackbirds and found no consistent directional or stabilizing selection on any of them. Correlational selection was also negligible. I used path analysis to explore potential relationships in more detail but found no direct or indirect effects of male traits on either within- or extrapair success. Males who were resident on the marsh for more years had higher within-pair success, primarily because they spent more of the season on their territory. Experimental manipulations of epaulet size and color and the extent of nonblack feathers in the black body plumage had no detectable effect on the number of within-pair mates, paternity, or the number of extrapair offspring sired in nearby territories. These results combine with data from other studies of red-winged blackbirds to suggest that, despite high variation in male mating success and hence a strong opportunity for sexual selection, several morphological attributes that differ between the sexes and vary among males are not under current sexual selection. The possible explanations for why add complexity to our understanding of how sexual selection operates.     

Pitfalls in understanding the functional significance of genital allometry

The male genitalia of arthropods consistently show negative static allometry (the genitalia of small males of a species are disproportionally large, and those of large males are disproportionally small). We discuss relations between the ‘one‐size‐fits‐all’ hypothesis to explain this allometry and the regimes of selection that may be acting on genitalia. We focus on the contrasts between directional vs. stabilizing selection, and natural vs. sexual selection. In addition, we point out some common methodological problems in studies of genital allometry. One‐size‐fits‐all types of arguments for negative allometry imply net stabilizing selection, but the effects of stabilizing selection on allometry will be weaker when the correlation between body size and the trait size is weaker. One‐size‐fits‐all arguments can involve natural as well as sexual selection, and negative allometry can also result from directional selection. Several practical problems make direct tests of whether directional or stabilizing selection is acting difficult. One common methodological problem in previous studies has been concentration on absolute rather than relative values of the allometric slopes of genitalia; there are many reasons to doubt the usefulness of comparing absolute slopes with the usual reference value of 1.00. Another problem has been the failure to recognize that size and shape are independent traits of genitalia; rapid divergence in the shape of genitalia is thus not paradoxical with respect to the reduced variation in their sizes that is commonly associated with negative allometric scaling.     

Size matters: genital allometry in an African mole-rat (Family: Bathyergidae)

Typically, sexually selected traits show positive allometry and high coefficients of variation (CV). To date, many studies on the allometry of genitalia have focused on insects. In addition, studies have largely ignored the potential for sexual selection on female genitalia, despite male and female structures presumably co-evolving. Insects tend to show negative allometry in both male and female genitalia, while in contrast, the few studies carried out in mammals (males only) show positive allometry. Reasons for these differences between the taxa still remain unclear. However, in mammals, three main mechanisms have been proposed for genital evolution, namely, sperm competition, female cryptic choice and sexual conflict. In the first such study that we are aware of, we examined intra-specific genital allometry in both males and females of a mammal, the subterranean solitary Cape dune mole-rat, Bathyergus suillus. We found positive allometry occurring in male genitalia, which is consistent with previous vertebrate studies. Similarly, we found that female genitalia also exhibited positive allometry further supporting the notion of co-evolution of male and female genitalia. Although it is difficult to distinguish between the forces or mechanisms determining this directional selection, we suggest that several reproductive advantages are incurred as a result of positive allometric relationship of the genitalia in B. suillus and such advantages are also likely in other subterranean mammals. Our study further highlights the differences in genital allometry across taxa.     

A natural experiment on the condition-dependence of achromatic plumage reflectance in black-capped chickadees

Honest advertisement models posit that only individuals in good health can produce and/or maintain ornamental traits. Even though disease has profound effects on condition, few studies have experimentally tested its effects on trait expression and even fewer have identified a mechanistic basis for these effects. Recent evidence suggests that black and white, but not grey, plumage colors of black-capped chickadees (Poecile atricapillus) are sexually selected. We therefore hypothesized that birds afflicted with avian keratin disorder, a condition that affects the beak and other keratinized tissues, would show reduced expression of black and white, but not grey, color. UV-vis spectrometry of black-capped chickadees affected and unaffected by avian keratin disorder revealed spectral differences between them consistent with this hypothesis. To elucidate the mechanistic bases of these differences, we used scanning electron microscopy (SEM), electron-dispersive x-ray spectroscopy (EDX) and a feather cleaning experiment. SEM showed extreme feather soiling in affected birds, and EDX revealed that this was most likely from external sources. Experimentally cleaning the feathers increased color expression of ornamental feathers of affected, but not unaffected, birds. These data provide strong evidence that black and white color is an honest indicator in chickadees, and that variation in feather dirtiness, likely due to differences in preening behavior is a mechanism for this association.     

The form of sexual selection on male genitalia cannot be inferred from within-population variance and allometry - a case study in Aquarius remigis

Male genital morphology in insects and arachnids is characterized by static hypoallometry and low intrapopulational levels of phenotypic variation relative to other male traits. The one-size-fits-all model of genital evolution attributes these patterns to stabilizing sexual selection. This model relies on the assumption that the observed patterns of variation and allometry reflect the form of sexual selection acting these traits. We test this by examining the patterns of scaling and trait variation for a set of genitalic and somatic morphological traits in male water striders (Aquarius remigis). This suite of traits is of particular interest because previous work has shown that the genitalic traits are under strong directional selection whereas the somatic traits are under either weak directional or stabilizing selection. Because the selection regime for these traits is known, we can, for the first time, test the purported relationship between trait variation, scaling, and the form of sexual selection. We show that the patterns of variation and scaling of these traits differ sharply from those predicted for traits experiencing strong directional sexual selection. Specifically, the male genital structures show static hypoallometry and low intrapopulational levels of phenotypic variation relative to other male traits, in spite of consistent, strong, directional sexual selection. These scaling relationships and levels of variation are typical of genital traits in other insect species, where they have been presumed to reflect stabilizing sexual selection. Our data clearly refute the assumption of the one-size-fits-all hypothesis that hypoallometric scaling of genitalic traits implies stabilizing selection. We discuss the implications of this finding and propose future directions for improving our current understanding of genital evolution in arthropods.