Simultaneous hermaphrodites maximize their fitness by optimizing their investment into male or female functions. Allocation of resources to male function (tissues, traits, and/or behaviours increasing paternity) is predicted to increase as density, and the associated level of sperm competition, increases. We tested whether the simultaneous hermaphroditic cestode Schistocephalus solidus uses cues of potential partner densities in its fish intermediate host to improve its male reproductive success in the final host. We had two worms, one originating from a multiple infection in the fish intermediate host and one from a single infection, sequentially compete to fertilize the eggs of a third worm. The fertilization rates of the two competitors nearly always differed from the 50–50 null expectation, sometimes considerably, implying there was a ‘winner’ in each experimental competition. However, we did not find a significant effect of density in the fish host (single vs multiple) or mating order on paternity. Additional work will be needed to identify the traits and environmental conditions that explain the high variance in male reproductive success observed in this experiment.

Sperm competition occurs when the gametes of two, or more, males compete for opportunities to fertilize a given set of ova. This study examined whether mammalian sperm morphology could be affected by sexual selection. Comparisons were made of linear and volumetric measurements of the sperm head, midpiece and flagellum for 123 species representing 71 genera of mammals having either single partner mating systems or multiple partner mating systems. Sperm midpiece volume was found to be significantly larger in those species were females mate with more than one male during a single peri-ovulatory period. A positive correlation was also found between relative testes sizes and sperm midpiece volumes, while no relationship was found with any other sperm measure. These results indicate that, within mammals, the evolution of sperm midpiece volume has been affected by selection pressures resulting from sperm competition. The significance of these findings may relate to the density of mitochondria within the sperm midpiece. The mitochondria provide the energy required for sperm motility, in the absence of glycolytic support. A higher mitochondrial loading may therefore be associated with enhanced sperm motility.

In this article I begin with a few current ideas on some physiological factors that influence mating choice in insects. Emphasis is placed on those proteins produced by the male reproductive accessory glands which increase female fecundity and reduce her receptivity to subsequent males. Strategies used by late-arriving males to favour their paternity are also mentioned. With a number of insect models as background, I then review what is currently known about several male factors in ticks (a capacitation factor, a male factor, an engorgement factor and a vitellogenesis stimulating factor) and suggest where we might focus our experimental activities in the future.

The renaissance of interest in sexual selection during the last decades has fuelled an extraordinary increase of scientific papers on the subject in spiders. Research has focused both on the process of sexual selection itself, for example on the signals and various modalities involved, and on the patterns, that is the outcome of mate choice and competition depending on certain parameters. Sexual selection has most clearly been demonstrated in cases involving visual and acoustical signals but most spiders are myopic and mute, relying rather on vibrations, chemical and tactile stimuli. This review argues that research has been biased towards modalities that are relatively easily accessible to the human observer. Circumstantial and comparative evidence indicates that sexual selection working via substrate-borne vibrations and tactile as well as chemical stimuli may be common and widespread in spiders. Pattern-oriented research has focused on several phenomena for which spiders offer excellent model objects, like sexual size dimorphism, nuptial feeding, sexual cannibalism, and sperm competition. The accumulating evidence argues for a highly complex set of explanations for seemingly uniform patterns like size dimorphism and sexual cannibalism. Sexual selection appears involved as well as natural selection and mechanisms that are adaptive in other contexts only. Sperm competition has resulted in a plethora of morphological and behavioural adaptations, and simplistic models like those linking reproductive morphology with behaviour and sperm priority patterns in a straightforward way are being replaced by complex models involving an array of parameters. Male mating costs are increasingly being documented in spiders, and sexual selection by male mate choice is discussed as a potential result. Research on sexual selection in spiders has come a long way since Darwin, whose spider examples are reanalysed in the context of contemporary knowledge, but the same biases and methodological constraints have persisted almost unchanged through the current boom of research.

In species of many taxa, males alter their investment in ejaculates depending on perceived levels of male–male competition and female mating history/age. Males of the tettigoniid Acanthoplus discoidalis produce a large sperm ampulla and associated spermatophylax, resulting in a long refractory period for males. Female A. discoidalis mate once before laying eggs, and then mate and lay eggs in a random order. The males, therefore, only have a high confidence of paternity when mating with virgins. Male A. discoidalis were studied to determine if they alter their investment in sperm count, spermatophylax mass and water content of spermatophylaces when mating with virgin and non-virgin females. No difference in investment in any spermatophore characteristics between virgins and non-virgins was found, but males do reject non-virgins more often, preferring to transfer spermatophores to virgin females. Males seem to prefer females of a lower mass, where higher mass may reflect increasing female age (and hence likelihood of previous matings) through advanced development of eggs. Outright rejection, rather than changes in spermatophore investment may be because of constraints in tailoring large spermatophores that develop over relatively long periods of time between matings.

The European bitterling Rhodeus sericeus Pallas is a freshwater fish belonging to the Acheilognathinae, a subfamily that has an unusual spawning symbiosis with freshwater mussels. Female bitterling possess long ovipositors that they use to place their eggs onto the gills of a mussel through the mussels' exhalant siphon. Males fertilize the eggs by releasing sperm into the inhalant siphon of the mussel. The embryos develop inside the mussel for approximately a month, eventually leaving the mussel as actively swimming larvae. Because they use a discrete spawning site that can be readily manipulated they have recently been used in studies to understand oviposition choice with respect to mate and spawning site quality, host–parasite coevolution, alternative male mating tactics, and linking behavioural decisions with population dynamics. The current state of knowledge of bitterling reproductive ecology is reviewed and synthesized, and future directions for research on this species are proposed.

Sperm competition is widespread, resulting in sexual selection when the gametes of two or more males compete for access to a given set of ova. This study shows, for the first time, that sexual selection has influenced the evolution of the mammalian vas deferens – that is, the muscular duct which rapidly transports spermatozoa from the cauda epididymis to the ampulla during sexual activity. Morphometric studies of 103 species, representing 70 genera of mammals, show that the vas deferens is on average 33% shorter and its muscular walls are 45% thicker, in those forms where sperm competition pressures are greatest. The outer and inner longitudinal muscle layers of the vas are thickened under these conditions, whereas the central, circular muscle layer is significantly reduced. Thus, mammals that have multiple partner mating systems, and large testes in relation to body weight, have shorter and more muscular vasa deferentia than mammals where single partner mating systems, and smaller relative testes sizes, are the norm. These differences are statistically significant after application of procedures to correct for possible phylogenetic biases in the data set. Sperm competition has played an important role in the evolution of the mammalian vasa deferentia, favouring muscular specializations that enhance sperm transport.

Odonates (dragonflies) are well known for the ability of the males to displace sperm stored in the female's sperm-storage organs during copulation. By this means, copulating males are able to increase their fertilization success. This ability has been used as an example to illustrate a conflict of interests between the sexes in which males have evolved sperm-displacement mechanisms whilst females have presumably evolved means to avoid sperm displacement. The present review has four aims: (1) to describe the copulatory mechanisms used during sperm displacement; (2) to analyse the causes of sperm usage patterns; (3) to discuss this information using current hypotheses on conflict between the sexes; and (4) to illuminate topics for further research. Four copulatory mechanisms are described: sperm removal (physical withdrawal of stored sperm), sperm repositioning (‘pushing’ of rival sperm to sites where its use will be least likely), female sensory stimulation to induce sperm ejection, and sperm flushing (displacement of sperm using the copulating male's sperm). Sperm-precedence studies in Odonata are scarce and their values vary considerably between species. In those species in which sperm displacement is incomplete, the last copulating male obtains a high but variable short-term fertilization success which decreases with time. Some male and female factors affecting sperm precedence patterns are mentioned: (1) male variation in genital morphology; (2) duration of copulation influenced by the male (the longer the copulation, the more stored sperm displaced); (3) adaptations of the sperm-storage organs that allow the female to manipulate the sperm she has received (i.e. avoiding sperm displacement, re-distributing sperm masses, favouring sperm located in certain sites and ejecting sperm after copulation). We suggest that male and female odonates have co-evolved at the level of genital function with the control of stored sperm as the focus of the conflict. The benefits for males in this co-evolution lie in maximizing their fertilization success. However, it is not clear what females obtain from storing sperm and making it unreachable during sperm displacement. Two hypothetical benefits that females may obtain for which some evidence has been gathered are genetic diversity and viability genes. It is finally suggested that odonates can become excellent subjects of study for testing current ideas related to sexual conflict and speciation processes through sexual selection.

Many simultaneously hermaphroditic land snails have sperm digesting glands and multiple sperm storage organs, which it is suggested give the sperm recipient control over the number of sperm accepted and hence fertilization. In order to increase fertilization success, the sperm donor should manipulate the recipient to accept more sperm, especially so in situations of sperm competition. In helicid snails, the love dart optionally pushed into the partner's body during courtship, has been suggested to be such a manipulative tool. We investigated the influence of the sperm donor's size, sperm volume transferred and dart pushing on the volume of allosperm stored by the recipient in virgin Arianta arbustorum. In addition, we were interested in sperm storage patterns within the spermathecae. Fifty per cent of the snails already possessed a dart and tried to use it. However, most of them missed the partner. The main tubule always contained more than 50% of the total amount of sperm stored. Otherwise sperm storage patterns were very variable. The volume of sperm stored was significantly influenced only by the number of spermathecal tubules. The significance of spermathecal morphology for sperm storage needs further exploration.

Antechinus agilis mate within a 2–3 week, highly synchronized period each year, with copulation consisting of short bouts of thrusting interrupted by longer bouts of rest during which time the male remains mounted. In the laboratory, mating can last as long as 8–12 h without any break in intromission, with captive paired animals generally copulating once per day (Woolley, 1966a,b). The mating programme used in this study examines the effects on copulatory behaviour of changing: (1) timing of access relative to ovulation; (2) order of mating; (3) the delay between the first and second males' mating access. The total mount time was divided into quarters and the changes in behavioural patterns assessed by examining changes in the frequency of five key activities; thrusting, pelvic side-to-side movements, walking, female resistance, and dismounts. No significant differences were observed in the time from initial pairing to first mount regardless of mating order or time of mating, nor was the total time mounted significantly affected by mating order. Significant reductions in the total time mounted were evident, however, for those males mating closer to the time of ovulation. The behavioural strategies associated with copulation in A. agilis significantly enhance arguments for equality between males and females in determining overall mating strategies.

When females are inseminated by more than one male (polyandry) sexual selection continues after insemination in the form of sperm competition and cryptic female choice. The sexually-selected sperm hypothesis proposes that, under the risk of sperm competition, additive variation in male traits determining fertilising efficiency will select for female propensity to be polyandrous in order to increase the probability of producing sons with superior fertilising efficiency. Two factors complicate this prediction: sex-biased transmission of male fertilising efficiency traits and sexual antagonism of sex-limited traits, fostered by sex-biased inheritance. Here, we (i) review the evidence that male traits contributing towards fertilising efficiency are heritable through sex-biased mechanisms, and (ii) explore the evolutionary implications for male and female reproductive strategies caused by both sex-biased transmission and sexual antagonism of fertilising efficiency traits. Many male fertilising efficiency traits are heritable through sex-biased mechanisms and may not necessarily increase female fitness. The predictions of the sexually-selected sperm hypothesis change dramatically under these different mechanisms of inheritance of fertilising efficiency traits, and different fitness pay-offs derived by females from the expression of such traits. Both sex-biased control of fertilising efficiency and sexual antagonism may also be important in explaining the maintenance of the genetic variance and selection potential of fertilising efficiency. We propose that a useful approach to test the sexually-selected sperm hypothesis is to combine studies which identify behavioural and physiological mechanisms explaining variation in reproductive success with artificial selection experiments to infer the underlying evolutionary patterns.

Comparative studies show that variation in sperm morphometry across taxa is associated with the environment in which sperm function, and the species' mating pattern dictating the risk of sperm competition. Accordingly, sperm have evolved to function in a non-self environment (in contrast to somatic cells) and sperm morphometry is predicted to be optimized independently of the individual male producing them, but is the result of selective forces arising directly from the fertilization and competitive environment in which sperm will operate. Males within a population are therefore under stabilizing selection to produce an optimal distribution of sperm sizes. The nature of this distribution was explored using consistent techniques to measure detailed sperm morphometry for 10 species in a range of taxa from insects to humans. Although we expected variance in sperm morphometry to be optimized by every individual male through stabilizing selection at a population or species level, we found the exact opposite; for every species examined there was significant variation between individual males in the total lengths of the sperm they produced. A significant variation is reported between individual males for every species in the sizes of each sperm head, mid-piece and flagellum component. The between-male variation exists consistently in wild, domestic and human populations, subject to a wide range of levels of inbreeding. In gryllid crickets sperm length is shown to be male-specific and is repeatable between successive ejaculates. Between-female variation in ova size (data are presented for trout) is explainable by individual female fecundity optimization strategies; however, the adaptive significance of widespread between-individual variance in male gamete size is counter-intuitive and difficult to interpret, particularly as the limited evidence available shows that sperm morphometry is not condition-dependent or resource-constrained. The differences, however, do suggest negligible influences from haploid expression in the development of sperm morphometry – if haplotypic expression were manifested we would expect more profound variation within a male's sperm population (to reflect the inherent within-male variance in haplotypes derived from recombination) rather than the significant between-male differences we found that suggests the diploid control of spermatozoal phenotype

The mouse-like marsupial Antechinus agilis is common to south-eastern Australia with breeding and life-history traits being highly synchronous. Mating activity is confined to a 10- to 15-day period in August, at the end of which all males die as a result of a stress-induced suppression of the immune system. Ovulation occurs at this time and females rely on stored sperm from specialized crypts in the lower isthmus of the oviduct for successful fertilization. Here we report a high incidence of mixed paternity litters, which can be attributed to sperm from multiple males being stored in the isthmic crypts. Data from this study also suggest a possible second male siring advantage in controlled ex situ sperm competition mating trials, irrespective of the delay between the two males being given mating access to the female or of the mating time relative to ovulation. In determining paternity through DNA profiling, population genetic data were obtained that showed significant differences in the genetic heterozygosity between unrelated adults, half siblings and full siblings. We suggest that the isthmic crypts, in addition to storing viable sperm, are capable of releasing a mix of sperm that increases the likelihood of mixed paternity litters. This allows all mating males the opportunity to sire young, increases female reproductive fitness and overall maintains high levels of population genetic heterozygosity in the face of total male mortality annually.

Sperm length varies considerably amongst closely related species and the variation can be positively related to interspecific variation in the risk of sperm competition. In addition, a comparative study has shown that traits involved in sexual selection may often be sex linked. This study examined the quantitative genetics of sperm length in the yellow dung fly Scathophaga stercoraria (L.). There was a significant heritability of sperm length, h2 = 0.673. In addition, a significant non-linear epistatic interaction between determinants on the X chromosome and on the autosomes was identified in a three-generation breeding experiment. The ratio of the additive to epistatic components was approx. 5 : 1, though this is probably an overestimate, and the total proportion of the variance in sperm length accounted for by these genetic factors was > 80%. I argue that selection acting in both males and females is relevant for the evolution of this trait.