EFFECTS OF SOCIAL AND EXTRA‐PAIR MATING ON SEXUAL SELECTION IN BLUE TITS (CYANISTES CAERULEUS)

The contribution of extra‐pair paternity (EPP) to sexual selection has received considerable attention, particularly in socially monogamous species. However, the importance of EPP remains difficult to assess quantitatively, especially when many extra‐pair young have unknown sires. Here, we combine measurements of the opportunity for selection (I), the opportunity for sexual selection (I_S), and the strength of selection on mating success (Bateman gradient, β_SS) with a novel simulation of random mating tailored to the specific mating system of the blue tit (Cyanistes caeruleus). In a population where social polygyny and EPP are common, the opportunity for sexual selection was significantly stronger and Bateman gradients significantly steeper for resident males than for females. In general, success with the social mate(s) contributed most to variation in male reproductive success. Effects of EPP were small, but significantly higher than expected under random mating. We used sibship analysis to estimate the number of unknown sires in our population. Under the assumption that the unknown sires are nonbreeding males, EPP reduced the variance in and the strength of selection on mating success, a possibility that hitherto has not been considered.     

Mating system, sexual dimorphism, and the opportunity for sexual selection in a territorial ungulate

In mammals, species with high sexual size dimorphism tend tohave highly polygynous mating systems associated with high variancein male lifetime reproductive success (LRS), leading to a highopportunity for sexual selection. However, little informationis available for species with weak sexual size dimorphism. Ina long-term study population, we used parentage analysis basedon 21 microsatellite markers to describe, for the first time,variance in male lifetime breeding success (LBS) of roe deer,a territorial ungulate where males weigh less than 10% morethan females. LBS ranged from 0 to 14 (mean = 4.54, variance= 15.5), and its distribution was highly skewed, with only afew males obtaining high LBS and many males failing to breedor siring only one fawn. As predicted for polygynous specieswith low sexual size dimorphism, the standardized variance inmale LBS was low (I_m = 0.75) and was only slightly higher thanthe standardized variance in female LRS (I_f = 0.53), suggestinga low opportunity for sexual selection. The I_m value reportedhere for roe deer is much lower than values reported for highlydimorphic ungulates such as red deer (I_m > 3). We suggestthat, along a continuum of opportunity for sexual selection,roe deer occupy a position closer to monogamous and monomorphicterritorial ungulates than to highly polygynous, sexually dimorphicungulates with dominance rank–based mating systems suchas harems or roving mating systems.     

Strong sexual selection in males against a mutation load that reduces offspring production in seed beetles

Theory predicts that sexual reproduction can increase population viability relative to asexual reproduction by allowing sexual selection in males to remove deleterious mutations from the population without large demographic costs. This requires that selection acts more strongly in males than females and that mutations affecting male reproductive success have pleiotropic effects on population productivity, but empirical support for these assumptions is mixed. We used the seed beetle Callosobruchus maculatus to implement a three‐generation breeding design where we induced mutations via ionizing radiation (IR) in the F₀ generation and measured mutational effects (relative to nonirradiated controls) on an estimate of population productivity in the F₁ and effects on sex‐specific competitive lifetime reproductive success (LRS) in the F₂. Regardless of whether mutations were induced via F₀ males or females, they had strong negative effects on male LRS, but a nonsignificant influence on female LRS, suggesting that selection is more efficient in removing deleterious alleles in males. Moreover, mutations had seemingly shared effects on population productivity and competitive LRS in both sexes. Thus, our results lend support to the hypothesis that strong sexual selection on males can act to remove the mutation load on population viability, thereby offering a benefit to sexual reproduction.     

Spatio-temporal variation of natural and sexual selection in breeding populations of the coreid bug,Colpula lativentris (Heteroptera: Coreidae)

Spatio-temporal variations of lifetime reproductive succes (LRS) of both male and female individuals of a coreid bugColpula lativentris were measured and analyzed using the multiple regression method of Arnold and Wade (1984a, b). The standardized variance of LRS was larger in males than that in females as males often to secure mates for a long period whereas females could easily find mates and oviposit simply dependent on ovarial maturation. LRS was partitioned into 4 consecutive fitness components: (1) reproductive lifespan, (2) copulating efficiency, (3) guarding efficiency (for males) or oviposition efficiency (for females), and (4) number of eggs per clutch. In males copulating efficiency was the largest determining factor of LRS, whereas in females reproductive lifespan was the most important factor. Such tendencies were stable on both a yearly and local basis. Patterns of relative contribution of natural selection (reproductive lifespan and number of eggs per clutch) and sexual selection (copulating efficiency and guarding or oviposition efficiency) to LRS were clearly different between males and females. This sexual difference is, at least to some extent, thought to be brought about by sexual selection among males for mating opportunity, though no physical fight was observed among males. Directional selection on body length was found only in relation to the clutch size of females because large females tended to lay larger clutches. No significant directional selection was found in other fitness components.     

The combined effects of pre‐ and post‐copulatory processes are masking sexual conflict over mating rate in Gerris buenoi

In polygynandrous animals, post‐copulatory processes likely interfere with precopulatory sexual selection. In water striders, sexual conflict over mating rate and post‐copulatory processes are well documented, but their combined effect on reproductive success has seldom been investigated. We combine genetic parentage analyses and behavioural observations conducted in a competitive reproductive environment to investigate how pre‐ and post‐copulatory processes influence reproductive success in Gerris buenoi Kirkaldy. Precopulatory struggles had antagonistic effects on male and female reproductive success: efficiently gaining copulations was beneficial for males, whereas efficiently avoiding copulations was profitable for females. Also, high mating rates and an intermediate optimal resistance level of females supported the hypothesis of convenience polyandry. Contrary to formal predictions, high mating rates (i.e. the number of copulations) did not increase reproductive success in males or decrease reproductive success in females. Instead, the reproductive success of both sexes was higher when offspring were produced with several partners and when there were few unnecessary matings. Thus, male and female G. buenoi displayed different interests in reproduction, but post‐copulatory processes were masking the effects of copulatory mating success on reproductive success. Given the high mating rates observed, sperm competition could easily counter the effect of mating rates, perhaps in interaction with cryptic female choice and/or fecundity selection. Our study presents a complex but realistic overview of sexual selection forces at work in a model organism for the study of sexual conflict, confirming that insights are gained from investigating all episodes in the reproduction cycle of polygynandrous animals.     

An ecological classification of odonate mating systems: the relative influence of natural, inter-and intra-sexual selection on males

We separate the mating systems of odonates into two main groups: non-resource and resource-based systems. These two groups comprise five classes of mating system: encounter-limited mating, free female choice, resource-limitation, resource-control and female-control. These classes are consistent with previous classifications of odonate mating systems and with the overall classification of mating systems by Emlen & Oring (1977: Science, 797: 215–223). Whereas Emlen & Oring's classification of mating systems was concerned with differences in sexual selection between mating systems, our classification of odonate mating systems also addresses the influence of inter-and intra-sexual selection on males within a mating system. Predictions about such relationships are useful in multivariate analysis of odonate lifetime reproduction success. Among most odonate mating systems, much of the sexual selection on males results from male-male competition for access to mates. Sexual selection via female choice is relatively less important or operates indirectly through females' choices of times or places to mate. We place resource-control and resource-limitation at opposite ends of a resource-defence continuum and postulate female choice will have greater influence in mating systems that are more like a resource-limitation system and less influence in mating systems that are more like resource-control. Sexual selection is likely to be weak in species that resort to encounter-limited mating where longevity is likely to contribute strongly to variation in reproductive success. Females have limited opportunity to exercise choice among males in the female-control mating system and in this system selection is most likely to operate on male characters which contribute to their efficiency in searching for and capturing mates. Predictions about the differences in the intensity of sexual selection between different odonate mating systems should be made on the basis of the variation in the number of potential fertilizations per male or even per ejaculate, rather than the number of fertilizable females per male. Very different mating systems could result in similar patterns of variation in male reproductive success.     

Genetic evidence for the mating system and reproductive success of black sea bream (Acanthopagrus schlegelii)

Understanding the mating system and reproductive success of a species provides evidence for sexual selection. We examined the mating system and the reproductive success of captive adult black sea bream (Acanthopagrus schlegelii), using parentage assignment based on two microsatellites multiplex PCR systems, with 91.5% accuracy in a mixed family (29 sires, 25 dams, and 200 offspring). Based on the parentage result, we found that 93.1% of males and 100% of females participated in reproduction. A total of 79% of males and 92% of females mated with multiple partners (only 1 sire and 1 dam were monogamous), indicating that polygynandry best described the genetic mating system of black sea bream. For males, maximizing the reproductive success by multiple mating was accorded with the sexual selection theory while the material benefits hypothesis may contribute to explain the multiple mating for females. For both sexes, there was a significant correlation between mating success and reproductive success and the variance in reproductive success of males was higher than females. Variation in mating success is the greatest determinant to variation in reproductive success when the relationship is strongly positive. The opportunity for sexual selection of males was twice that of females, as well as the higher slope of the Bateman curve in males suggested that the intensity of intrasexual selection of males was higher than females. Thus, male–male competition would lead to the greater variation of mating success for males, which caused greater variation in reproductive success in males. The effective population number of breeders (N_b) was 33, and the N_b/N ratio was 0.61, slightly higher than the general ratio in polygynandrous fish populations which possibly because most individuals mated and had offspring with a low variance. The relatively high N_b contributes to the maintenance of genetic diversity in farmed black sea bream populations.     

Sexual selection: harem size and the variance in male reproductive success

Sexual selection is potentially stronger than natural selection when the variance in male reproductive fitness exceeds all other components of fitness variance combined. However, measuring the variance in male reproductive fitness is difficult when nonmating males are absent, inconspicuous, or otherwise difficult to find. Omitting the nonmating males inflates estimates of average male reproductive success and diminishes the variance, leading to underestimates of the potential strength of sexual selection. We show that, in theory, the proportion of the total variance in male fitness owing to sexual selection is approximately equal to H, the mean harem size, as long as H is large and females are randomly distributed across mating males (i.e., Vharem=H). In this case, mean harem size not only provides an easy way to estimate the potential strength of sexual selection but also equals the opportunity for sexual selection, I(mates). In nature, however, females may be overdispersed with VharemH. We show that H+(k-1) is a good measure of the opportunity for sexual selection, where k is the ratio Vharem/H. A review of mating system data reveals that in nature the median ratio for Vharem/H is 1.04, but as H increases, females tend to become more aggregated across mating males with V(harem) two to three times larger than H.     

Paternity analyses in wild‐caught and laboratory‐reared Caribbean cricket females reveal the influence of mating environment on post‐copulatory sexual selection

Polyandry is ubiquitous in insects and provides the conditions necessary for male‐ and female‐driven forms of post‐copulatory sexual selection to arise. Populations of Amphiacusta sanctaecrucis exhibit significant divergence in portions of the male genitalia that are inserted directly into the female reproductive tract, suggesting that males may exercise some post‐copulatory control over fertilization success. We examine the potential for male–male and male–female post‐copulatory interactions to influence paternity in wild‐caught females of A. sanctaecrucis and contrast our findings with those obtained from females reared in a high‐density laboratory environment. We find that female A. sanctaecrucis exercise control by mating multiple times (females mount males), but that male–male post‐copulatory interactions may influence paternity success. Moreover, post‐copulatory interactions that affect reproductive success of males are not independent of mating environment: clutches of wild‐caught females exhibit higher sire diversity and lower paternity skew than clutches of laboratory‐reared females. There was no strong evidence for last male precedence in either case. Most attempts at disentangling the contributions of male–male and male–female interactions towards post‐copulatory sexual selection have been undertaken in a laboratory setting and may not capture the full context in which they take place – such as the relationship between premating and post‐mating interactions. Our results reinforce the importance of designing studies that can capture the multifaceted nature of sexual selection for elucidating the role of post‐copulatory sexual selection in driving the evolution of male and female reproductive traits, especially when different components (e.g. precopulatory and post‐copulatory interactions) do not exert independent effects on reproductive outcomes.     

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.     

Sexual conflict and the evolution of female mate choice and male social dominance

Conflicts between the sexes over control of reproduction are thought to lead to a cost of sexual selection through the evolution of male traits that manipulate female reproductive physiology and behaviour, and female traits that resist this manipulation. Although studies have begun to document negative fitness effects of sexual conflict, studies showing the expected association between sexual conflict and the specific behavioural mechanisms of sexual selection are lacking. Here we experimentally manipulated the opportunity for sexual conflict in the cockroach. Nauphoeta cinerea and showed that, for this species, odour cues in the social environment influence the behavioural strategies and fitness of males and females during sexual selection. Females provided with the opportunity for discriminating between males but not necessarily mating with preferred males produced fewer male offspring than females mated at random. The number of female offspring produced was not affected, nor was the viability of the offspring. Experimental modification of the composition of the males' pheromone showed that the fecundity effects were caused by exposure to the pheromone component that makes males attractive to females but also makes males less likely to be dominant. Female mate choice therefore carries a demographic cost but functions to avoid male manipulation and aggression. Male-male competition appears to function to circumvent mate choice rather than directly manipulating females, as the mate choice can be cryptic. The dynamic struggle between the sexes for control of mating opportunities and outcomes in N. cinerea therefore reveals a unique role for sexual conflict in the evolution of the behavioural components of sexual selection.     

Sexual selection and mating patterns in a mammal with female-biased sexual size dimorphism

In mammals, species with highly male-biased sexual size dimorphismtend to have high variance in male reproductive success. However,little information is available on patterns of sexual selection,variation in male and female reproductive success, and bodysize and mating success in species with female-biased size dimorphism.We used parentage data from microsatellite DNA loci to examinethese issues in the yellow-pine chipmunk (Tamias amoenus), asmall ground squirrel with female-biased sexual size dimorphism.Chipmunks were monitored over 3 years in the Kananaskis Valley,Alberta, Canada. We found evidence of high levels of multiplepaternity within litters. Variation in male and female reproductivesuccess was equal, and the opportunity for sexual selectionwas only marginally higher in males than females. Male and femalereproductive success both depended on mating success. We foundno evidence that the number of genetic mates a male had dependedon body size. Our results are consistent with a promiscuousmating system in which males and female mate with multiple partners.Low variation in male reproductive success may be a generalfeature of mammalian species in which females are larger thanmales.     

Pre‐ and postcopulatory sexual selection favor aggressive,young males in polyandrous groups of red junglefowl

A challenge in evolutionary biology is to understand the operation of sexual selection on males in polyandrous groups, where sexual selection occurs before and after mating. Here, we combine fine‐grained behavioral information (>41,000 interactions) with molecular parentage data to study sexual selection in replicated, age‐structured groups of polyandrous red junglefowl, Gallus gallus. Male reproductive success was determined by the number of females mated (precopulatory sexual selection) and his paternity share, which was driven by the polyandry of his female partners (postcopulatory sexual selection). Pre‐ and postcopulatory components of male reproductive success covaried positively; males with high mating success also had high paternity share. Two male phenotypes affected male pre‐ and postcopulatory performance: average aggressiveness toward rival males and age. Aggressive males mated with more females and more often with individual females, resulting in higher sexual exclusivity. Similarly, younger males mated with more females and more often with individual females, suffering less intense sperm competition than older males. Older males had a lower paternity share even allowing for their limited sexual exclusivity, indicating they may produce less competitive ejaculates. These results show that—in these populations—postcopulatory sexual selection reinforces precopulatory sexual selection, consistently promoting younger and more aggressive males.     

A STATISTICAL STUDY OF MATE CHOICE: SEXUAL SELECTION IN A PLETHODONTID SALAMANDER (DESMOGNATHUS OCHROPHAEUS)

Our experiment revealed the existence of significant variation in mating success in a salamander species in which males do not provide courtship feeding, nest sites, or parental care. Differences in mating success were based on natural variation among adult males and females, rather than on traits of an artificially selected set of potential mates. Courtship encounters deliberately involved only one male and one female, thus eliminating the potentially confounding effects of male-male competition and variations in mate encounter rate. Even after eliminating these effects and random error, some females were more likely than others to elicit spermatophore deposition by a male, and some males were more likely than others to inseminate a female. Such variation among individuals represents an opportunity for sexual selection to act on phenotypic characters that affect mating success. We advocate the use of a factorial experimental design to analyze sexual selection. This approach permits the statistical evaluation of separate male and female effects, interaction between these effects, and random effects. Designs which combine the evaluations of mating success and courtship behaviors could estimate the force of sexual selection on behavior.     

Phenology of scramble polygyny in a wild population of chrysolemid beetles: the opportunity for and the strength of sexual selection

Recent debate has highlighted the importance of estimating both the strength of sexual selection on phenotypic traits, and the opportunity for sexual selection. We describe seasonal fluctuations in mating dynamics of Leptinotarsa undecimlineata (Coleoptera: Chrysomelidae). We compared several estimates of the opportunity for, and the strength of, sexual selection and male precopulatory competition over the reproductive season. First, using a null model, we suggest that the ratio between observed values of the opportunity for sexual selections and their expected value under random mating results in unbiased estimates of the actual nonrandom mating behavior of the population. Second, we found that estimates for the whole reproductive season often misrepresent the actual value at any given time period. Third, mating differentials on male size and mobility, frequency of male fighting and three estimates of the opportunity for sexual selection provide contrasting but complementary information. More intense sexual selection associated to male mobility, but not to male size, was observed in periods with high opportunity for sexual selection and high frequency of male fights. Fourth, based on parameters of spatial and temporal aggregation of female receptivity, we describe the mating system of L. undecimlineata as a scramble mating polygyny in which the opportunity for sexual selection varies widely throughout the season, but the strength of sexual selection on male size remains fairly weak, while male mobility inversely covaries with mating success. We suggest that different estimates for the opportunity for, and intensity of, sexual selection should be applied in order to discriminate how different behavioral and demographic factors shape the reproductive dynamic of populations.     

Sexual selection in a lekking bird: the relative opportunity for selection by female choice and male competition

Leks are classic models for studies of sexual selection due to extreme variance in male reproductive success, but the relative influence of intrasexual competition and female mate choice in creating this skew is debatable. In the lekking lance-tailed manakin (Chiroxiphia lanceolata), these selective episodes are temporally separated into intrasexual competition for alpha status and female mate choice among alpha males that rarely interact. Variance in reproductive success between status classes of adult males (alpha versus non-alpha) can therefore be attributed to male-male competition whereas that within status largely reflects female mate choice. This provides an excellent opportunity for quantifying the relative contribution of each of these mechanisms of sexual selection to the overall opportunity for sexual selection on males (I males). To calculate variance in actual reproductive success, we assigned genetic paternity to 92.3% of 447 chicks sampled in seven years. Reproduction by non-alphas was rare and apparently reflected status misclassifications or opportunistic copulations en route to attaining alpha status rather than alternative mating strategies. On average 31% (range 7-44%, n=6 years) of the total I males was due to variance in reproductive success between alphas and non-alphas. Similarly, in a cohort of same-aged males followed for six years, 44-58% of the total I males was attributed to variance between males of different status. Thus, both intrasexual competition for status and female mate choice among lekking alpha males contribute substantially to the potential for sexual selection in this species.     

A measure of sexual selection for interspecific comparisons of species with diversified mating systems and different mortality schedules

Based on variances in components of lifetime reproductive success (LRS), a new index of the intensity of sexual selection was proposed mainly useful for interspecific comparisons. The index was defined as the male to female ratio of the standardized variance in mating efficiency during mating period. The index can to a large extent exclude the effect from natural selection which commonly act on both sexes in mating period and also from mortality in pre-reproductive period. This empirical measure has some defects in the strict sense (i.e., dichotomy of natural and sexual selection), however, it enables interspecific comparisons of the intensity of sexual selection among different taxonomic groups of animals with various mating systems and mortality schedules.     

The Effects of Perceived Mating Opportunities on Patterns of Reproductive Investment by Male Guppies

Males pay considerable reproductive costs in acquiring mates (precopulatory sexual selection) and in producing ejaculates that are effective at fertilising eggs in the presence of competing ejaculates (postcopulatory sexual selection). Given these costs, males must balance their reproductive investment in a given mating to optimise their future reproductive potential. Males are therefore expected to invest in reproduction prudently according to the likelihood of obtaining future matings. In this study we tested this prediction by determining whether male reproductive investment varies with expected future mating opportunities, which were experimentally manipulated by visually exposing male guppies (Poecilia reticulata) to high or low numbers of females in the absence of competing males. Our experiment did not reveal consistent effects of perceived future mating opportunity on either precopulatory (male mate choice and mating behaviour) or postcopulatory (sperm quality and quantity) investment. However, we did find that male size and female availability interacted to influence mating behaviour; large males visually deprived of females during the treatment phase became more choosy and showed greater interest in their preferred female than those given continuous visual access to females. Overall, our results suggest males tailor pre- rather than postcopulatory traits according to local female availability, but critically, these effects depend on male size.     

Sexual selection without sexual dimorphism: Bateman gradients in a simultaneous hermaphrodite

One of the most general patterns in sexual selection is stronger selection on mating activity in males than in females. This asymmetry is thought to result from the higher energetic cost of producing one female compared to one male gamete (anisogamy). However, most studies focused on gonochoric species with strong sexual dimorphism, in which males and females are necessarily under different selection regimes. The question remains whether anisogamy alone would suffice to produce such differences. In simultaneous hermaphrodites one can compare sexual selection on the male and female functions in the absence of sexual dimorphism. Here we quantify sexual selection in the hermaphroditic freshwater snail Physa acuta under laboratory conditions. We combine exhaustive behavioral records of mating activity in mating groups and molecular paternity assignment to measure the mating success and reproductive success of 120 individuals. Our results validate the prediction of stronger selection to gain mating partners in the male than in the female function. Moreover, we did not detect cross‐sex effects on fitness, or correlations between male and female production of offspring over the course of our experiment. We conclude that with respect to sexual selection P. acuta is comparable to gonochorists, confirming that anisogamy is a sufficient explanation for the differences in sexual selection regimes between sexes.     

Genetic covariance between components of male reproductive success: within‐pair vs. extra‐pair paternity in song sparrows

The evolutionary trajectories of reproductive systems, including both male and female multiple mating and hence polygyny and polyandry, are expected to depend on the additive genetic variances and covariances in and among components of male reproductive success achieved through different reproductive tactics. However, genetic covariances among key components of male reproductive success have not been estimated in wild populations. We used comprehensive paternity data from socially monogamous but genetically polygynandrous song sparrows (Melospiza melodia) to estimate additive genetic variance and covariance in the total number of offspring a male sired per year outside his social pairings (i.e. his total extra‐pair reproductive success achieved through multiple mating) and his liability to sire offspring produced by his socially paired female (i.e. his success in defending within‐pair paternity). Both components of male fitness showed nonzero additive genetic variance, and the estimated genetic covariance was positive, implying that males with high additive genetic value for extra‐pair reproduction also have high additive genetic propensity to sire their socially paired female's offspring. There was consequently no evidence of a genetic or phenotypic trade‐off between male within‐pair paternity success and extra‐pair reproductive success. Such positive genetic covariance might be expected to facilitate ongoing evolution of polygyny and could also shape the ongoing evolution of polyandry through indirect selection.