Sexual selection is stabilizing selection in pupfish [Cyprinodon pecosensis)

One of the predictions of the 'good genes' model of sexual selection is that reproductively successful males with well-developed indicator traits should show smaller variances for non-indicator traits, that are not directly associated with mating success, when compared to non-breeding males and females. Thus sexual selection should reinforce stabilizing natural selection in reducing the variance in quantitative traits. This prediction is tested by analysing variation in eight morphological traits of breeding males, non-breeding males, and females of pupfish (Cyprinodon pecosensis). Breeding males tended to be less variable than non-breeding males for all principal component factors, and for all morphological traits except for depth, although these differences were statistically significant only for PC2, and PC5 and for pelvic fin length, number of pelvic fin rays and number of preopercular and preorbital pores. Similarly, breeding males tended to be less variable than females for all principal component factors and for all morphological traits except for number of preopercular pores. These differences were statistically significant for PC2, and for depth, pelvic fin length, number of preorbital pores and pectoral fin rays. The overall pattern of reduced variability in independent traits of breeding males revealed by principal component analysis is very consistent and highly significant (P<10⁵). These results support the prediction of the 'good genes' model and show that reproductively active males are subject to more severe stabilizing selection for several quantitative traits than non-breeding males and females. Thus sexual selection, through male-male competition, female choice, or an interaction of both selective processes, results in stabilizing selection on quantitative morphological traits.     

Factors driving extreme sexual size dimorphism of a sit-and-wait predator under low density

Sexual size dimorphism is often a likely outcome of the interplay between natural selection and sexual selection, with female size dictated primarily by natural selection that maximizes fecundity and male size by sexual selection that maximizes reproductive opportunities. Attention to male fitness has focused heavily on direct male-male conflict selecting for superior male size and/or fighting ability, although male reproductive traits vary immensely among animals. An alternative, advanced by Michael Ghiselin, posits highly mobile dwarf males as a strategy for finding relatively immobile females in low-density populations. Adult male crab spiders Misumena vatia , sit-and-wait predators, are strikingly smaller, much more active, and relatively longer-legged than their females. This size difference results largely from males having two fewer instars than females, which simultaneously results in marked protandry. Populations of M. vatia often were small and of low density, with a female-biased sex ratio and an operational sex ratio that changed strikingly over the season. Sexual selection through scramble competition (locating the female first) should favour this suite of characters in males of low-density populations. Although direct male-male contests favoured large males, the low densities of adult males and the dispersed, relatively immobile females led to low levels of direct intrasexual contest. Females exaggerated the problem of males in finding them by providing few cues to their presence, a pattern consistent with indirect mate choice. In addition to favouring high mobility, scramble competition favoured males that selected flowers attracting many prey, the sites most often occupied by females.     

Sexual selection, stabilizing selection and fluctuating asymmetry in two populations of pupfish (Cyprinodon pecosensis)

Fluctuating asymmetry of morphological traits is thought to reflect the capacity of a genotype to produce an integrated, functional phenotype. I tested three predictions. (1) In a polygynous breeding system, under intense sexual selection on males, breeding males should show greater symmetry in bilaterally symmetrical traits than non-breeding males or females. (2) If these traits are under stabilizing selection, highly symmetrical individuals also should be modal phenotypes, thus near the mean value for that trait, whereas individuals with increased asymmetry should represent marginal phenotypes, near the extremes of the distribution for that trait. (3) Differences in the intensity of sexual selection should be reflected in differences in the degree of fluctuating asymmetry between sexes among populations. I examined the relationship between male breeding status and the degree of fluctuating asymmetry of four bilaterally symmetrical- traits, preorbital and preopercular pores and pectoral and pelvic fin rays, in two populations of Pecos pupfish which differed in the intensity of sexual selection. These traits do not function in male-male competition or female choice, thus are not directly affected by sexual selection. In Mirror Lake breeding males, as a group, were most symmetrical for all four traits, while non-breeding males and females showed higher levels of fluctuating asymmetry. Similarly, symmetrical individuals also represented modal phenotypes for four traits (breeding males), and for three traits (non-breeding males and females). These patterns were not seen in the Lake Francis population, where breeding males were as asymmetrical as non-breeding males and females, and the degree of fluctuating symmetry did not differ between modal and marginal phenotypes for any of the four traits. When ecological conditions favour intense sexual selection, either through female choice, male-male competition, or both, breeding males represent the most fit phenotypes. Thus sexual selection reinforces the effects of stabilizing selection on characters that do not function as secondary sexual traits. However, when sexual selection is relaxed, differences between sexes disappear.     

Female lizards ignore the sweet scent of success: Male characteristics implicated in female mate preference

Sexual selection molds the morphology, physiology and behavior of males in many animals. At first glance, it seems reasonable to assume that females would use the same male traits and signals in mate choice as males do during male-male competition. However, intra- and intersexual competition may affect traits in the same or the opposite direction, with differing strength. We investigated which color, morphometric and performance traits are selected for through male-male competition and whether female mate preference is based on these same traits and/or dominance status in the three male color morphs of the lizard Podarcis melisellensis. Males with relatively bigger heads and relatively higher bite forces were more likely to win fights and orange males were always dominant over the other morphs. Females, however, preferred scents of bigger males that were in better body condition, and surprisingly had lower bite force capacities. They did not show a preference for scents of any particular color morph or for scents of the more dominant males. These results indicate that intra- and intersexual competition may result in selection for different secondary sexual traits in P. melisellensis.     

Armaments and ornaments: an evolutionary explanation of traits of dual Utility

Secondary sexual characters in many species function both in male-male competition and as cues for female choice. Based on a literature compilation of existing knowledge of traits with this dual function, we propose that they commonly arise through intersexual selection processes and serve as honest signals to other males regarding fighting ability or dominance. Faking these traits, here called armaments, (i.e. weapons and status badges) is difficult, as they are constantly put to trial in male-male contests. Females that subsequently utilize them as indicators of male phenotypic quality when selecting a partner will benefit by acquiring males of higher quality to father their offspring. Thus, evolution of armaments through male-male competition is seen as a usually initiating process, whereas female choice later may assume a role as an additional selective factor. The reverse, that males use information from traits evolved through female choice, is, however, also possible. The traditional view of independently evolved and temporarily unordered intra- and intersexual selection processes fails to explain dual trait functions. Moreover, our model may more satisfyingly than traditional ones explain how trait honesty and trait genetic variance are maintained: theoretical and empirical evidence suggests that such honesty and variation are more easily maintained under male-male competition than under female choice.     

Year-round resource defence and the evolution of male and female song in suboscine birds: social armaments are mutual ornaments

The evolution of sexually monomorphic (i.e. mutual) ornamentation has attracted growing attention as a 'blind-spot' in evolutionary biology. The popular consensus is that female ornaments are subject to the same modes of sexual selection as males: intrasexual competition and mate choice. However, it remains unclear how these forces interact within and between sexes, or whether they fully capture selection on female traits. One possibility is that the 'armament-ornament' model - which proposes that traits used primarily in male-male contests are also co-opted by females as indicators of male quality - can be extended to explain signal evolution in both sexes. We examine this idea by testing the function of acoustic signals in two species of duetting antbirds. Behavioural observations and playback experiments suggest that male and female songs function primarily as armaments in competitive interactions. Removal experiments reveal that song is also a classic ornament used by unpaired males and females to advertise for mates. These results indicate that 'armament-ornament' processes may operate in reciprocal format, potentially explaining widespread mutual ornamentation in species with elevated intrasexual competition for resources. In addition, given that songs mediate competition between species outside the breeding season, our findings suggest that processes shaping monomorphic ornaments extend beyond the traditional definitions of sexual selection and are best understood in the broader framework of social selection.     

Sexual selection for male dominance reduces opportunities for female mate choice in the European bitterling (Rhodeus sericeus)

Sexual selection involves two main mechanisms: intrasexual competition for mates and intersexual mate choice. We experimentally separated intrasexual (male-male interference competition) and intersexual (female choice) components of sexual selection in a freshwater fish, the European bitterling (Rhodeus sericeus). We compared the roles of multiple morphological and behavioural traits in male success in both components of sexual competition, and their relation to male reproductive success, measured as paternity of offspring. Body size was important for both female choice and male-male competition, though females also preferred males that courted more vigorously. However, dominant males often monopolized females regardless of female preference. Subordinate males were not excluded from reproduction and sired some offspring, possibly through sneaked ejaculations. Male dominance and a greater intensity of carotenoid-based red colouration in their iris were the best predictors of male reproductive success. The extent of red iris colouration and parasite load did not have significant effects on female choice, male dominance or male reproductive success. No effect of parasite load on the expression of red eye colouration was detected, though this may have been due to low parasite prevalence in males overall. In conclusion, we showed that even though larger body size was favoured in both intersexual and intrasexual selection, male-male interference competition reduced opportunities for female choice. Females, despite being choosy, had limited control over the paternity of their offspring. Our study highlights the need for reliable measures of male reproductive success in studies of sexual selection.     

The potential for sexual selection in males: Effect of sex ratio and spatiotemporal distribution of receptive females

Summary An animal mating system characterized by male-male competition and active searching for sexually receptive females was modelled to study how varying sex ratio and spatiotemporal distribution of receptive females can affect the variance in male mating success (i. e. potential for sexual selection) in males. The temporal distribution of female receptivity periods appeared to be the variable that had the most pronounced effect on the potential for sexual selection in males. The potential for sexual selection increased monotonically as the degree of asynchrony of female reproduction increased. Female spatial distribution and sex ratio were important only when female reproduction was asynchronous. Then, the potential for sexual selection in males was at its peak when females were overdispersed in space and the sex ratio was female biased. Some of the results derived from the model analysis contrast with predictions from previous studies. The deviating results are most likely caused by different assumptions about modes of mate acquisition in males.     

Intrasexual selection in Verreaux's sifaka (Propithecus verreauxi verreauxi)

Studies of sexual selection show that both female choice and male-male competition can influence the evolution and expression of male phenotypes. In this regard, it is important to determine the functional basis through which male traits influence variation in male reproductive success. In this study, we estimate the strength and type of sexual selection acting on adult males in a population of wild lemur, Verreaux's sifaka (Propithecus verreauxi verreauxi). The data used in this study were collected at Beza Mahafaly Special Reserve, southwest Madagascar. We conducted paternity analyses on 70 males in order to estimate the distribution of reproductive success in this population. Paternity data were combined with morphometric data in order to determine which morphological traits covary with male fitness. Five morphological traits were defined in this analysis: body size, canine size, torso shape, arm shape, and leg shape. We utilized phenotypic selection models in order to determine the strength and type of selection acting directly on each trait. Our results show that directional selection acts on leg shape (a trait that is functionally related to locomotor performance), stabilizing selection acts on body mass and torso shape, and negative correlational selection acts on body mass and leg shape. We draw from biomechanical and kinematic studies of sifaka locomotion to provide a functional context for how these traits influence male mating competition within an arboreal environment. Verreaux's sifaka and many other gregarious lemurs are sexually monomorphic in body mass and canine size, despite a high frequency and intensity of male-male aggressive competition. Our results provide some insight into this paradox: in our population, there is no directional selection acting on body mass or canine size in males. The total pattern of selection implicates that behaviors relating to locomotor performance are more important than behaviors relating to fighting ability during intrasexual contests.     

Operational sex ratio and density do not affect directional selection on male sexual ornaments and behavior

Demographic parameters including operational sex ratio (OSR) and population density may influence the opportunity for, and strength of sexual selection. Traditionally, male-biased OSRs and high population densities have been thought to increase the opportunity for sexual selection on male sexual traits due to increased male competition for mates. Recent experimental evidence, however, suggests that male-biased OSRs might reduce the opportunity for sexual selection due to increased sexual coercion experienced by females. How OSR, density, and any resultant changes in the opportunity for sexual selection actually affect selection on male sexual traits is unclear. In this study, we independently manipulated OSR and density in the guppy (Poecilia reticulata) without altering the number of males present. We recorded male and female behavior and used DNA microsatellite data to assign paternity to offspring and estimate male reproductive success. We then used linear selection analyses to examine the effects of OSR and density on directional sexual selection on male behavioral and morphological traits. We found that females were pursued more by males in male-biased treatments, despite no change in individual male behavior. There were no differences in sexual behavior experienced by females or performed by males in relation to density. Neither OSR nor density significantly altered the opportunity for sexual selection. Also, Although there was significant multivariate linear selection operating on males, neither OSR nor density altered the pattern of sexual selection on male traits. Our results suggest that differences in either OSR or density (independent of the number of males present) are unlikely to alter directional evolutionary change in male sexual traits.     

Morphological Correlates of a Combat Performance Trait in the Forked Fungus Beetle, Bolitotherus cornutus

Combat traits are thought to have arisen due to intense male-male competition for access to females. While large and elaborate weapons used in attacking other males have often been the focus of sexual selection studies, defensive traits (both morphological and performance) have received less attention. However, if defensive traits help males restrict access to females, their role in the process of sexual selection could also be important. Here we examine the morphological correlates of grip strength, a defensive combat trait involved in mate guarding, in the tenebrionid beetle Bolitotherus cornutus. We found that grip strength was repeatable and differed between the sexes. However, these differences in performance were largely explained by body size and a non-additive interaction between size and leg length that differed between males and females. Our results suggest that leg size and body size interact as part of an integrated suite of defensive combat traits.     

A novel method of comparing mating success and survival reveals similar sexual and viability selection for mobility traits in female tree crickets

The relationship between sexual and viability selection in females is necessarily different than that in males, as investment in sexual traits potentially comes at the expense of both fecundity and survival. Accordingly, females do not usually invest in sexually selected traits. However, direct benefits obtained from mating, such as nuptial gifts, may encourage competition among females and subsidize investment into sexually selected traits. We compared sexual and viability selection on female tree crickets Oecanthus nigricornis, a species where females mate frequently to obtain nuptial gifts and sexual selection on females is likely. If male choice determines female mating success in this species, we expect sexual selection for fecundity traits, as males of many species prefer more fecund females. Alternatively, intrasexual scramble or combat competition on females may select for larger jumping legs or wider heads (respectively). We estimated mating success in wild caught crickets using microsatellite analysis of stored sperm and estimated relative viability by comparing surviving female O. nigricornis to those captured by a common wasp predator. In support of the scramble competition hypothesis, we found sexual selection for females with larger hind legs and narrower heads. We also found stabilizing viability selection for intermediate head width and hind leg size. As predicted, traits under viability and sexual selection were very similar, and the direction of that selection was not opposing. However, because the shape of sexual and viability selection differs, these episodes of selection may favour slightly different trait sizes.     

Path analysis and the relative importance of male-female conflict, female choice and male-male competition in water striders

Three major behavioural mechanisms underlying sexual selection in many systems are male-male competition, female choice and male-female conflict. We used path analysis to evaluate the relative importance of these mechanisms in generating sexual selection on a suite of male traits in the stream-dwelling water strider, Aquarius remigis. To gather data for the analysis, we quantified the morphology (total length, limb length, forefemur width, genital length), behaviour (activity), mating frequency, mean mating duration and overall mating success of 96 adult males over a 10-day period. Males varied considerably in mating success. Over the 10 days, individual males mated 0-14 times, spending 0-67% of their time in copula. Path analysis showed that male-male competition was important in favouring larger males with longer limbs that tended to be more active. Active males should encounter females more frequently. In water striders, almost all male-female encounters result in intense premating struggles pitting female resistance to mating against male insistence. This male-female conflict favoured larger males with longer genitalia. These traits help males overcome female resistance. Female choice was expressed via female-controlled variation in mate-guarding durations. Female choice favoured smaller males perhaps because they are lighter, less expensive ‘shields’ against harassment by other males. Male size thus affected mating success via all three behavioural mechanisms. Weak, overall sexual selection favouring larger males was the product of conflicting effects: male-female conflict strongly favouring larger males and male-male competition weakly favouring larger males, offset by female choice favouring smaller males. Separate analyses for warmer versus colder days showed that on colder days (daily high below 20 °C), male-male competition was the main mechanism generating variation in mating frequency, while on warmer days, both male-male competition and male-female conflict were important. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Sexual selection is influenced by both developmental and adult environments

Sexual selection is often assumed to be strong and consistent, yet increasing research shows it can fluctuate over space and time. Few experimental studies have examined changes in sexual selection in response to natural environmental variation. Here, we use a difference in resource quality to test for the influence of past environmental conditions and current environmental conditions on male and female mate choice and resulting selection gradients for leaf‐footed cactus bugs, Narnia femorata. We raised juveniles on natural high‐ and low‐quality diets, cactus pads with and without ripe cactus fruits. New adults were again assigned a cactus pad with or without fruit, paired with a potential mate, and observed for mating behaviors. We found developmental and adult encounter environments affected mating decisions and the resulting patterns of sexual selection for both males and females. Males were not choosy in the low‐quality encounter environment, cactus without fruit, but they avoided mating with small females in the high‐quality encounter environment. Females were choosy in both encounter environments, avoiding mating with small males. However, they were the choosiest when they were in the low‐quality encounter environment. Female mate choice was also context dependent by male developmental environment. Females were more likely to mate with males that had developed on cactus with fruit when they were currently in the cactus with fruit environment. This pattern disappeared when females were in the cactus without fruit environment. Altogether, these results experimentally demonstrate context‐dependent mate choice by both males and females. Furthermore, we demonstrate that simple, seasonal changes in resources can lead to fluctuations in sexual selection.     

Influence of male morphology on male mating status and behavior during interunit encounters in western lowland gorillas

The western lowland gorilla (Gorilla gorilla gorilla) is one of the most sexually dimorphic primate species. Mature males are twice the size of females and have grey fur on their backs and a fibrous, adipose crest on their heads. Such traits are likely to have evolved by sexual selection, either because they confer advantages during male-male fights or because females prefer males with more dimorphic traits. We developed photogrammetric methods for distance collection of morphological data from silverback males frequenting the Lokoué forest clearing in Odzala-Kokoua National Park, Republic of the Congo. Body length, head-crest size, musculature development, and extent of the grey color on the back were assessed in 87 nonbreeding and breeding mature males. Behavioral data were also collected during 312 male-male encounters involving 67 mature males in order to estimate their level of aggressiveness. The number of females belonging to a mature male positively correlated with the male crest size, body length, and musculature. Whereas morphological variables did not significantly affect the intensity of male-male encounters, the number of females attending male-male encounters strongly affected the number of agonistic displays by the two males. We discuss the mechanisms through which males with more exaggerated traits could obtain a mating advantage, namely male-male fights or female mate choice.     

Operational sex ratio influences the opportunity for sexual selection in guppies

The opportunity for sexual selection was greater when the operational sex ratio (OSR) in guppies Poecilia reticulata was biased towards males. This could be due to an increase in both male-male competition and female mate choice under male-biased OSR.     

Correlational selection leads to genetic integration of body size and an attractive plumage trait in dark-eyed juncos

When a trait's effect on fitness depends on its interaction with other traits, the resultant selection is correlational and may lead to the integration of functionally related traits. In relation to sexual selection, when an ornamental trait interacts with phenotypic quality to determine mating success, correlational sexual selection should generate genetic correlations between the ornament and quality, leading to the evolution of honest signals. Despite its potential importance in the evolution of signal honesty, correlational sexual selection has rarely been measured in natural populations. In the dark-eyed junco (Junco hyemalis), males with experimentally elevated values of a plumage trait (whiteness in the tail or "tail white") are more attractive to females and dominant in aggressive encounters over resources. We used restricted maximum-likelihood analysis of a long-term dataset to measure the heritability of tail white and two components of body size (wing length and tail length), as well as genetic correlations between pairs of these traits. We then used multiple regression to assess directional, quadratic, and correlational selection as they acted on tail white and body size via four components of lifetime fitness (juvenile and adult survival, mating success, and fecundity). We found a positive genetic correlation between tail white and body size (as measured by wing length), which indicates past correlational selection. Correlational selection, which was largely due to sexual selection on males, was also found to be currently acting on the same pair of traits. Larger males with whiter tails sired young with more females, most likely due to a combination of female choice, which favors males with whiter tails, and male-male competition, which favors both tail white and larger body size. To our knowledge, this is the first study to show both genetic correlations between sexually selected traits and currently acting correlational sexual selection, and we suggest that correlational sexual selection frequently may be an important mechanism for maintaining the honesty of sexual signals.     

Sexual dimorphism in lizard body shape: the roles of sexual selection and fecundity selection

Sexual dimorphism is widespread in lizards, with the most consistently dimorphic traits being head size (males have larger heads) and trunk length (the distance between the front and hind legs is greater in females). These dimorphisms have generally been interpreted as follows: (1) large heads in males evolve through male-male rivalry (sexual selection); and (2) larger interlimb lengths in females provide space for more eggs (fecundity selection). In an Australian lizard (the snow skink, Niveoscincus microlepidotus), we found no evidence for ongoing selection on head size. Trunk length, however, was under positive fecundity selection in females and under negative sexual selection in males. Thus, fecundity selection and sexual selection work in concert to drive the evolution of sexual dimorphism in trunk length in snow skinks.     

Measuring sexual selection on females in sex‐role‐reversed Mormon crickets (Anabrus simplex,Orthoptera: Tettigoniidae)

Although many studies examine the form of sexual selection in males, studies characterizing this selection in females remain sparse. Sexual selection on females is predicted for sex‐role‐reversed Mormon crickets, Anabrus simplex, where males are choosy of mates and nutrient‐deprived females compete for matings and nutritious nuptial gifts. We used selection analyses to describe the strength and form of sexual selection on female morphology. There was no positive linear sexual selection on the female body size traits predicted to be associated with male preferences and female competition. Instead, we detected selection for decreasing head width and mandible length, with stabilizing selection as the dominant form of nonlinear selection. Additionally, we tested the validity of a commonly used instantaneous measure of mating success by comparing selection results with those determined using cumulative mating rate. The two fitness measures yielded similar patterns of selection, supporting the common sampling method comparing mated and unmated fractions.     

Sexual selection and its evolutionary consequences in female animals

For sexual selection to act on a given sex, there must exist variation in the reproductive success of that sex as a result of differential access to mates or fertilisations. The mechanisms and consequences of sexual selection acting on male animals are well documented, but research on sexual selection acting on females has only recently received attention. Controversy still exists over whether sexual selection acts on females in the traditional sense, and over whether to modify the existing definition of sexual selection (to include resource competition) or to invoke alternative mechanisms (usually social selection) to explain selection acting on females in connection with reproduction. However, substantial evidence exists of females bearing characters or exhibiting behaviours that result in differential reproductive success that are analogous to those attributed to sexual selection in males. Here we summarise the literature and provide substantial evidence of female intrasexual competition for access to mates, female intersexual signalling to potential mates, and postcopulatory mechanisms such as competition between eggs for access to sperm and cryptic male allocation. Our review makes clear that sexual selection acts on females and males in similar ways but sometimes to differing extents: the ceiling for the elaboration of costly traits may be lower in females than in males. We predict that current and future research on female sexual selection will provide increasing support for the parsimony and utility of the existing definition of sexual selection.