Correlates of male mating success in the lekking great snipe (Gallinago media): results from a four-year study

To investigate behavioral or morphological traits importantas mate choice cues, we measured selection differentials (s)as the covariances between each trait and male mating success,and directional selection gradients (J3) from multiple linearregression of the standardized traits on male mating success.Data from two leks in four consecutive years were included,and the annual data were analyzed separately. The main findingsare: (1) the distribution of male mating success proved to beless skewed than those found in many other lekking species,(2) only a few traits yielded significant selection gradients,(3) the importance of age on male mating success changed acrossyears, (4) females may use traits with a high variance as matechoice cues, and (5) individual males achieved similar matingsuccesses between years. Attendance and age were the traitsmost consistently correlated with male mating success, but notraits showed significant selection gradients in all years.Our results indicate that variable sexual selection pressuresexisted between years, but the high correlation found betweenthe mating success of individual males in successive seasonsalso indicates that permanent differences in male traits areimportant. Key words: lek, mate choice, sexual selection.     

MULTIVARIATE ANALYSIS OF SELECTION IN EXPERIMENTAL POPULATIONS DERIVED FROM HYBRIDIZATION OF TWO ECOTYPES OF THE ANNUAL PLANT DIODIA TERES W. (RUBIACEAE)

Morphologically variable F₂ genotypes derived from hybridization of coastal and inland ecotypes of the annual plant Diodia teres were used to identify selection on morphological traits in the natural habitat of each ecotype. These ecotypes occur in very different habitats, and have evolved pronounced morphological differentiation. Selection analysis can suggest whether present patterns of selection can explain morphological differences between ecotypes. F₂ genotypes were characterized morphologically, clonally replicated, and transplanted into the habitat of each ecotype. Selection was measured on six morphological traits. Directional and stabilizing selection occurred on many traits; direction and strength of selection varied sharply at different stages of growth, as revealed by a path-analysis approach that divided selection into a set of independent components. Directional selection favored traits of the native population at the coastal habitat, but less so at the inland habitat. Selection was of sufficient strength to create the observed morphological differences between ecotypes in 25–100 generations, given constant selection and sufficient genetic variation. In effects on fitness, most traits were neither independent nor consistently interactive with other traits. Rather, many traits entered into strong but evanescent interactions affecting particular components of fitness. Observed interactions did not support the hypothesis that the morphology of each ecotype was functionally integrated to a high degree.     

Female Mate Choice, Calling Song and Genetic Variance in the Cricket, Gryllodes sigillatus

Female preferences for song patterns of males of Gryllodes sigillatus and genetic variance of morphological traits correlated with them were analyzed. Females preferred short pulses associated with large males. The males’ thorax width, wing length and femur III length showed stronger relationship with the song pulse duration, whereas the relationship between pulse duration and wing width was not significant. Interestingly, this last trait was the only one that showed significant levels of genetic variance. Perhaps these results could be explained by the evolutionary response to sexual selection. Sexual selection could deplete the genetic variance in the male traits related to male‐mating success.     

Do male secondary sexual characters signal ejaculate quality? A meta‐analysis

There are two reasons why researchers are interested in the phenotypic relationship between the expression of male secondary sexual characters (SSCs) and ‘ejaculate quality’ (defined as sperm/ejaculate traits that are widely assumed to increase female fertility and/or sperm competitiveness). First, if the relationship is positive then females could gain a direct benefit by choosing more attractive males for fertility assurance reasons (‘the phenotype‐linked fertility’ hypothesis). Second, there is much interest in the direction of the correlation between traits favoured by pre‐copulatory sexual selection (i.e. affecting mating success) and those favoured by post‐copulatory sexual selection (i.e. increasing sperm competitiveness). If the relationship is negative this could lead to the two forms of selection counteracting each other. Theory predicts that the direction of the relationship could be either positive or negative depending on the underlying genetic variance and covariance in each trait, the extent of variation among males in condition (resources available to allocate to reproductive traits), and variation among males in the cost or rate of mating. We conducted a meta‐analysis to determine the average relationship between the expression of behavioural and morphological male secondary sexual characters and four assays of ejaculate quality (sperm number, viability, swimming speed and size). Regardless of how the data were partitioned the mean relationship was consistently positive, but always statistically non‐significant. The only exception was that secondary sexual character expression was weakly but significantly positively correlated with sperm viability (r = 0.07, P < 0.05). There was no significant difference in the strength or direction of the relationship between behavioural and morphological SSCs, nor among relationships using the four ejaculate quality assays. The implications of our findings are discussed.     

The Developmental Instability—Sexual Selection Hypothesis: A General Evaluation and Case Study

Developmental instability results from small, random perturbations to developmental processes of individual traits. Phenotypic outcomes of developmental instability include fluctuating asymmetry (FA, subtle deviations from perfect bilateral symmetry) and phenodeviance (minor morphological abnormalities). A great deal of research over the past 18 years has focused on the role of developmental instability in sexual selection. A driving force behind this research has been the developmental instability-sexual selection hypothesis, which posits that symmetry and lack of phenodeviance in secondary sexual traits are assessed by mates and rivals because they provide a reliable cue of individual genetic quality. The present article tests this hypothesis by evaluating its five main predictions using published results: expressions of developmental instability in secondary sexual traits should be (1) negatively correlated with mating success; (2) directly assessed by mates and sexual rivals; (3) heritable; (4) condition-dependent; and (5) negatively correlated with ornament size. The first two predictions receive considerable, though not ubiquitous, support from a range of animal species. However, FA in secondary sexual traits is generally not significantly heritable, indicating that FA is unlikely to reveal genetic quality that can be transmitted to offspring. Similarly, there is little evidence to support the predictions that FA is condition dependent, and that it is negatively phenotypically or genetically correlated with sexual trait size. Based on an evaluation of the evidence overall, it is concluded that this hypothesis is unlikely to be viable; it appears unlikely that mate choice for symmetry evolves by “good genes” sexual selection. Hypotheses that do not require asymmetry and phenodeviance to reveal heritable genetic quality may explain observed links between FA/phenodeviance and mating success. Results of a case study of Drosophila bipectinata are summarized, which reinforce this general conclusion. It is suggested that nonadditive genetic variation arising from an interaction between trait-specific developmental genes and genetic background may drive sexual selection for reducing developmental instability in some cases. Levels of developmental instability variation in a population may need to surpass a critical threshold for sexual selection to operate, possibly explaining some of the pronounced heterogeneity in the effect of developmental instability on sexual selection reported in the literature.     

Male morphology,performance and female mate choice of a swarming insect

1. Females of many species select mates based on important morphological and performance traits. This behaviour likely facilitates reproductive success thus exhibiting sexual selection. Most such studies have evaluated a single morphological variable, and only a minority of them studies the effects of behaviour and performance (functional capacity: Irschick et al., Evolutionary Ecology Research, 10 , 177–196, 2008) at all.
2. This study compares male morphological and performance traits differing in three variables that may correlate with fitness: condition index, flight ascent speed and resistance to torpor in relation to male mating success in the parasitoid wasp species Alabagrus texanus (Braconidae), a species where males swarm about emerging virgin females that display both choosy and non-choosy behavioural phenotypes.
3. Males that successfully mated with choosy females exhibited higher condition indices and somewhat stronger resistance to torpor than other males. Conversely, males that mated with non-choosy females did not differ in any trait from other males we measured. Early-arriving males did not have higher condition indices or greater mating success than other males.
4. Thus, both morphological and performance traits contributed to male success, which acted through female choice, indicating a role for sexual selection. Patterns of choice further differed among females, independent of male traits.

     

Sexual selection on flight endurance,flight-related morphology and physiology in a scrambling damselfly

We have limited knowledge on the mechanistic base of sexual selection, especially in scrambling species. This asks for a functional approach that explores the link between each component of the phenotype-performance-fitness axis and that includes both morphological and physiological traits. We explored the phenotype-performance-fitness axis in the scrambling damselfly Coenagrion puella by studying the links between a set of physiological and morphological traits, flight performance (flight speed and flight endurance), and short-term mating success. As expected for scrambling competition, there was sexual selection for increased flight endurance rather than for increased flight speed. For fat content, we could demonstrate the full phenotype-performance-fitness axis, where selection for a higher fat content could be explained by the sexual selection for a higher flight endurance and the positive covariation between fat content and flight endurance. For three other traits (size, relative flight muscle mass and wing loading), however, we detected selection that could not be explained via their effect on flight performance, generating novel testable hypotheses about how the covariation between these traits and mating success is generated. This also urges caution when using morphological traits as proxies for flight speed and flight endurance in phenotypic selection studies.     

Sexual selection on morphological and physiological traits and fluctuating asymmetry in the yellow dung fly

Previous univariate studies of the yellow dung fly (Scathophaga stercoraria) have demonstrated strong sexual selection, in terms of mating success, on male size (estimated as hind tibia length). To identify specific target(s) of selection on body size and possible conflicting selection pressures on particular body parts, two multivariate field studies of sexual selection were conducted. In one study using point samples from three populations, we assessed several morphological traits, including genital traits and measures of fluctuating asymmetry (FA) of all paired traits. There was sexual selection for large male size in general, confirming previous, univariate studies. With the possible exception of thorax width, which was selected in the opposite direction, no main target of selection was identified, as most morphological traits were highly correlated. There was no detectable sexual selection on the male external genital structures assessed. In a second study using multiple samples from one population, we included physiological measures of energy reserves (lipids, glucose and glycogen) known to affect mating success, in addition to trait size and FA of wings and legs. Inclusion of physiological traits is rare in phenomenological studies of selection. This study again confirmed the mating advantage of large males, and additionally showed independent positive influences of lipid and glucose but not glycogen levels. FA in paired traits generally did not affect male mating success, but was negatively correlated with energy reserves. Our study suggests that inclusion of physiological measures and genital traits in phenomenological studies of selection would be fruitful in other species.     

Sexual selection at varying population densities in male field crickets,Gryllus veletis andG. pennsylvanicus

Male field crickets call and attract females or they silently search for females in the vicinity of calling males. At high population densities, fewer calling sites are available, defense of calling sites is costly, and an increased proportion of matings should result from searching behavior. To test these predictions, individually marked field crickets, Gryllus veletisand G. pennsylvanicus,were observed for 10 h nightly in large outdoor arenas at relatively high and low densities (2020 and 55, males and females). Data were gathered on body weight, calling duration, movement, and mating frequency for individual crickets. These observations showed that variance in male mating success was significantly greater at a low density in G. pennsylvanicus,and calling duration correlated with mating success at this density. Direct selection on a trait was estimated as the partial regression coefficient (selection gradient, ) and the total selection was estimated (direct and indirect selection on correlated traits) as the covariance (standardized intensity of selection, s) of the trait on the relative mating success. Direct selection favored increased movement at a high density in G. veletis,and direct and total selection favored increased calling duration at a low density in G. pennsylvanicus.Most other comparisons were not statistically significant. The data are discussed in terms of density-dependent fluctuations in sexual selection on correlated male traits.     

DIRECT AND INDIRECT SEXUAL SELECTION AND QUANTITATIVE GENETICS OF MALE TRAITS IN GUPPIES (POECILIA RETICULATA)

Abstract.— The ornamentation and displays on which sexual attractiveness and thus mating success are based may be complex and comprise several traits. Predicting the outcome of sexual selection on such complex phenotypes requires an understanding of both the direct operation of selection on each trait and the indirect consequences of selection operating directly on genetically correlated traits. Here we report the results of a quantitative genetic analysis of the ornamentation, sexual attractiveness, and mating success of male guppies (Poecilia reticulata). We analyze male ornamentation both from the point of view of single ornamental traits (e.g., the area of each color) and of composite measures of the way the entire pattern is likely to be perceived by females (e.g., the mean and contrast in chroma). We demonstrate that there is substantial additive genetic variation in almost all measures of male ornamentation and that much of this variation may be Y linked. Attractiveness and mating success are positively correlated at the phenotypic and genetic level. Orange area and chroma, the area of a male's tail, and the color contrast of his pattern overall are positively correlated with attractiveness and/or mating success at the phenotypic and genetic levels. Using attractiveness and mating success as measures of fitness, we estimate gradients of linear directional sexual selection operating on each male trait and use equations of multivariate evolutionary change to predict the response of male ornamentation to this sexual selection. From these analyses, we predict that indirect selection may have important effects on the evolution of male guppy color patterns.     

OPERATIONAL SEX RATIO BUT NOT DENSITY AFFECTS SEXUAL SELECTION IN A FISH

The operational sex ratio (OSR) and density are considered important factors affecting the strength of sexual selection. Although there is increasing evidence that OSR and density affect the potential for sexual selection, few studies have addressed whether this is realized in phenotypic selection and how the two factors interact. We manipulated OSR (three levels) and male density (two levels) in 36 experimental breeding populations of Gobiusculus flavescens—a fish with paternal care. We measured mating competition behavior, the opportunity for selection (I), and selection on four morphological traits in males. We found sexual selection on two male traits, with the strongest selection being 20% of I. As predicted from OSR theory, increasing female scarcity caused males to become more competitive, concomitant with an increase in I and selection on morphological traits. Model simulations of I based on random mating (I_min) and maximum mate monopolization (I_max) demonstrated that the potential for sexual selection was close to its theoretical maximum across the range of OSRs. However, male density and its interaction with the OSR did not affect sexual selection. We argue that a multifaceted approach, combining mating behavior and selection analyses, can help us to understand how ecological factors affect sexual selection.     

Inbreeding depression in the competitive fertilization success of male crickets

Mating between close relatives generally results in offspring of decreased fitness. Inbreeding depression is generally greater for life-history traits than for morphological traits, and recent studies of traits subject to sexual selection suggest that these may suffer the greatest inbreeding depression. Sexual selection continues after mating in the form of sperm competition and cryptic female choice, imposing strong selection on male competitive fertilization success. Here, I examine the effects of a single generation of full-sib mating on competitive fertilization success in a cricket, Teleogryllus oceanicus. The estimated coefficient of inbreeding depression in competitive fertilization success was 0.37, higher than that for other life-history and morphological traits. Such intense inbreeding depression coupled with little or no additive genetic variance for this trait is consistent with strong directional selection on male competitive fertilization success generating high levels of dominance variance, and provides an adaptive explanation for the evolution of inbreeding avoidance found in this species.     

THE IMPORTANCE OF FEMALE CHOICE,MALE–MALE COMPETITION,AND SIGNAL TRANSMISSION AS CAUSES OF SELECTION ON MALE MATING SIGNALS

Selection on advertisement signals arises from interacting sources including female choice, male–male competition, and the communication channel (i.e., the signaling environment). To identify the contribution of individual sources of selection, we used previously quantified relationships between signal traits and each putative source to predict relationships between signal variation and fitness in Enchenopa binotata treehoppers (Hemiptera: Membracidae). We then measured phenotypic selection on signals and compared predicted and realized relationships between signal traits and mating success. We recorded male signals, then measured lifetime mating success at two population densities in a realistic environment in which sources of selection could interact. We identified which sources best predicted the relationship between signal variation and mating success using a multiple regression approach. All signal traits were under selection in at least one of the two breeding seasons measured, and in some cases selection was variable between years. Female preference was the strongest source of selection shaping male signals. The E. binotata species complex is a model of ecological speciation initiated by host shifts. Signal and preference divergence contribute to behavioral isolation within the complex, and the finding that female mate preferences drive signal evolution suggests that speciation in this group results from both ecological divergence and sexual selection.     

Courtship success and multivariate analysis of sexual selection on morphometric traits inDrosophila buzzatii (Diptera: Drosophilidae)

Using wild-reared flies, we examined sexual selection on five phenotypic traits (thorax length, wing length, wing width, head width, and face width) inDrosophila buzzatii, by scoring copulatory status in nine mass mating cages. Only male face width was identified as a direct target of sexual selection in an analysis of selection gradient, while indirect selection was present on all other studied traits, as expected from their correlations with face width. In contrast to males, there was no indication of selection in females. Nor was there evidence of assortative mating. The suggested direct selection on face width seems to take place during licking behavior of the courtship and might be related to courtship feeding. This study suggests that courtship success gives rise to indirect selection on body size.     

Impact of body melanization on mating success in Drosophila melanogaster

Mating speed and copulation duration respond rapidly to laboratory selection in Drosophila melanogaster Meigen (Diptera: Drosophilidae), but there is a lack of data on the evolutionary response to natural selection in the wild. Further, it is not clear whether body melanization and mating behavior are correlated traits. Accordingly, we tested whether variation in body color impacts on mating latency, copulation duration, and fecundity in latitudinal populations of D. melanogaster. We observed geographical variation (cline) for mating propensity, i.e., mating speed as well as copulation duration increased along latitude. Phenotypic plastic responses for body melanization at 17 and 25 °C also showed significant correlations with mating latency and copulation duration. Within‐population analysis based on assorted dark and light flies of five geographical populations showed significant positive correlations of copulation duration and fecundity with body melanization. To assess the role of males and/or females on mating speed and copulation duration, we used atypical body color strains (i.e., dark and light males of D. melanogaster) for no‐choice mating tests. Our data showed a major influence of males for copulation duration and of females for mating speed. Furthermore, a difference in impact of body melanization on mating speed and copulation duration was demonstrated between species, i.e., low melanization in Drosophila ananassae Doleschall is correlated with lower mating speed and shorter copulation duration than in D. melanogaster. Geographical changes in mating propensity were significantly correlated with body melanization at three levels, i.e., within and between populations and between species. Thus, we have shown that a relationship exists between body melanization and mating success. Further, we found seasonal changes in temperature and humidity to confer selection pressures on mating‐related traits.     

SEXUAL SELECTION THROUGH FEMALE CHOICE IN LAWES' PAROTIA,A LEK-MATING BIRD OF PARADISE

We studied sexual selection in Lawes' Parotia, a lek-mating bird of paradise, during 1981–1983 in Papua New Guinea. There was a high variance in mating success among males, with fewer than half of the individuals mating in any one year. This variance was independent of male-male interactions and disruptions. A role of female choice in sexual selection was suggested by the patterns of female visitation to courts and statistical correlations across males between phenotypic traits and mating success. Females repeatedly visited most males in their home ranges and began visiting males up to six weeks before mating. In one or more years, six aspects of male behavior and one morphological variable were positively correlated with mating success, but the probability values were not significant using a simultaneous inference test. Calculation of combined probability values across all three years revealed that one aspect of male display behavior, the probability of display, positively and significantly influenced mating status. The probability of display was also significantly correlated with relative mating success among males. Females showed strong fidelity to mates, both within and between seasons. Display sites of male Lawes' Parotia are variably dispersed, but mating success did not differ for grouped and solitary males. These data confirm an important role of female choice in sexual selection in birds of paradise but also suggest that female choice may be unrelated to the process of lek-initiation in this species.     

SEXUAL SELECTION ON BODY SIZE AND SHAPE IN THE WESTERN HARVESTER ANT,POGONOMYRMEX OCCIDENTALIS CRESSON

Mating in social insects has generally been studied in relation to reproductive allocation and relatedness. Despite the tremendous morphological diversity in social insects, little is known about how individual morphology affects mating success. We examined the correlation of male size and shape with mating success in the western harvester ant, Pogonomyrmex occidentalis. Larger males had significantly higher mating success in two independent collections of males at mating aggregations. We also detected significant linear and nonlinear selection on aspects of male shape that were consistent across years. These shape components are independent of size, suggesting that male mating success is a complex function of size and shape. Successful males had elongate thoraxes and short mandibles relative to males collected at random at the lek. Overall, mated males also had longer postpetioles relative to body size, but there was also evidence of nonlinear selection on relative postpetiole length in both years. We found no evidence of assortative mating based on size or multivariate shape measures in either year, but in one year we found weak assortative mating based on some univariate traits.     

Heat knockdown resistance and chill‐coma recovery as correlated responses to selection on mating success at high temperature in Drosophila buzzatii

Reproduction and related traits such as mating success are strongly affected by thermal stress. We tested direct and correlated responses to artificial selection in replicated lines of Drosophila buzzatii that were selected for mating success at high temperature. Knockdown resistance at high temperature (KRHT) and chill‐coma recovery (CCR) were tested as correlated selection responses. Virgin flies were allowed to mate for four hours at 33°C in three replicated lines (S lines) to obtain the selected flies and then returned at 25°C to lay eggs. Other three replicated lines were maintained at 25°C without any selection as control (C lines). After 15 selection generations, KRHT and CCR were measured. Both traits were assessed in flies that did not receive any hardening pretreatments as well as in flies that were either heat or cold hardened. Thermotolerance traits showed significant correlated responses with higher KRHT in S than in C lines, both with a heat‐hardening pretreatment and without a heat‐hardening pretreatment. CCR time was longer in S than in C lines both with a cold‐hardening pretreatment and without a cold‐hardening pretreatment. Hardening treatments improved both KRHT and CCR in all cases excepting KRHT in C lines. Overall, KRHT and CCR showed an antagonistic pattern of correlated responses to our selection regime, suggesting either pleiotropy or tightly linked trait‐specific genes partially affecting KRHT and CCR.     

Mating Success in the Polyandrous Social Wasp Vespula maculifrons

The mating decisions made by social insect males and females profoundly affect the structure of colonies and populations. However, few studies have used experimental approaches to understand mating behavior and mate choice in social insect taxa. This study investigated mating success in the polyandrous social wasp Vespula maculifrons. Mating trials were designed to test predictions that characteristics of body size and colony‐of‐origin would affect mating success. We first investigated if size differences existed among individuals and found that males from different colonies differed significantly in the size of nine morphological traits. However, male trait size was not significantly associated with male mating success. In contrast, females from different colonies differed significantly in only six of the nine measured traits, and four of these traits were associated with successful mating behaviors. Specifically, the correlated traits of gaster length, third tergum length, antennal length, and total length were positively associated with female mating success. Thus, long females experience mating advantages over females that are short. We also found that males and females from one particular colony displayed significantly greater mating activity than individuals from other colonies. Thus, the colony from which individuals originate plays an important role in determining mating success. Finally, our experiments failed to detect any evidence of nestmate avoidance during the mating trials. Overall, our data suggest that social insect reproductives may experience differential mating success based on their phenotype or developmental environment.     

The complex interplay between macronutrient intake,cuticular hydrocarbon expression and mating success in male decorated crickets

The condition dependence of male sexual traits plays a central role in sexual selection theory. Relatively little, however, is known about the condition dependence of chemical signals used in mate choice and their subsequent effects on male mating success. Furthermore, few studies have isolated the specific nutrients responsible for condition‐dependent variation in male sexual traits. Here, we used nutritional geometry to determine the effect of protein (P) and carbohydrate (C) intake on male cuticular hydrocarbon (CHC) expression and mating success in male decorated crickets (Gryllodes sigillatus). We show that both traits are maximized at a moderate‐to‐high intake of nutrients in a P:C ratio of 1 : 1.5. We also show that female precopulatory mate choice exerts a complex pattern of linear and quadratic sexual selection on this condition‐dependent variation in male CHC expression. Structural equation modelling revealed that although the effect of nutrient intake on mating success is mediated through condition‐dependent CHC expression, it is not exclusively so, suggesting that other traits must also play an important role. Collectively, our results suggest that the complex interplay between nutrient intake, CHC expression and mating success plays an important role in the operation of sexual selection in G. sigillatus.