Positive correlations between pre‐ and post‐copulatory sexual traits in warblers

Theoretical models predict that investment in pre‐copulatory and post‐copulatory sexually selected traits should trade‐off. At the macroevolutionary scale, the majority of studies to date have focused on male weaponry as the target of pre‐copulatory sexual selection, but the trade‐off should equally apply to traits used to attract females, such as bird song and plumage. We studied the Old World leaf warblers (Phylloscopidae), a group of socially monogamous songbirds that experience relatively high levels of sperm competition. We examined the relationships between song duration and number of elements in the song with sperm length across 21 species, and between the same song variables and combined testes mass in a subset of these species (n = 10). Across species, these song variables and testes mass/sperm length were generally positively correlated, albeit not statistically significantly so or with borderline significance. In contrast to theory, we found no evidence for negative associations between pre‐ and post‐copulatory traits. We argue that this is a consequence of males of some species investing more into overall fertilization success (i.e. the sum of pre‐ and post‐copulatory sexual selection) than males of other species, and high fertilization success is achieved through investment into both mate attraction and sperm competition.     

Rapid evolution of testis size relative to sperm morphology suggests that post‐copulatory selection targets sperm number in Anolis lizards

Post‐copulatory sexual selection is thought to be responsible for much of the extraordinary diversity in sperm morphology across metazoans. However, the extent to which post‐copulatory selection targets sperm morphology versus sperm production is generally unknown. To address this issue, we simultaneously characterized the evolution of sperm morphology (length of the sperm head, midpiece and flagellum) and testis size (a proxy for sperm production) across 26 species of Anolis lizards, a group in which sperm competition is likely. We found that the length of the sperm midpiece has evolved 2–3 times faster than that of the sperm head or flagellum, suggesting that midpiece size may be the most important aspect of sperm morphology with respect to post‐copulatory sexual selection. However, testis size has evolved faster than any aspect of sperm morphology or body size, supporting the hypothesis that post‐copulatory sexual selection acts more strongly upon sperm production than upon sperm morphology. Likewise, evolutionary increases in testis size, which typically indicate increased sperm competition, are not associated with predictable changes in sperm morphology, suggesting that any effects of post‐copulatory selection on sperm morphology are either weak or variable in direction across anoles. Collectively, our results suggest that sperm production is the primary target of post‐copulatory sexual selection in this lineage.     

The total opportunity for sexual selection and the integration of pre‐ and post‐mating episodes of sexual selection in a complex world

It is well known that sexual selection can target reproductive traits during successive pre‐ and post‐mating episodes of selection. A key focus of recent studies has been to understand and quantify how these episodes of sexual selection interact to determine overall variance in reproductive success. In this article, we review empirical developments in this field but also highlight the considerable variability in patterns of pre‐ and post‐mating sexual selection, attributable to variation in patterns of resource acquisition and allocation, ecological and social factors, genotype‐by‐environment interaction and possible methodological factors that might obscure such patterns. Our aim is to highlight how (co)variances in pre‐ and post‐mating sexually selected traits can be sensitive to changes in a range of ecological and environmental variables. We argue that failure to capture this variation when quantifying the opportunity for sexual selection may lead to erroneous conclusions about the strength, direction or form of sexual selection operating on pre‐ and post‐mating traits. Overall, we advocate for approaches that combine measures of pre‐ and post‐mating selection across contrasting environmental or ecological gradients to better understand the dynamics of sexual selection in polyandrous species. We also discuss some directions for future research in this area.     

Evaluating the post‐copulatory sexual selection hypothesis for genital evolution reveals evidence for pleiotropic harm exerted by the male genital spines of Drosophila ananassae

The contemporary explanation for the rapid evolutionary diversification of animal genitalia is that such traits evolve by post‐copulatory sexual selection. Here, we test the hypothesis that the male genital spines of Drosophila ananassae play an adaptive role in post‐copulatory sexual selection. Whereas previous work on two Drosophila species shows that these spines function in precopulatory sexual selection to initiate genital coupling and promote male competitive copulation success, further research is needed to evaluate the potential for Drosophila genital spines to have a post‐copulatory function. Using a precision micron‐scale laser surgery technique, we test the effect of spine length reduction on copulation duration, male competitive fertilization success, female fecundity and female remating behaviour. We find no evidence that male genital spines in this species have a post‐copulatory adaptive function. Instead, females mated to males with surgically reduced/blunted genital spines exhibited comparatively greater short‐term fecundity relative to those mated by control males, indicating that the natural (i.e. unaltered) form of the trait may be harmful to females. In the absence of an effect of genital spine reduction on measured components of post‐copulatory fitness, the harm seems to be a pleiotropic side effect rather than adaptive. Results are discussed in the context of sexual conflict mediating the evolution of male genital spines in this species and likely other Drosophila.     

Reproductive delays in mammals: an unexplored avenue for post‐copulatory sexual selection

Numerous mammalian taxa exhibit reproductive delays, pauses in reproduction that occur between mating and fertilization, between fertilization and implantation of the embryo, or after an embryo has implanted. Of the 27 mammalian orders, 9 are known to exhibit reproductive delays, including Diptrotodontia, Dasyuromorphia, Eulipotyphyta, Cingulata, Carnivora, Rodentia, Chiroptera, Lagomorpha and Cetartiodactyla. Most researchers interested in delays have focused on their evolutionary origins. However, the consequences of these delays have not been considered fully. Given the lengthening of the period over which reproduction occurs, it is possible that this unique aspect of reproduction facilitates post‐copulatory sexual selection. When considered in the context of sexual selection, delays may allow sperm competition and female manipulation of fertilization (cryptic female choice) as well as other post‐copulatory processes. We investigate the potential for reproductive delays to facilitate post‐copulatory sexual selection and suggest avenues for research that may further our knowledge of sexual selection. We also provide a general review of reproductive delays in mammals.     

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.     

Old‐male paternity advantage is a function of accumulating sperm and last‐male precedence in a butterfly

Old‐male mating advantage has been convincingly demonstrated in Bicyclus anynana butterflies. This intriguing pattern may be explained by two alternative hypotheses: (i) an increased aggressiveness and persistence of older males during courtship, being caused by the older males' low residual reproductive value; and (ii) an active preference of females towards older males what reflects a good genes hypothesis. Against this background, we here investigate postcopulatory sexual selection by double‐mating Bicyclus anynana females to older and younger males, thus allowing for sperm competition and cryptic mate choice, and by genotyping the resulting offspring. Virgin females were mated with a younger virgin (2–3 days old) and afterwards an older virgin male (12–13 days old) or vice versa. Older males had a higher paternity success than younger ones, but only when being the second (=last) mating partner, while paternity success was equal among older and younger males when older males were the first mating partner. Older males produced larger spermatophores with much higher numbers of fertile sperm than younger males. Thus, we found no evidence for cryptic female mate choice. Rather, the findings reported here seem to result from a combination of last‐male precedence and the number of sperm transferred upon mating, both increasing paternity success.     

Polyandrous females produce sons that are successful at post‐copulatory competition

Some of the genetic benefit hypotheses put forward to explain multiple male mating (polyandry) predict that sons of polyandrous females will have an increased competitive ability under precopulatory or post‐copulatory competition via paternally inherited traits, such as attractiveness or fertilization efficiency. Here, we tested these predictions by comparing the competitive ability of sons of experimentally monandrous and polyandrous female bank voles (Myodes glareolus), while controlling for potential material and maternal effects. In female choice experiments, we found no clear preference for sons of either monandrous or polyandrous mothers. Moreover, neither male type was dominant over the other, indicating no advantage in precopulatory male contest competition. However, in competitive matings, sons of polyandrous mothers significantly increased their mating efforts (mating duration, intromission number). In line with this, paternity success was biased towards sons of polyandrous mothers. Because there was no evidence for maternal effects, our results suggest that female bank voles gain genetic benefits from polyandry.     

Genetic variation but weak genetic covariation between pre‐ and post‐copulatory episodes of sexual selection in Drosophila melanogaster

When females mate polyandrously, male reproductive success depends both on the male's ability to attain matings and on his ability to outcompete rival males in the fertilization of ova post‐copulation. Increased investment in  ejaculate components may trade off with investment in precopulatory traits due to resource allocation. Alternatively, pre‐ and post‐copulatory traits could be positively related if individuals can afford to invest heavily in traits advantageous at both episodes of selection. There is empirical evidence for both positive and negative associations between pre‐ and post‐copulatory episodes, but little is known about the genetic basis of these correlations. In this study, we measured morphological, chemical and behavioural precopulatory male traits and investigated their relationship with measures of male fitness (male mating success, remating inhibition and offensive sperm competitiveness) across 40 isofemale lines of Drosophila melanogaster. We found significant variation among isofemale lines, indicating a genetic basis for most of the traits investigated. However, we found weak evidence for genetic correlations between precopulatory traits and our indices of male fitness. Moreover, pre‐ and post‐copulatory episodes of selection were uncorrelated, suggesting selection may act independently at the different episodes to maximize male reproductive success.     

Determinants of sperm precedence in a noctuid moth Heliothis virescens: a role for male age

1. Females of the noctuid moth Heliothis virescens F. mate more than once. Thus, sperm from two or more males normally compete for fertilisations within the female reproductive tract. The eggs are typically fertilised by sperm from only one male, either the female's last mate or an earlier mate. Twice‐mated females store only one ejaculate's worth of fertilising sperm (eupyrene) but nearly two ejaculates' worth of a nonfertilising sperm morph (apyrene), which is thought to play a role in sperm competition. 2. The mechanism of sperm use in H. virescens was investigated by examining factors that vary with paternity, which was assigned based on allozyme variation. The factors included male and female body masses and ages, male genital characters, the size of the sperm package, and the number of sperm stored by the female. 3. One male typically gained sperm precedence; this was nearly twice as likely to be the second male as it was to be the first. Two factors were found to vary significantly with paternity: female mass and male age. The second male to mate was more likely to gain sperm precedence if the female was larger and if the male was older than the female's first mate. 4. The significance of male age and female mass to several hypothetical models of the mechanism of sperm use is discussed.     

Sexual selection and sperm quantity: meta‐analyses of strategic ejaculation

Multiple mating or group spawning leads to post‐copulatory sexual selection, which generally favours ejaculates that are more competitive under sperm competition. In four meta‐analyses we quantify the evidence that sperm competition (SC) favours greater sperm number using data from studies of strategic ejaculation. Differential investment into each ejaculate emerges at the individual level if males exhibit phenotypic plasticity in ejaculate properties in response to the likely risk and/or intensity of sperm competition after a given mating. Over the last twenty years, a series of theoretical models have been developed that predict how ejaculate size will be strategically adjusted in relation to: (a) the number of immediate rival males, with a distinction made between 0 versus 1 rival (‘risk’ of SC) and 1 versus several rivals (‘intensity’ of SC); (b) female mating status (virgin or previously mated); and (c) female phenotypic quality (e.g. female size or condition). Some well‐known studies have reported large adjustments in ejaculate size depending on the relevant social context and this has led to widespread acceptance of the claim that strategic sperm allocation occurs in response to each of these factors. It is necessary, however, to test each claim separately because it is easy to overlook studies with weak or negative findings. Compiling information on the variation in outcomes among species is potentially informative about the relevance of these assumptions in different taxa or mating systems. We found strong evidence that, on average, males transfer larger ejaculates to higher quality females. The effect of female mating status was less straightforward and depended on how ejaculate size was measured (i.e. use of proxy or direct measure). There is strong evidence that ejaculate size increased when males were exposed to a single rival, which is often described as a response to the risk of SC. There is, however, no evidence for the general prediction that ejaculate size decreases as the number of rivals increases from one to several males (i.e. in response to a higher intensity of SC which lowers the rate of return per sperm released). Our results highlight how meta‐analysis can reveal unintentional biases in narrative literature reviews. We note that several assumptions of theoretical models can alter an outcome's predicted direction in a given species (e.g. the effect of female mating status depends on whether there is first‐ or last‐male sperm priority). Many studies do not provide this background information and fail to make strong a priori predictions about the expected response of ejaculate size to manipulation of the mating context. Researchers should be explicit about which model they are testing to ensure that future meta‐analyses can better partition studies into different categories, or control for continuous moderator variables.     

Beyond sex allocation: the role of mating systems in sexual selection in parasitoid wasps

Despite the diverse array of mating systems and life histories which characterise the parasitic Hymenoptera, sexual selection and sexual conflict in this taxon have been somewhat overlooked. For instance, parasitoid mating systems have typically been studied in terms of how mating structure affects sex allocation. In the past decade, however, some studies have sought to address sexual selection in the parasitoid wasps more explicitly and found that, despite the lack of obvious secondary sexual traits, sexual selection has the potential to shape a range of aspects of parasitoid reproductive behaviour and ecology. Moreover, various characteristics fundamental to the parasitoid way of life may provide innovative new ways to investigate different processes of sexual selection. The overall aim of this review therefore is to re‐examine parasitoid biology with sexual selection in mind, for both parasitoid biologists and also researchers interested in sexual selection and the evolution of mating systems more generally. We will consider aspects of particular relevance that have already been well studied including local mating structure, sex allocation and sperm depletion. We go on to review what we already know about sexual selection in the parasitoid wasps and highlight areas which may prove fruitful for further investigation. In particular, sperm depletion and the costs of inbreeding under chromosomal sex determination provide novel opportunities for testing the role of direct and indirect benefits for the evolution of mate choice.     

Sexual rest and post‐meiotic sperm ageing in house mice

Fertilization by aged sperm can result in adverse fitness consequences for both males and females. Sperm storage during male sexual rest could provide an environment for post‐meiotic sperm senescence causing a deterioration in the quality of stored sperm, possibly impacting on both sperm performance (e.g. swimming ability) and DNA quality. Here, we compared the proportion of sperm with fragmented DNA, an indicator of structural damage of DNA within the sperm cell, among males that had been sexually rested for approximately 2 months, to that of males that had mated recently. We found no evidence of intra‐epididymal sperm DNA damage or any impairment in sperm performance, and consequently no evidence of post‐meiotic sperm senescence. Our results suggest that male house mice are likely to possess mechanisms that function to ensure that their sperm reserves remain stocked with ‘young’, viable sperm during periods of sexual inactivity. We also discuss the possibility that our experimental design leads to no difference in the age of sperm among males from the two mating treatments. Post‐meiotic sperm senescence is especially relevant under sperm competition. Thus, we sourced mice from populations that differed in their levels of post‐copulatory sexual selection, enabling us to gain insight into how selection for higher sperm production influences the rate of sperm ageing and levels of DNA fragmentation. We found that males from the population that produced the highest number of sperm also had the smallest proportion of DNA‐fragmented sperm and discuss this outcome in relation to selection acting upon males to ensure that they produce ejaculates with high‐quality sperm that are successful in achieving fertilizations under competitive conditions.     

Increased postcopulatory sexual selection reduces the intramale variation in sperm design

Sperm competition is an important force driving the evolution of sperm design and function. Inter- and intraspecific variation in sperm design are strongly influenced by the risk of sperm competition in many taxa. In contrast, the variation among sperm of one male (intramale variation) is less well understood. We investigated intramale variation in sperm design in passerine birds and found that risk of sperm competition is negatively associated with intramale variation. This result is the first clear evidence that variation among sperm within an individual male is influenced by postcopulatory sexual selection. Our finding has important implications for male traits under pre- and postcopulatory sexual selection.     

Effects of two seminal fluid transcripts on post‐mating behaviour in the simultaneously hermaphroditic flatworm Macrostomum lignano

The seminal fluid proteins (SFPs) transferred to mating partners along with sperm often play crucial roles in mediating post‐mating sexual selection. One way in which sperm donors can maximize their own reproductive success is by modifying the partner's (sperm recipient's) post‐copulatory behaviour to prevent or delay re‐mating, thereby decreasing the likelihood or intensity of sperm competition. Here, we adopted a quantitative genetic approach combining gene expression and behavioural data to identify candidates that could mediate such a response in the simultaneously hermaphroditic flatworm Macrostomum lignano. We identified two putative SFPs—Mlig‐pro46 and Mlig‐pro63—linked to both mating frequency and ‘suck’ frequency, a distinctive behaviour, in which, upon ejaculate receipt, the worm places its pharynx over its female genital opening and apparently attempts to remove the received ejaculate. We, therefore, performed a manipulative experiment using RNA interference‐induced knockdown to ask how the loss of Mlig‐pro46 and Mlig‐pro63 expression, singly and in combination, affects mating frequency, partner suck propensity and sperm competitive ability. None of the knockdown treatments impacted strongly on the mating frequency or sperm competitive ability, but knockdown of Mlig‐pro63 resulted in a significantly decreased suck propensity of mating partners. This suggests that Mlig‐pro63 may normally act as a cue in the ejaculate to trigger recipient suck behaviour and—given that other proteins in the ejaculate have the opposite effect—could be one component of an ongoing arms race between donors and recipients over the control of ejaculate fate. However, the adaptive significance of Mlig‐pro46 and Mlig‐pro63 from a donor perspective remains enigmatic.     

Polyandrous females provide sons with more competitive sperm: Support for the sexy‐sperm hypothesis in the rattlebox moth (Utetheisa ornatrix)

Given the costs of multiple mating, why has female polyandry evolved? Utetheisa ornatrix moths are well suited for studying multiple mating in females because females are highly polyandrous over their life span, with each male mate transferring a substantial spermatophore with both genetic and nongenetic material. The accumulation of resources might explain the prevalence of polyandry in this species, but another, not mutually exclusive, possibility is that females mate multiply to increase the probability that their sons will inherit more‐competitive sperm. This latter “sexy‐sperm” hypothesis posits that female multiple mating and male sperm competitiveness coevolve via a Fisherian runaway process. We tested the sexy‐sperm hypothesis by using competitive double matings to compare the sperm competition success of sons of polyandrous versus monandrous females. In accordance with sexy‐sperm theory, we found that in 511 offspring across 17 families, the male whose polyandrous mother mated once with each of three different males sired significantly more of all total offspring (81%) than did the male whose monandrous mother was mated thrice to a single male. Interestingly, sons of polyandrous mothers had a significantly biased sex ratio of their brood toward sons, also in support of the hypothesis.     

The accumulation of reproductive isolation in early stages of divergence supports a role for sexual selection

Models of speciation by sexual selection propose that male–female coevolution leads to the rapid evolution of behavioural reproductive isolation. Here, we compare the strength of behavioural isolation to ecological isolation, gametic incompatibility and hybrid inviability in a group of dichromatic stream fishes. In addition, we examine whether any of these individual barriers, or a combined measure of total isolation, is predicted by body shape differences, male colour differences, environmental differences or genetic distance. Behavioural isolation reaches the highest values of any barrier and is significantly greater than ecological isolation. No individual reproductive barrier is associated with any of the predictor variables. However, marginally significant relationships between male colour and body shape differences with ecological and behavioural isolation are discussed. Differences in male colour and body shape predict total reproductive isolation between species; hierarchical partitioning of these two variables' effects suggests a stronger role for male colour differences. Together, these results suggest an important role for divergent sexual selection in darter speciation but raise new questions about the mechanisms of sexual selection at play and the role of male nuptial ornaments.