Sperm competition games: a prospective analysis of risk assessment

Sperm competition games: the risk model can generate higher sperm allocation to virgin females

We examine the risk model in sperm competition games for cases where female fertility increases significantly with sperm numbers (sperm limitation). Without sperm competition, sperm allocation increases with sperm limitation. We define 'average risk' as the probability q that females in the population mate twice, and 'perceived risk' as the information males gain about the sperm competition probability with individual females. If males obtain no information from individual females, sperm numbers increase with q unless sperm limitation is high and one of the two competing ejaculates is strongly disfavoured. If males can distinguish between virgin and mated females, greater sperm allocation to virgins is favoured by high sperm limitation, high q, and by the second male's ejaculate being disfavoured. With high sperm limitation, sperm allocation to virgins increases and to mated females decreases with q at high q levels. With perfect information about female mating pattern, sperm allocation (i) to virgins that will mate again exceeds that to mated females and to virgins that will mate only once, (ii) to virgins that mate only once exceeds that for mated females if q is high and there is high second male disadvantage and (iii) to each type of female can decrease with q if sperm limitation is high, although the average allocation increases at least across low q levels. In general, higher sperm allocation to virgins is favoured by: strong disadvantage to the second ejaculate, high sperm limitation, high average risk and increased information (perceived risk). These conditions may apply in a few species, especially spiders.

SHORTER SPERM CONFER HIGHER COMPETITIVE FERTILIZATION SUCCESS

Spermatozoa exhibit taxonomically widespread patterns of divergent morphological evolution. However, the adaptive significance of variation in sperm morphology remains unclear. In this study we examine the role of natural variation in sperm length on fertilization success in the dung beetle Onthophagus taurus. We conducted sperm competition trials between males that differed in the length of their sperm and determined the paternity of resulting offspring using amplified fragment length polymorphism (AFLP) markers. We also quantified variation in the size and shape of the female's sperm storage organ to determine whether female morphology influenced the competitiveness of different sperm morphologies. We found that fertilization success was biased toward males with relatively shorter sperm, but that selection on sperm length was dependent on female tract morphology; selection was directional for reduced sperm length across most of the spermathecal size range, but stabilizing in females with the smallest spermathecae. Our data provide empirical support for the theory that sperm competition should favor the evolution of numerous tiny sperm. Moreover, because sperm length is both heritable and genetically correlated with condition, our results are consistent with a process by which females can accrue genetic benefits for their offspring from the incitement of sperm competition and/or cryptic female choice, as proposed by the "sexy sperm" and "good sperm" models for the evolution of polyandry.

Sperm competition games: optimal sperm allocation in response to the size of competing ejaculates

Sperm competition theory predicts that when males are certain of sperm competition, they should decrease sperm investment in matings with an increasing number of competing ejaculates. How males should allocate sperm when competing with differently sized ejaculates, however, has not yet been examined. Here, we report the outcomes of two models assuming variation in males' sperm reserves and males being faced with different amounts of competing sperm. In the first 'spawning model', two males compete instantaneously and both are able to assess the sperm competitive ability of each other. In the second 'sperm storage model', males are sequentially confronted with situations involving different levels of sperm competition, for instance different amounts of sperm already stored by the female mating partner. In both of the models, we found that optimal sperm allocation will strongly depend on the size of the male's sperm reserve. Males should always invest maximally in competition with other males that are equally strong competitors. That is, for males with small sperm reserves, our model predicts a negative correlation between sperm allocation and sperm competition intensity, whereas for males with large sperm reserves, this correlation is predicted to be positive.

Sexual Selection and Genital Evolution in Mammals: A Phylogenetic Analysis of Baculum Length

Studies in invertebrate taxa suggest that postcopulatory sexual selection is an important factor in genital evolution. However, despite wide interspecific variation in genital morphology, evidence for an influence of sexual selection on mammalian genitalia is equivocal. Here I conduct phylogenetically controlled comparative analyses across four mammalian orders to assess how one aspect of this variation--male genital length--scales with (a) male body mass and (b) relative testis mass, the latter providing an index of the intensity of sperm competition. In all four orders, baculum (=os penis) length is found to scale only weakly with male body mass. Both baculum and glans penis length in rodents and baculum length in carnivores are found to vary positively with relative testis mass. In contrast, there is no evidence to support an association between baculum length and relative testis mass in either bats or primates. These results suggest that postcopulatory sexual selection influences genital length in at least some mammals, but significant questions remain both as to why selection on the baculum should differ between mammalian groups and as to the precise mechanistic basis through which males benefit from increased genital length.

Sperm competition, mating rate and the evolution of testis and ejaculate sizes: a population model

There are various ways to estimate ejaculate expenditure. Ejaculate size or sperm number (s) is an absolute number of units of ejaculate. Relative ejaculate expenditure (E) is the expenditure on the ejaculate as the proportion of the total expenditure on all aspects of the mating, including finding and acquiring a female, and so on. Relative testis size or gonadosomatic index (sigma) is testes mass divided by body mass; it is assumed to reflect the product of mating rate (M) and ejaculate mass (s). In a new model, where mating rate, sperm competition and sperm allocation interact, and where the female's inter-clutch interval is assumed to be independent of s or M, we show that sigma is directly proportional to the mean E for a species; across species sigma and E increase monotonically with sperm competition. However, the relation between s and sperm competition across species depends on the range of sperm competition (low risk or high intensity): s increases with sperm competition at low risk levels, but decreases with sperm competition at high intensity levels. This situation arises because s alpha E/M; both E and M increase with sperm competition, but E increases differently with sperm competition in its two ranges.

Image content influences men's semen quality

There is increasing evidence from non-human animals that males adjust their ejaculate expenditure according to the risk of sperm competition. In this study we show that, after controlling for lifestyle factors known to influence semen quality, human males viewing images depicting sperm competition had a higher percentage of motile sperm in their ejaculates. Many lifestyle variables were confirmed to influence semen quality, including the recent suggestion that storage of mobile phones close to the testes can decrease semen quality.

Chemosensory assessment of sperm competition levels and the evolution of internal spermatophore guarding

Males of many species adjust their reproductive behaviour according to the perceived risk of sperm competition. Although this phenomenon is widespread in insects and other animals, the mechanisms that allow mates to assess sperm competition levels remain largely unexplored. In this study, we analysed the mating behaviour of pairs of Tenebrio molitor beetles under three odour treatments representing increasing levels of sperm competition risk (SCR) and sperm competition intensity (SCI). Copula duration and male and female post-copulatory behaviour varied significantly with odour treatment. Both copula duration and post-copulatory associations (PCAs) increased significantly in odour treatments reflecting high male density. To our knowledge, this is the first study to report that insects may assess the actual density of potential competitors at the time of mating, a cue to SCR and SCI, on the basis of chemical cues. In T. molitor, males inhibit sperm release from the spermatophore of a rival male when remating takes place at short intervals. We show that, when sperm competition levels are high, PCAs increase female remating interval just above that necessary to prevent spermatophore inhibition by rival males. This finding strongly suggests that strategic male behaviour plays a 'spermatophore guarding' role in this species. Although common in insects with external spermatophore transfer, spermatophore guarding is not expected in species with rapid ejaculate transfer and internal spermatophore delivery. Our results reveal that spermatophore guarding may evolve, even under these circumstances, as an evolutionary response to short-term spermatophore inhibition or displacement mechanisms.

Theoretical influence of female mating status and remating propensity on male sperm allocation patterns

Theoretical models predict that males should allocate more sperm in matings where the immediate risk of sperm competition is high. It has therefore often been argued that males should invest less sperm in matings with virgin females compared with matings with already mated females. However, with relatively polyandrous females, high sperm competition risk will covary with high sperm competition intensity leading to more unpredictable conditions, as high competition intensity should favour smaller ejaculates. With the use of a genetic algorithm, we found that males should allocate more sperm in matings with virgin females when female mating frequency is relatively high, whereas low remating rates will select for higher effort in matings with nonvirgin females. At higher remating rates, first male sperm precedence favours larger ejaculates in matings with virgin females and second male precedence favours the reverse. These results shed some light on several findings that have been difficult to explain adaptively by the hitherto developed theory on sperm allocation.

Intra-specific variation in strategic ejaculation according to level of polyandry in Callosobruchus chinensis

Optimal sperm allocation should differ according to the level of polyandry within a population, because the risk of sperm competition depends on the re-mating frequency of females. We compared the number of sperm ejaculated by males into the female reproductive organ between strains with different levels of polyandry in the adzuki bean beetle, Callosobruchus chinensis (Coleoptera: Bruchidae) when males were reared in different larval densities in a bean. The results showed that males derived from a population with a higher level of polyandry increased ejaculatory expenditure when they were reared under higher larval densities. We discuss the evolutionary correlation of ejaculatory expenditure to the level of polyandry.

Psychological and Physiological Adaptations to Sperm Competition in Humans

Postcopulatory competition between males, in the form of sperm competition, is a widespread phenomenon in many animal species. The extent to which sperm competition has been an important selective pressure during human evolution remains controversial, however. The authors review critically the evidence that human males and females have psychological, behavioral, and physiological adaptations that evolved in response to selection pressures associated with sperm competition. The authors consider, using evidence from contemporary societies, whether sperm competition is likely to have been a significant adaptive problem for ancestral humans and examine the evidence suggesting that human males have physiological and psychological mechanisms that allow for “prudent” sperm allocation in response to variations in the risk of sperm competition.

Sperm competition and the evolution of male reproductive anatomy in rodents

Sperm competition is a pervasive selective force in evolution, shaping reproductive anatomy, physiology and behaviour. Here, we present comparative evidence that varying sperm competition levels account for variation in the male reproductive anatomy of rodents, the largest and most diverse mammalian order. We focus on the sperm-producing testes and the accessory reproductive glands, which produce the seminal fluid fraction of the ejaculate. We demonstrate a positive association between relative testis size and the prevalence of within-litter multiple paternity, consistent with previous analyses in which relative testis size has been found to correlate with sperm competition levels inferred from social organization and mating systems. We further demonstrate an association between sperm competition level and the relative size of at least two accessory reproductive glands: the seminal vesicles and anterior prostate. The size of the major product of these glands-the copulatory plug-is also found to vary with sperm competition level. Our findings thus suggest that selection for larger plugs under sperm competition may explain variation in accessory gland size, and highlight the need to consider both sperm and non-sperm components of the male ejaculate in the context of post-copulatory sexual selection.

Antlers honestly advertise sperm production and quality

Evolutionary theory proposes that exaggerated male traits have evolved via sexual selection, either through female mate choice or male-male competition. While female preferences for ornamented males have been amply demonstrated in other taxa, among mammals sexual characters are commonly regarded as weapons whose main function is to enhance male competitiveness in agonistic encounters. One particularly controversial hypothesis to explain the function of male sexual characters proposes that they advertise male fertility. We test this hypothesis in red deer (Cervus elaphus), a species where sexual characters (antlers) reach an extreme degree of elaboration. We find that a global measure of relative antler size and complexity is associated with relative testes size and sperm velocity. Our results exclude the possibility that condition dependence, age or time of culling, drive these associations. Red deer antlers could signal male fertility to females, the ability to avoid sperm depletion throughout the reproductive season and/or the competitive ability of ejaculates. By contrast, male antlers could also signal to other males not only their competitive ability at the behavioural level (fighting ability) but also at the physiological level (sperm competition).

Sperm viability matters in insect sperm competition

Experimental studies in insects have shown how sperm competition can be a potent selective force acting on an array of male reproductive traits . However, the role of sperm quality in determining paternity in insects has been neglected, despite the fact that sperm quality has been shown to influence the outcome of sperm competition in vertebrates . A recent comparative analysis found that males of polyandrous insect species show a higher proportion of live sperm in their stores . Here, we test the hypothesis that sperm viability influences paternity at the within-species level. We use the cricket Teleogryllus oceanicus to conduct sperm competition trials involving prescreened males that differ in the viability of their sperm. We find that paternity success is determined by the proportion of live sperm in a male's ejaculate. Furthermore, we were able to predict the paternity patterns observed on the basis of the males' relative representation of viable sperm in the female's sperm-storage organ. Our findings provide the first experimental evidence for the theory that sperm competition selects for higher sperm quality in insects. Between-male variation in sperm quality needs to be considered in theoretical and experimental studies of insect sperm competition.

Co-evolution of male and female reproductive characters across the Scathophagidae (Diptera)

Sperm morphometry is extremely variable across species, but a general adaptive explanation for this diversity is lacking. As sperm must function within the female, variation in sperm form may be associated with variation in female reproductive tract morphology. We investigated this and other potential evolutionary associations between male and female reproductive characters across the Scathophagidae. Sperm length was positively associated with the length of the spermathecal (sperm store) ducts, indicating correlated evolution between the two. No association was found between sperm length and spermathecal size. However, the size of the spermathecae was positively associated with testis size indicating co-evolution between male investment in sperm production and female sperm storage capacity. Furthermore, species with a higher degree of polyandry (larger testes) had longer spermathecal ducts. However, no associations between sperm length or length variation and testis size were found which suggests greater sperm competition sensu stricto does not select for longer sperm.

Sperm competition in mammals

Although Darwin identified the evolutionary significance of competition between males in the context of reproduction, it is only in the past few decades that we have begun to appreciate the importance of competition at the gametic level. Sperm competition, defined as competition between the sperm of two or more males for fertilization of the same set of ova, is now recognised as a key selective force shaping male reproductive anatomy, physiology and behaviour across diverse animal groups, including mammals. The aim of this article is to provide a brief review of the selective consequences of sperm competition in mammals, with emphasis on recent theoretical advances and empirical controversies. Evidence of female influences on sperm competition outcomes in mammals is also discussed, and it is concluded that understanding the selective pressures driving coevolution between male and female reproductive traits remains a major challenge for researchers in this field.

Pitfalls in experiments testing predictions from sperm competition theory

As females of many species mate with more than one male, ejaculates often face competition from the sperm of other males. In recent years, numerous papers have been published on theoretical predictions of evolutionary, behavioural and physiological responses to variation in the strength of sperm competition (SC). These theoretical predictions have also been extensively tested. However, although predictions from SC theory are relatively straightforward, extra caution has to be paid in the design of experiments testing them. One difficulty is for example to disentangle immediate and mean SC risk and intensity. Without carefully designed experiments, it is also very easy to simultaneously increase SC risk and the probability of intense SC--a situation for which we currently have no clear predictions, as the theoretical models to date only assume variation in either SC risk or intensity. In this paper, we discuss these and some other pitfalls related to manipulations of SC risk and intensity and suggest how to avoid them.

Sperm competition and male ejaculate investment in Nauphoeta cinerea: effects of social environment during development

Selective pressure arising from sperm competition has been predicted to influence evolutionary and behavioural adjustment of ejaculate investment, but also may influence developmental adjustment of ejaculate investment. Immature males able to target resources strategically based on the competitive environment they will experience when they become sexually mature should be at a selective advantage. In our study we investigated how the presence of potential competitors or mates affects ejaculate and testes investment during development in the cockroach Nauphoeta cinerea, a species where males control female remating via their ejaculate size (large spermatophores prevent females from remating and therefore function to avoid sperm competition for males) and females store sperm. Our aim was to determine whether the social environment influences developmental adjustment of ejaculate investment and the relative importance of ejaculate components with different functions; avoidance of or engagement in sperm competition. We conclude that the social environment can influence developmental and behavioural flexibility in specific ejaculate components that may function to avoid or engage in sperm competition.

Male mammals respond to a risk of sperm competition conveyed by odours of conspecific males

Sperm competition occurs when a female copulates with two or more males and the sperm of those males compete within the female's reproductive tract to fertilize her eggs. The frequent occurrence of sperm competition has forced males of many species to develop different strategies to overcome the sperm of competing males. A prevalent strategy is for males to increase their sperm investment (total number of sperm allocated by a male to a particular female) after detecting a risk of sperm competition. It has been shown that the proportion of sperm that one male contributes to the sperm pool of a female is correlated with the proportion of offspring sired by that male. Therefore, by increasing his sperm investment a male may bias a potential sperm competition in his favour. Here we show that male meadow voles, Microtus pennsylvanicus, increase their sperm investment when they mate in the presence of another male's odours. Such an increase in sperm investment does not occur by augmenting the frequency of ejaculations, but by increasing the amount of sperm in a similar number of ejaculations.

Infertile matings and sperm competition: the effect of "nonsperm representation" on intraspecific variation in sperm precedence patterns

In theoretical and experimental approaches to the study of sperm competition, it is often assumed that ejaculates always contain enough sperm of good quality and that they are successfully transferred and used for fertilization. However, this view neglects the potential effects of infertility and sperm limitation. Permanent or temporal male infertility due to male sterility, insemination failures, or failures to fertilize the ova implies that some males do not achieve sperm representation in the female reproductive tract after mating. A review of the literature suggests that rates of nonsperm representation may be high; values for the proportion of infertile matings across 30 insect species vary between 0% and 63%, with the median being 22%. I simulated P2 (the proportion of offspring fathered by the second male to copulate with a female in a double-mating trial) distributions under a mechanism of random sperm mixing when sample sizes and rates of male infertility varied. The results show that nonsperm representation can be responsible for high intraspecific variance in sperm precedence patterns and that it can generate misleading interpretations about the mechanism of sperm competition. Nonsperm representation might be a common obstacle in the studies of sperm competition and postcopulatory female choice.

FEMALE RESISTANCE TO MALE HARM EVOLVES IN RESPONSE TO MANIPULATION OF SEXUAL CONFLICT

The interests of males and females over reproduction rarely coincide and conflicts between the sexes over mate choice, mating frequency, reproductive investment, and parental care are common in many taxa. In Drosophila melanogaster, the optimum mating frequency is higher for males than it is for females. Furthermore, females that mate at high frequencies suffer significant mating costs due to the actions of male seminal fluid proteins. Sexual conflict is predicted to lead to sexually antagonistic coevolution, in which selection for adaptations that benefit males but harm females is balanced by counterselection in females to minimize the extent of male-induced harm. We tested the prediction that elevated sexual conflict should select for increased female resistance to male-induced harm and vice versa. We manipulated the intensity of sexual conflict by experimentally altering adult sex ratio. We created replicated lines of D. melanogaster in which the adult sex ratio was male biased (high conflict lines), equal (intermediate conflict lines), or female biased (low conflict lines). As predicted, females from high sexual conflict lines lived significantly longer in the presence of males than did females from low conflict lines. Our conclusion that the evolutionary response in females was to the level of male-induced harm is supported by the finding that there were no female longevity differences in the absence of males. Differences between males in female harming ability were not detected. This suggests that the response in females was to differences between selection treatments in mating frequency, and not to differences in male harmfulness.

Determinants of sperm transfer in the scorpionfly Panorpa cognate: male variation, female condition and copulation duration

Recent studies suggest that sperm production and transfer may have significant costs to males. Male sperm investment into a current copulation may therefore influence resources available for future matings, which selects for male strategic mating investment. In addition, females may also benefit from actively or passively altering the number of sperm transferred by males. In the scorpionfly Panorpa cognata, the number of sperm transferred during copulation depended on copulation duration and males in good condition (residual weight) copulated longer and also transferred more sperm. Moreover, sperm transferred and stored per unit time was higher in copulations with females in good condition than in copulations with females in poor condition. Males varied greatly and consistently in their sperm transfer rate, indicative of costs associated with this trait. The duration of the pairing prelude also varied between males and correlated negatively with the male's sperm transfer rate, but no other male character correlated significantly with male sperm transfer rate. The results are consistent with strategic mating effort but sperm transfer could also be facilitated by the physical size of females and/or females in good condition may be more cooperative during sperm transfer.

Sperm competition games: sperm selection by females

We analyse a co-evolutionary sexual conflict game, in which males compete for fertilizations (sperm competition) and females operate sperm selection against unfavourable ejaculates (cryptic female choice). For simplicity, each female mates with two males per reproductive event, and the competing ejaculates are of two types, favourable (having high viability or success) or unfavourable (where progeny are less successful). Over evolutionary time, females can increase their level of sperm selection (measured as the proportion of unfavourable sperm eliminated) by paying a fecundity cost. Males can regulate sperm allocations depending on whether they will be favoured or disfavoured, but increasing sperm allocation reduces their mating rate. The resolution of this game depends on whether males are equal, or unequal. Males could be equal: each is favoured with probability, p, reflecting the proportion of females in the population that favour his ejaculate (the 'random-roles' model); different males are favoured by different sets of females. Alternatively, males could be unequal: given males are perceived consistently by all females as two distinct types, favoured and disfavoured, where p is now the frequency of the favoured male type in the population (the 'constant-types' model). In both cases, the evolutionarily stable strategy (ESS) is for females initially to increase sperm selection from zero as the viability of offspring from unfavourable ejaculates falls below that of favourable ejaculates. But in the random-roles model, sperm selection decreases again towards zero as the unfavourable ejaculates become disastrous (i.e. as their progeny viability decreases towards zero). This occurs because males avoid expenditure in unfavourable matings, to conserve sperm for matings in the favoured role where their offspring have high viability, thus allowing females to relax sperm selection. If sperm selection is costly to females, ESS sperm selection is high across a region of intermediate viabilities. If it is uncostly, there is no ESS in this region unless sperm limitation (i.e. some eggs fail to be fertilized because sperm numbers are too low) is included into the model. In the constant-types model, no relaxation of sperm selection occurs at very low viabilities of disfavoured male progeny. If sperm selection is sufficiently costly, ESS sperm selection increases as progeny viability decreases down towards zero; but if it is uncostly, there is no ESS at the lowest viabilities, and unlike the random-roles model, this cannot be stabilized by including sperm limitation. Sperm allocations in the ESS regions differ between the two models. With random roles, males always allocate more sperm in the favoured role. With constant types, the male type that is favoured allocates less sperm than the disfavoured type. These results suggests that empiricists studying cryptic female choice and sperm allocation patterns need to determine whether sperm selection is applied differently, or consistently, on given males by different females in the same population.

Experimental evidence for the evolution of numerous, tiny sperm via sperm competition

Sperm competition, when sperm from different males compete to fertilize a female's ova, is a widespread and fundamental force in the evolution of animal reproduction. The earliest prediction of sperm competition theory was that sperm competition selected for the evolution of numerous, tiny sperm, and that this force maintained anisogamy. Here, we empirically test this prediction directly by using selective breeding to generate controlled and independent variance in sperm size and number traits in the cricket Gryllus bimaculatus. We find that sperm size and number are male specific and vary independently and significantly. We can therefore noninvasively screen individuals and then run sperm competition experiments between males that differ specifically in sperm size and number traits. Paternity success across 77 two-male sperm competitions (each running over 30-day oviposition periods) shows that males producing both relatively small sperm and relatively numerous sperm win competitions for fertilization. Decreased sperm size and increased sperm number both independently predicted sperm precedence. Our findings provide direct experimental support for the theory that sperm competition selects for maximal numbers of miniaturized sperm. However, our study does not explain why G. bimaculatus sperm length persists naturally at approximately 1 mm; we discuss possibilities for this sperm size maintenance.

Relative testis size and sperm morphometry across mammals: no evidence for an association between sperm competition and sperm length

Understanding why there is extensive variation in sperm form and function across taxa has been a challenge because sperm are specialized cells operating at a microscopic level in a complex environment. This comparative study collates published data to determine whether the evolution of sperm morphometry (sperm total length and separate component dimensions) is associated with sperm competition (when different males' sperm mix and compete for a female's ova) across 83 mammalian species. We use relative testes mass as an indicator of the intensity of sperm competition across taxa: relative investment into testes is widely accepted to predict the level of sperm competition that a species or population endures. Although we found evidence for positive associations between relative testes mass (controlling for allometry) and sperm morphometry across 83 mammalian species, these relationships were phylogenetically dependent. When we appropriately controlled for phylogenetic association using multiple regression within a phylogenetic framework, there was no relationship between relative testes mass and sperm length across mammals. Furthermore, we found no evidence for associations between relative testes mass and sperm head, mid-piece or flagellar lengths, nor was there a relationship with mid-piece or mitochondrial volumes. Results, therefore, indicate that sperm competition does not select for longer or shorter sperm across mammals, and alternative forces selecting on sperm form and function are discussed.

Milt quality, parasites, and immune function in dominant and subordinate Arctic charr

Within a species, different males may display different mating strategies. For example, some males may be selected to invest in attractiveness and mate guarding, whereas others are selected for increased sperm production and sneaky breeding. In systems with a hierarchical structure, dominant males are expected to adopt mate-guarding behaviour and subordinate males sneaky-breeding behaviour. In this study, we kept wild-caught and sexually ripe male Arctic charr (Salvelinus alpinus) in size-matched pairs and determined social rank from the number of aggressive encounters. After 4 days, subordinate males showed symptoms of stress, with higher blood glucose and erythrocyte levels than dominant males. There were no differences between dominant and subordinate males in parasite intensities or immune activity, measured as levels of granulocytes and lymphocytes in blood. Although subordinate males had smaller testes than dominant males, they still had a higher density of sperm cells and higher sperm numbers relative to the size of their testes. These results can be explained as indicating adaptation of subordinate males for reproduction in an unfavourable role, always exposed to sperm competition and out of synchrony with females' egg release. Our results suggest that rapid changes in social rank may affect ejaculate production.

Sexual selection and speciation in mammals, butterflies and spiders

Recently refined evolutionary theories propose that sexual selection and reproductive conflict could be drivers of speciation. Male and female reproductive optima invariably differ because the potential reproductive rate of males almost always exceeds that of females: females are selected to maximize mate 'quality', while males can increase fitness through mate 'quantity'. A dynamic, sexually selected conflict therefore exists in which 'competitive' males are selected to override the preference tactics evolved by 'choosy' females. The wide variation across taxa in mating systems therefore generates variance in the outcome of intrasexual conflict and the strength of sexual selection: monandry constrains reproductive heterozygosity and allows female choice to select and maintain particular (preferred) genes; polyandry promotes reproductive heterozygosity and will more likely override female choice. Two different theories predict how sexual selection might influence speciation. Traditional ideas indicate that increased sexual selection (and hence conflict) generates a greater diversity of male reproductive strategies to be counteracted by female mate preferences, thus providing elevated potentials for speciation as more evolutionary avenues of male-female interaction are created. A less intuitively obvious theory proposes that increased sexual selection and conflict constrains speciation by reducing the opportunities for female mate choice under polyandry. We use a comparative approach to test these theories by investigating whether two general measures of sexual selection and the potential for sexual conflict have influenced speciation. Sexual size dimorphism (across 480 mammalian genera, 105 butterfly genera and 148 spider genera) and degree of polyandry (measured as relative testes size in mammals (72 genera) and mating frequency in female butterflies (54 genera)) showed no associations with the variance in speciosity. Our results therefore show that speciation occurs independently of sexual selection.