Measures Of Sexual Selection Research Articles

Latitudinal clines in sexual selection, sexual size dimorphism and sex-specific genetic dispersal during a poleward range expansion.

Range expansions can be shaped by sex differences in behaviours and other phenotypic traits affecting dispersal and reproduction. Here, we investigate sex differences in morphology, behaviour and genomic population differentiation along a climate-mediated range expansion in the common bluetail damselfly (Ischnura elegans) in northern Europe. We sampled 65 sites along a 583-km gradient spanning the I. elegans range in Sweden and quantified latitudinal gradients in site relative abundance, sex ratio and sex-specific shifts in body size and mating status (a measure of sexual selection). Using single nucleotide polymorphism (SNP) data for 426 individuals from 25 sites, we further investigated sex-specific landscape and climatic effects on neutral genetic connectivity and migration patterns. We found evidence for sex differences associated with the I. elegans range expansion, namely (a) increased male body size with latitude, but no latitudinal effect on female body size, resulting in reduced sexual dimorphism towards the range limit, (b) a steeper decline in male genetic similarity with increasing geographic distance than in females, (c) male-biased genetic migration propensity and (d) a latitudinal cline in migration distance (increasing migratory distances towards the range margin), which was stronger in males. Cooler mean annual temperatures towards the range limit were associated with increased resistance to gene flow in both sexes. Sex ratios became increasingly male biased towards the range limit, and there was evidence for a changed sexual selection regime shifting from favouring larger males in the south to favouring smaller males in the north. Our findings suggest sex-specific spatial phenotype sorting at the range limit, where larger males disperse more under higher landscape resistance associated with cooler climates. The combination of latitudinal gradients in sex-biased dispersal, increasing male body size and (reduced) sexual size dimorphism should have emergent consequences for sexual selection dynamics and the mating system at the expanding range front. Our study illustrates the importance of considering sex differences in the study of range expansions driven by ongoing climate change.

Measurement of sexual selection in plants made easier

A recommendation of: Dorken, ME and Perry LE Correlated paternity measures mate monopolization and scales with the magnitude of sexual selection 10.1111/jeb.13013

Male pheomelanin pigmentation and breeding onset in Barn Swallows Hirundo rustica gutturalis

Plumage color is a composite trait and each component can provide information regarding individual quality. Melanin-based color is one of the most common plumage coloration in birds. This color comprises two types of melanin pigments: eumelanin (black pigment) and pheomelanin (yellow-reddish pigment), and it is affected by several post-molting processes such as UV damage, staining, and preen oils. In some birds, pheomelanin-based plumage color is related to several measures of sexual selection; however, pheomelanin is almost always expressed together with eumelanin and affected by post-molting processes. Therefore, it is still unclear whether (and to what extent) pheomelanin can explain the observed relationship between plumage color and the measure of sexual selection. Here we examined the melanin (both eumelanin and pheomelanin) concentration in relation to breeding onset, as a fitness component associated with sexual selection in male Barn Swallows (Hirundo rustica gutturalis). We found that throat feathers in males contained more pheomelanin than those in females. The amount of pheomelanin, but not eumelanin, declined throughout the sampling period, indicating that pheomelanin pigmentation conveys different information than eumelanin. Even after correcting for depigmentation of melanin, males with more pheomelanin bred earlier than the others. Together with the results from previous studies, these findings indicate that pheomelanin-based coloration may have evolved via sexual selection for pheomelanin pigmentation in Barn Swallows.

Precopulatory sexual selection in the seed bug Lygaeus equestris: a comparison of choice and no-choice paradigms

Mate choice has long been appreciated as a key component of sexual selection. However, how we measure mate choice, both in the field and in the laboratory, remains problematic. Mating preferences may be tested using either no-choice or choice tests, but explicit comparisons between these two experimental paradigms remain limited. It has been suggested that preferences may be stronger in choice tests as they allow simultaneous comparison, and some studies have indeed found stronger mating preferences in choice tests compared to no-choice tests. Here we explicitly tested the effect of experimental choice paradigm on the measurement of sexual selection on male and female morphology in the promiscuous seed bug Lygaeus equestris (Heteroptera, Lygaeidae). We performed mating trials in which we varied the amount of choice presented to each sex, giving four choice treatments: no-choice, male choice, female choice and mutual choice. Overall we found evidence for significant positive directional selection on female body length and stabilizing selection on an overall measure of male body size. However, we found no significant effect of choice paradigm on the patterns of sexual selection for males or females. We suggest this may be because of the method of mate assessment in L. equestris, which appears to be primarily via contact cues, which may limit simultaneous comparison between options. • We investigated sexual selection on male and female morphology in a lygaeid bug. • We varied the level of choice available to males and females during mating trials. • We found significant sexual selection on male and female morphology. • However we found no effect of choice paradigm on the strength of selection. • The mechanism of mate choice in this species may limit comparison between options.

The measurement of sexual selection on females and males

Abstract As in any field of research, the study of sexual selection is subject to ongoing debate over definitions and interpretations of the fundamental concepts involved. These arguments generally promote progress, as they highlight areas where current explanations are incomplete. Here we briefly review two ongoing discussions in the sexual selection literature. First, the definition of sexual selection has received renewed interest in light of increasing research effort into when and how it operates in females. Second, how best to measure sexual selection is an ongoing subject of debate; in practice, recognition that the appropriate measures depend on the focus of the specific study, and that multiple measures should be employed wherever possible, seems to provide the most informative approach. The wide scope of recent empirical work in these and related areas, with the application of new techniques and approaches, reflects that the field of sexual selection is being constantly expanded and enriched.

Genetic versus Census Estimators of the Opportunity for Sexual Selection in the Wild

Abstract The existence of a direct link between intensity of sexual selection and mating-system type is widely accepted. However, the quantification of sexual selection has proven problematic. Several measures of sexual selection have been proposed, including the operational sex ratio (OSR), the breeding sex ratio (BSR), and the opportunity for sexual selection (I(mates)). For a wild population of pronghorn (Antilocapra americana), we calculated OSR and BSR. We estimated I(mates) from census data on the spatial and temporal distribution of receptive females in rut and from a multigenerational genetic pedigree. OSR and BSR indicated weak sexual selection on males, but census and pedigree I(mates) suggested stronger sexual selection on males than on females. OSR and BSR correlated with census but not pedigree estimates of I(mates), and census I(mates) did not correlate with pedigree estimates. This suggests that the behavioral mating system, as deduced from the spatial and temporal distribution of females, does not predict the genetic mating system of pronghorn. The differences we observed between estimators were primarily due to female mate sampling and choice and to the sex ratio. For most species, behavioral data are not perfectly accurate and therefore will be an insufficient alternative to using multigenerational pedigrees to quantify sexual selection.

Who to include in measures of sexual selection is no trivial matter

In many animals acquiring limited reproductive opportunities involves competition for resources, mates and opposite-sex gametes. There is ambiguity in which competitive steps are included in measures of sexual selection: individuals who fail to obtain resources necessary for reproduction are often excluded. We illustrate the implications of variation in who is included in measures of selection. We quantified selection on male length and the opportunity for selection associated with nest acquisition, mate acquisition, and fertility of mates at two levels of density and two levels of nest availability in the sand goby. Both measures varied significantly across the three episodes of selection. Nest and mate acquisition contributed substantially to the overall opportunity for selection and selection on male size. Focusing only on males with nests led to lower estimates of selection. The effects of density and nest availability depended on the selective episodes considered. While there is nothing wrong with focusing on particular episodes of interest, inconsistency in who is included in measures of sexual selection across studies will make it difficult to answer broad research questions.

Sexual dimorphism in primate aerobic capacity: a phylogenetic test

Male intrasexual competition should favour increased male physical prowess. This should in turn result in greater aerobic capacity in males than in females (i.e. sexual dimorphism) and a correlation between sexual dimorphism in aerobic capacity and the strength of sexual selection among species. However, physiological scaling laws predict that aerobic capacity should be lower per unit body mass in larger than in smaller animals, potentially reducing or reversing the sex difference and its association with measures of sexual selection. We used measures of haematocrit and red blood cell (RBC) counts from 45 species of primates to test four predictions related to sexual selection and body mass: (i) on average, males should have higher aerobic capacity than females, (ii) aerobic capacity should be higher in adult than juvenile males, (iii) aerobic capacity should increase with increasing sexual selection, but also that (iv) measures of aerobic capacity should co-vary negatively with body mass. For the first two predictions, we used a phylogenetic paired t-test developed for this study. We found support for predictions (i) and (ii). For prediction (iii), however, we found a negative correlation between the degree of sexual selection and aerobic capacity, which was opposite to our prediction. Prediction (iv) was generally supported. We also investigated whether substrate use, basal metabolic rate and agility influenced physiological measures of oxygen transport, but we found only weak evidence for a correlation between RBC count and agility.

Nutritional correlates and mate acquisition role of multiple sexual traits in male collared flycatchers

The information content of a sexual signal may predict its importance in a multiple signal system. Many studies have correlated sexual signal expression with the absolute levels of nutrient reserves. In contrast, the changes of nutrient reserves associated with signal expression are largely unknown in the wild due to technical limitations although they are important determinants of signal information content. We compared two visual and eight acoustic sexual traits in male collared flycatchers to see whether the nutritional correlates of expression predict the role of the signal in sexual selection. We used single point assays of plasma lipid metabolites to estimate short-term changes in nutritional state in relation to sexual trait expression during courtship. As a measure of sexual selection, we estimated the relationship with pairing latency after arrival in a 4-year dataset. Males which found a mate rapidly were characterized by large wing and forehead patches, but small song strophe complexity and small figure repertoire size. Traits more strongly related to pairing latency were also more closely related to changes in nutrient reserves. This indicates a link between signal role and information content. Small wing patches and, surprisingly, complex songs seemed to indicate poor phenotypic quality and were apparently disfavoured at mate acquisition in our population. Future studies of the information content of sexual traits, especially dynamic traits such as song, may benefit from the use of plasma metabolite profiles as non-invasive indicators of short-term changes in body condition.

The mismeasurement of sexual selection

Sexual selection can explain major micro- and macro-evolutionary patterns. Much of current theory predicts that the strength of sexual selection (i) is driven by the relative abundance of males and females prepared to mate (i.e. the operational sex ratio, OSR) and (ii) can be generally estimated by calculating intra-sexual variation in mating success (e.g. the opportunity for sexual selection, I(s)). Here, we demonstrate the problematic nature of these predictions. The OSR and I(s) only accurately predict sexual selection under a limited set of circumstances, and more specifically, only when mate monopolization is extremely strong. If mate monopolization is not strong, using OSR or I(s) as proxies or measures of sexual selection is expected to produce spurious results that lead to the false conclusion that sexual selection is strong when it is actually weak. These findings call into question the validity of empirical conclusions based on these measures of sexual selection.

Bateman (1948): pioneer in the measurement of sexual selection

Bateman (1948): pioneer in the measurement of sexual selection

A measure of sexual selection in hermaphroditic animals: parentage skew and the opportunity for selection

The role of sexual selection in shaping the mating system of hermaphrodites is currently widely accepted. However, a quantification of the intensity of sexual selection in hermaphroditic animals has never been accomplished. We evaluated the opportunity for sexual selection for both the female and the male functions in the simultaneous outcrossing hermaphrodite Ophryotrocha diadema by measuring focal hermaphrodites' paternal and maternal offspring in experimental replicated monogamous and promiscuous populations, using genetic markers to estimate paternity. Opportunity for sexual selection for each of the two sexual functions was quantified by means of the Crow's index, i.e. the ratio of variance in progeny number to the squared mean number of progeny. In addition, the extent to which the reproductive success was shared among competing individuals was estimated by means of the Nonacs's B index. We documented that the strength of selection on the male and female function in hermaphrodites with external fertilization depends on the reproductive context. Under a promiscuous regime, hermaphrodites have higher opportunities for selection for both the male and the female function than under the monogamous regime. Moreover, the reproductive skew for the female function becomes greater than that for the male function, moving from monogamy to promiscuity. In our model system, allocation to one sexual function is opposed by any degree of allocation to the other, indicating that sex-specific patterns of selection operate in this model species.

Evolution of an avian pigmentation gene correlates with a measure of sexual selection.

The extravagant plumage traits of male birds are a favourite example of sexual selection. However, to date the units that selection is acting upon, the genes themselves have been a 'black box'. Here, we report evidence of change driven by sexual selection at a pigmentation gene locus in the galliform birds. Across species, we find a correlation between the rate of amino acid change (dN/dS) at this locus (MC1R) and the degree of sexual dichromatism, which we use as a measure of the strength of sexual selection. There is no evidence for a similar pattern in any of five other loci (four candidate and one control locus). This is consistent with previous work on colour polymorphisms and suggests that MC1R may be a key target for selection acting on plumage colour. The pattern of selection at MC1R seems to be consistent with the continuous or cyclical evolution of traits and preferences that is the outcome of several Fisherian and good-genes models of sexual selection. In contrast, we found no support for models of sexual selection that predict an increase in purifying selection as a result of purging of deleterious mutations or for models that predict an increased rate of mutation in association with stronger sexual selection.

Of isopods and Hollywood stars: the measure of sexual selection

Of isopods and Hollywood stars: the measure of sexual selection

Size Dimorphism, Intrasexual Competition, and Sexual Selection in Wattled Jacana (Jacana Jacana), A Sex-Role-Reversed Shorebird in Panama

AbstractWe studied sexual size dimorphism, intrasexual competition, and sexual selection in an individually marked population of Wattled Jacanas (Jacana jacana) in the Republic of Panama. Males are the sole incubators of eggs (28-day incubation) and primary providers of chick care (50–60 days). Females were 48% heavier than, and behaviorally dominant over, males. Females also showed greater development of secondary sexual characters (fleshy facial ornamentation and wing spurs) than males. Both sexes defended territories throughout the year against same-sex conspecifics. Competition for territorial space was intense, and many individuals of both sexes did not become breeders. Resident females further competed with one another to accumulate multiple mates, resulting in a mating system of simultaneous polyandry. Female and male residents (territory holders) were larger, heavier, and more ornamented than adult floaters of the same sex. Larger and heavier females also had more mates than smaller females. Body size was thus a critical predictor of success in intrasexual competition for territories (both sexes) and for mates (females). Three measures of sexual selection—(1) sex difference in the opportunity for sexual selection, (2) female-to-male ratio of potential reproductive rates, and (3) operational sex ratio—each indicated that sexual selection is currently operating more strongly on females than on males (female-to-male ratios ranged from 1.43:1 to 2.22:1). Values of 1.61:1 and 1.43:1 represent the first published quantitative estimates of the opportunity for sexual selection for any sex-role-reversed bird. Our study supports the theory that when increased parental care entails reduced opportunities for future reproduction, asymmetries in parental care behaviors of the sexes can influence the intensity of competition for mates and the direction and strength of sexual selection.

Sexual selection in primates: New and comparative perspectives

List of contributors Foreword Robert L. Trivers Preface Part I. Introduction: 1. Sexual selection in primates: review and selective preview Peter M. Kappeler and Carel P. van Schaik 2. What is sexual selection? Tim H. Clutton-Brock 3. Sex roles, contests for the control of reproduction and sexual selection Patricia Adair Gowaty Part II. Sexual Signals: Substrates and Function: 4. Sexual selection and communication Charles T. Snowdon 5. Sexual selection and exaggerated sexual swellings of female primates Dietmar P. Zinner, Charles L. Nunn, Carel P. van Schaik and Peter M. Kappeler 6. Female multiple mating and genetic benefits in humans: investigations of design Steven W. Gangestad and Randy Thornhill Part III. Sexual Selection in Action: 7. Sexual selection, behaviour and sexually transmitted diseases Charles L. Nunn and Sonia M. Altizer 8. Mating conflict in primates: infanticide, sexual harassment and female sexuality Carel P. van Schaik, Gauri R. Pradhan and Maria A. van Noordwijk 9. Post-copulatory sexual selection in birds and primates Tim R. Birkhead and Peter M. Kappeler Part IV. Development and Consequences: 10. Development and sexual selection in primates Joanna M. Setchell and Phyllis C. Lee 11. Alternative male reproductive strategies: male bimaturism in orangutans Suci Atmoko Utami and Jan A. R. A. M. van Hooff 12. Sexual selection and the careers of primate males: paternity concentration, dominance acquisition tactics and transfer decisions Maria A. van Noordwijk and Carel P. van Schaik 13. Sexual selection, measures of sexual selection and sexual dimorphism in primates J. Michael Plavcan 14. Sex ratios in primate groups Joan B. Silk and Gillian R. Brown 15. Natural and sexual selection and the evolution of multi-level societies: insights from zebras with comparisons to primates Daniel I. Rubenstein and Mace Hack Index.

Bateman's Principles and the Measurement of Sexual Selection in Plants and Animals

A. J. Bateman's approach to animal mating systems is generalized to provide a framework for studying sexual selection in plants and hermaphroditic animals. The key feature of this generalization is the realization that the relationship between fecundity and mating success can take a variety of forms beyond those observed in Bateman's data. Thus, under many circumstances, female animals, or the female function of a hermaphroditic plant or animal, can experience sexual selection. Path diagrams are used to show that sexual selection is a component of fecundity selection. Finally, statistical procedures are discussed for measuring sexual selection in plant and animal populations with separate or combined sexes.

A biomechanical perspective on the use of forelimb length as a measure of sexual selection in frogs

AbstractDifferences in forelimb length between male and female frogs and between amplexing and non‐amplexing males have been interpreted to be the results of sexual selection on forelimb length. The causal feature of the forelimb that has been posited to cause such selection is the observation that non‐amplexing males attempt to disrupt breeding by prying amplexing males from females. A biomechanical model of forelimb function suggests that total length per se may not be the most appropriate measure to use. There are more functionally significant aspects of forelimb morphology, such as lever arm lengths, that should influence amplexing ability and may make measures of overall forelimb length misleading. This example highlights the relevance of functional analysis to current questions in evolutionary biology that rely on postulated roles for morphological structures under selection.

On the Measurement of Sexual Selection

On the Measurement of Sexual Selection