Genital damage in the orb-web spider Argiope bruennichi (Araneae: Araneidae) increases paternity success

The morphology of male genitalia often suggests functions besidessperm transfer that may have evolved under natural or sexualselection. In several species of sexually cannibalistic spiders,males damage their paired genitalia during mating, limitingthem to one copulation per pedipalp. Using a triple-mating experiment,we tested if genital damage in the orb-web spider Argiope bruennichiincreases male fitness either through facilitating his escapefrom an aggressive female or by obstructing the female's inseminationducts against future copulation attempts from other males. Wefound no survival advantage for males damaging their pedipalps;however, copulations into a previously used insemination ductwere significantly shorter when the previous male had left partsof his genitalia inside the insemination duct. Because copulationduration determines paternity in this species, our result suggeststhat male genital damage in A. bruennichi is sexually selected.By breaking off parts of their intromittent organs inside avirgin female, males can reduce sperm competition and therebyincrease their paternity success.     

A mating plug protein reduces early female remating in Drosophila melanogaster

Mating plugs are formed within the female reproductive tract during mating from male ejaculate constituents or even from male genitalia themselves. Across species, mating plugs have roles in sperm storage and the prevention of female remating. In the fruitfly Drosophila melanogaster, accessory gland proteins such as the sex peptide are known to reduce female remating, however this effect can take some time to establish, hence other ejaculate components must also be involved. We hypothesised a role for the PEBII mating plug protein in the prevention of early female remating. Using RNA interference we produced PEBII knockdown males. We found that these males were significantly less able to prevent female remating in the 4 h following mating. The mating plugs produced by PEBII knockdown males also showed lower levels of autofluorescence in the first 10 min after the start of mating, suggesting they differed in composition to those of control males. Reduced levels of PEBII had no effect, however, on fecundity, progeny production or egg-adult viability in the first 24 after mating, suggesting there were no short-term effects of PEB II on sperm transfer, storage or use. Our results show that PEBII has a subtle but significant role in the prevention of early female remating.     

Longer exaggerated male genitalia confer defensive sperm-competitive benefits in an earwig

As evidence mounts that male genitalia can affect relative fertilisation success, the role that sexual selection has played in the rapid and divergent evolution of genitalia is becoming increasingly recognized. Unfortunately, the limited functional understanding of these complex structures and their interactions with the female reproductive tract often limit interpretation regarding their evolution. Here, we address this issue using the earwig Euborellia brunneri, where both the male intromittent organ and the female spermatheca are highly exaggerated in length yet structurally simple. In a double mating design, we use the sterile male technique to study how sperm precedence patterns are affected by male genital length, male age, and the size of the male sperm storage organ, the seminal vesicle. Relative fertilisation success exhibited considerable variation around modest last-male paternity. Only an interaction between first and second male genital length affected paternity, where males gained reduced paternity when preceded by rivals with longer genitalia. Longer genitalia confer defensive benefits in sperm competition by apparently depositing ejaculate deeper in the tubular spermatheca, safe from removal by rivals. Paternity similarly depended on an interaction between the ages of both males, likely mediated by sperm traits as testes size decreased with age. Seminal vesicle size showed positive allometry but did not affect paternity; instead, greater seminal vesicle size in last males expedited oviposition. The exaggerated yet relatively simple genitalia of E. brunneri facilitate an unusually clear example of post-copulatory selection on phenotypic variation in multiple reproductive traits.     

Male copulatory sensory stimulation induces female ejection of rival sperm in a damselfly

Male damselflies possess very specialized genitalia. Females mate multiply and store sperm in two sperm storage organs, the bursa copulatrix and the spermatheca. During copulation, males physically remove the sperm stored in these organs using their genitalia. I document a novel mechanism by which males gain access to the spermatheca in Calopteryx haemorrhoidalis asturica. The mechanism is based on male stimulation of the female sensory system that controls egg fertilization and laying. During copulation, the aedeagus (a male genitalic structure indirectly involved in sperm transfer) distorts the cuticular plates in the female genital tract that bear mechanoreceptive sensilla. This stimulation results in sperm ejection from the spermatheca. Aedeagus width is positively correlated with the amount of sperm ejected. I propose that males have exploited a pre-existing female sensory bias to gain access to otherwise physically unreachable sperm. These results shed light on the issue of the origin of female preferences in current models of sexual selection and on the evolution of genitalia via sexual selection. It is postulated that females might use this process as a form of post-copulatory sexual selection on the basis of males'' genitalia.     

Copulatory behaviour and the process of intromission in Anastrepha ludens (Diptera: Tephritidae)

Complex genitalia occur in many arthropods and in some species extreme female morphologies lead to serious mechanical difficulties for males. Tephritid flies offer examples of such complex genitalia. Because of their economic importance and the extensive use of sterile male releases for tephritid control in Texas and Mexico, studies have been done on various aspects of their basic reproductive biology, but the process of intromission has received little attention. The distiphallus of the male of Anastrepha ludens is complex. One membranous sac on the distiphallus is capable of rhythmic cycles of inflation and deflation. Inflations of the sac near the base of the distiphallus probably help propel the aedeagus deeper into the female along with stiffening of the basiphallus and may drive the genital rod (which does not transfer sperm) into the ventral receptacle. We were unable to establish an association between some of the behaviours displayed by males during mating and intromission process.     

Factors Affecting Sperm Quality Before and After Mating of Calopterygid Damselflies

Damselflies (Odonata: Zygoptera) have a more complex sperm transfer system than other internally ejaculating insects. Males translocate sperm from the internal reproductive organs to the specific sperm vesicles, a small cavity on the body surface, and then transfer them into the female. To examine how the additional steps of sperm transfer contribute to decreases in sperm quality, we assessed sperm viability (the proportion of live sperm) at each stage of mating and after different storage times in male and female reproductive organs in two damselfly species, Mnais pruinosa and Calopteryx cornelia. Viability of stored sperm in females was lower than that of male stores even just after copulation. Male sperm vesicles were not equipped to maintain sperm quality for longer periods than the internal reproductive organs. However, the sperm vesicles were only used for short-term storage; therefore, this process appeared unlikely to reduce sperm viability when transferred to the female. Males remove rival sperm prior to transfer of their own ejaculate using a peculiar-shaped aedeagus, but sperm removal by males is not always complete. Thus, dilution occurs between newly received sperm and aged sperm already stored in the female, causing lower viability of sperm inside the female than that of sperm transferred by males. If females do not remate, sperm viability gradually decreases with the duration of storage. Frequent mating of females may therefore contribute to the maintenance of high sperm quality.     

Female mating status does not affect male mating behavior in the West Indian sweetpotato weevil, Euscepes postfasciatus

Because multiple mating by females encourages sperm competition, the assessment of female mating status before insemination is important for males in order to avoid the risk of sperm competition or to intensify sperm competition. When interacting with females before sperm transfer, males can alter their mating tactics according to the risk or intensity of sperm competition. Information on how mating systems are associated with sperm competition is essential for sterile insect technique eradication programs, which depend on successful mating of released sterile males with wild females. We tested whether males of the West Indian sweetpotato weevil, Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae), adjusted their mating behavior in response to female mating experience and/or age. As virgin/young female weevils accepted males more easily than non-virgin/old females, assessing females before insemination can be adaptive for male weevils. We found that E. postfasciatus males were unable to adjust their mating tactics in response to female mating status. Although this mating strategy would be costly for individual male weevils, the ability in males to discriminate female mating status can prevent the chance of mating with already-mated females containing the sperm of wild male(s). Therefore, the mating tactics of male E. postfasciatus are advantageous for sterile insect technique eradication programs.     

Strategic ejaculation in the common dung fly Sepsis cynipsea

Sperm competition has been a major selective force acting on male and female behaviour. Theory predicts that when sperm compete numerically, selection will favour males that vary the number of sperm they transfer with sperm competition risk. This often leads to increased copula duration when sperm competition risk is high, the selective advantage of which is increased paternity. We investigated the copulatory behaviour of the common dung fly Sepsis cynipsea in relation to male and female size, female mating status, age, and presence or absence of dung. This fly is unusual in that males mate-guard before copula while females use the sperm of previous males for their current clutch. Body size had no effect on copula duration, but duration of first copulations depended on female age, with older females having longer copulations. For females that copulated twice, there was an interaction between female age and mating status influencing copula duration: old females had longer copulations than young females, but second copulas were longer for young females. Residual testis size of nonvirgin males was smaller than for virgins, and testis shrinkage was significantly associated with copula duration, which indicates that males transfer more ejaculate with longer copulations. We therefore conclude that copulation duration and ejaculate transfer vary in accordance with sperm competition theory.     

A phylogenetic analysis of the evolution of sexual dimorphism and mating systems in water striders (Hemiptera: Gerridae)

Conflicts over mating decisions characterize the sexual behaviour of many insects, in particular when males encounter females that already carry enough sperm to fertilize their eggs, since a mating often will inflict greater costs than benefits upon females. Therefore, coevolutionary models predict adaptation and counter-adaptation by the sexes in a battle to control the outcome of sexual encounters. A phylogenetic analysis was performed on patterns of sexual dimorphism and mating systems within water striders (Hemiptera, Gerridae). Phylogenetic effects or 'constraints' have significantly shaped patterns of sexual dimorphism in female/ male size ratios, legs and genitalia of males, and the structure of the female abdomen. Males of ancestral gerrids were probably slightly smaller than conspecific females, had powerful fore legs adapted to grasp the female's thorax during mating, and had clasping genitalic structures suited to grasp or pinch the female posteriorly. Most gerrids have a female/male size ratio between 1.05 and 1.14, but more pronounced sexual size ratios (above 1.25) have independently evolved several times in the family, usually in association with extended post-copulatory mate guarding. The comparative, phylogenetic analysis suggests coevolution of female anticlasper and male clasping devices for the clade comprising the subfamilies Cylindrostethinae, Ptilomerinae, and Halobatinae while female anticlasper devices have evolved in the absence of male clasping genitalia in the Gerrinae. The ancestral and most common mating system in gerrids is 'scramble competition polygyny' from which has evolved 'resource defence polygyny' at least four times independently of each other. The phylogenetic effects on patterns of mating behaviour are much less obvious, as exemplified by the large amount of interspecific variation in some genera.     

Incidental Sanctions and the Evolution of Direct Benefits

Recent research has shown that a variety of traits that increase male success in mating and sperm competition can impose costs on females, resulting in antagonistic coevolution between the sexes. Yet, in many animals, females are known to receive direct benefit from their mates, including many in which female multiple mating results in intense sperm competition among males. The most common explanation for the evolution of male‐provided direct benefits is pre‐mating female choice based on benefit quality. This explanation is insufficient, however, for those direct benefits that females cannot directly assess prior to mating. Given that intrasexual selection will often favor male traits that increase female mating costs, many types of direct benefits can thus be difficult to explain. In this paper, we review four additional hypotheses for the evolution of male‐provided direct benefits, and present a fifth hypothesis that has received little attention. This latter hypothesis proposes that selection often favors female reproductive tactics that are conditional upon the past costs and benefits of mating. These conditional female reproductive tactics should evolve because the quality of the benefit provided by a previous mate can change the costs and benefits of alternative reproductive decisions. Furthermore, many of the conditional reproductive tactics we might expect females to express should incidentally penalize males which provide lower quality direct benefits. These conditional reproductive tactics may thus play an important role in determining whether females incur costs or receive benefits from their mates. In addition to favoring the evolution of direct benefits, we argue that conditional female reproductive tactics may also favor reliable signaling of benefit quality. The most common explanation for reliable signaling is the handicap mechanism, which proposes that differential costs of signaling prevent low quality males from deceptively producing attractive signals. For direct benefits, however, there is a second type of deception: males which produce attractive signals and can afford to provide high quality direct benefits may choose to cheat on the advertised benefit. The handicap mechanism does nothing to prevent cheating on direct benefits by males which can afford to produce attractive signals, and is thus insufficient for ensuring reliable signaling of benefit quality. In contrast, conditional female reproductive tactics that incidentally penalize low benefit males should also penalize males which cheat on the benefits advertised by their signals.     

Male scorpionflies assess the amount of rival sperm transferred by females' previous mates

Theory predicts that when sperm compete numerically, selection will favor males who vary the number of sperm they transfer with the immediate level of sperm competition. In this study, I measured male mating investment in response to both female mating status (virgin vs. mated) and the number of foreign sperm stored by females in a previous mating in the scorpionfly Panorpa cognata. Female sperm storage was manipulated by interrupting copulations at different time points. Female mating status did not significantly influence male mating investment, but resource-limited males invested strategically in relation to the amount of sperm stored by females in a previous mating. I found continuously decreasing male investment in response to increasing amounts of competing sperm. These results demonstrate an unprecedented male ability to assess the number of sperm stored by females. As a result, males are capable of an extraordinarily fine-tuned reaction to the intensity of sperm competition.     

Copulatory mechanism in a sexually cannibalistic spider with genital mutilation (Araneae: Araneidae: Argiope bruennichi)

Genitalia are among the fastest evolving morphological traits as evidenced by their common function as diagnostic traits in species identification. Even though the main function of genitalia is the successful transfer of spermatozoa, the presence of diverse structures that are obviously not necessary for this suggests that genitalia are a target of sexual selection. The male genitalia of many spider species are extremely complex and equipped with numerous sclerites, plates and spines whose functions are largely unknown. Selection on male genitalia may be particularly strong in sexually cannibalistic spiders, where mating success of males is restricted to a single female. We investigated the copulatory mechanism of the sexually cannibalistic orb weaving spider Argiope bruennichi by shock freezing mating pairs and revealed a complicated interaction between the appendices and sclerites that make up the male gonopods (paired pedipalps). The plate that covers the female genital opening (scape) is secured between two appendices of the male genital bulb, while three sclerites that bear the sperm duct are unfolded and extended into the female copulatory opening. During copulation, females attack and cannibalise the male and males mutilate their genitalia in about 80% of cases. Our study demonstrates that (i) genital coupling is largely accomplished on the external part of the female genitalia, (ii) that the mechanism requires an interaction between several non-sperm-transferring structures and (iii) that there are two predetermined breaking points in the male genitalia. Further comparative work on the genus Argiope will test if the copulatory mechanism with genital mutilation indeed is an adaptation to sexual cannibalism or if cannibalism is a female counter adaptation to male monopolisation through genital plugging.     

Female behaviour and the interaction of male and female genital traits mediate sperm transfer during mating

Natural selection and post‐copulatory sexual selection, including sexual conflict, contribute to genital diversification. Fundamental first steps in understanding how these processes shape the evolution of specific genital traits are to determine their function experimentally and to understand the interactions between female and male genitalia during copulation. Our experimental manipulations of male and female genitalia in red‐sided garter snakes (Thamnophis sirtalis parietalis) reveal that copulation duration and copulatory plug deposition, as well as total and oviductal/vaginal sperm counts, are influenced by the interaction between male and female genital traits and female behaviour during copulation. By mating females with anesthetized cloacae to males with spine‐ablated hemipenes using a fully factorial design, we identified significant female–male copulatory trait interactions and found that females prevent sperm from entering their oviducts by contracting their vaginal pouch. Furthermore, these muscular contractions limit copulatory plug size, whereas the basal spine of the male hemipene aids in sperm and plug transfer. Our results are consistent with a role of sexual conflict in mating interactions and highlight the evolutionary importance of female resistance to reproductive outcomes.     

Structure and allometry of genitalia in males and females of a social African ground squirrel with high polygynandry

The few studies that have looked at genital allometry in mammals have typically shown a positively allometric relationship with body size and high coefficients of variation. Cryptic female choice, sexual conflict or sperm competition are mechanisms underlying genital evolution and as these are not mutually exclusive, they are often difficult to disentangle. In addition, these mechanisms are affected by both male and female social structure and/or mating strategies and, as such, pre- and post-copulatory behaviours have been shown to alter selection on genitalia. We examined genital traits and allometry in a polygynandrous and social ground squirrel Xerus inauris . We found that male testes are positively allometric and account for 1.5% of their body weight, one of the highest percentages known for sciurids. The penis, at 42.4% of head/body length, was isometric while the female reproductive tract, 22.4% head/body length, demonstrated no such relationship. Based on the allometric relationships of both males and females presented here, in conjunction with high levels of competition for females and lack of male aggression and territoriality, we suggest that sperm competition is the most likely mechanism for the evolution of the extremely large genitalia in this species.     

Mating duration and sperm precedence in the spider <Emphasis Type="Italic">Linyphia triangularis</Emphasis>

In many animal species, mating behaviour is highly ritualised, which may allow us to relate some of its consequences, e.g. male paternity and female receptivity, to the progression of phases in the mating sequence; at the same time, ritualisation raises the question of to what extent the partners, especially the males, are able to influence the outcome of mating for their own benefit. We studied the linyphiid spider Linyphia triangularis in which mating follows a strict sequence during which the male inducts two droplets of sperm and transfers them to the female. We performed sperm competition experiments (sterile-male technique) including four treatments, in which the copulation of the first male was interrupted at prescribed phases of the mating sequence, while the second male was allowed a complete mating. Second males spent a shorter time than first males on the behaviours prior to sperm transfer, but the amount of sperm (2 droplets) and the time spent in sperm transfer were independent of the females’ mating status. The proportion of females accepting the second male depended on the mating duration of the first male, i.e. whether the first male had transferred one or two sperm droplets. After a complete first mating, most females accepted no further males. A last-male sperm precedence was apparent if only half of the first sperm droplet had been transferred by the first male, but this switched to a first male precedence if one full sperm droplet had been transferred. Thus, even in the face of sperm competition, it is sufficient for the first male to transfer one sperm droplet. The second sperm droplet and the extended copulatory courtship associated with its transfer may serve to induce a lack of receptivity in the female, but the males seem unable to enhance their reproductive success through variable copulatory tactics.     

Quantitative genetic insights into the coevolutionary dynamics of male and female genitalia

The spectacular variability that typically characterizes male genital traits has largely been attributed to the role of sexual selection. Among the evolutionary mechanisms proposed to account for this diversity, two processes in particular have generated considerable interest. On the one hand, females may exploit postcopulatory mechanisms of selection to favour males with preferred genital traits (cryptic female choice; CFC), while on the other hand females may evolve structures or behaviours that mitigate the direct costs imposed by male genitalia (sexual conflict; SC). A critical but rarely explored assumption underlying both processes is that male and female reproductive traits coevolve, either via the classic Fisherian model of preference-trait coevolution (CFC) or through sexually antagonistic selection (SC). Here, we provide evidence for this prediction in the guppy (Poecilia reticulata), a polyandrous livebearing fish in which males transfer sperm internally to females via consensual and forced matings. Our results from a paternal half-sibling breeding design reveal substantial levels of additive genetic variation underlying male genital size and morphology—two traits known to predict mating success during non-consensual matings. Our subsequent finding that physically interacting female genital traits exhibit corresponding levels of genetic (co)variation reveals the potential intersexual coevolutionary dynamics of male and female genitalia, thereby fulfilling a fundamental assumption underlying CFC and SC theory.     

Experimental modifications imply a stimulatory function for male tsetse fly genitalia,supporting cryptic female choice theory

One of the most sweeping of all patterns in morphological evolution is that animal genitalia tend to diverge more rapidly than do other structures. Abundant indirect evidence supports the cryptic female choice (CFC) explanation of this pattern, which supposes that male genitalia often function to court females during copulation; but direct experimental demonstrations of a stimulatory function have been lacking. In this study, we altered the form of two male genital structures that squeeze the female’s abdomen rhythmically in Glossina pallidipes flies. As predicted by theory, this induced CFC against the male: ovulation and sperm storage decreased, while female remating increased. Further experiments showed that these effects were due to changes in tactile stimuli received by the female from the male’s altered genitalia, and were not due to other possible changes in the males due to alteration of their genital form. Stimulation from male genital structures also induces females to permit copulation to occur. Together with previous studies of tsetse reproductive physiology, these data constitute the most complete experimental confirmation that sexual selection (probably by CFC) acts on the stimulatory properties of male genitalia.     

Male mating behaviour and sperm production characteristics under varying sperm competition risk in guppies

Since natural populations of guppies, Poecilia reticulata, often differ from one another in social structure, the intensity of sperm competition is likely to vary between localities. Guppies are promiscuous, with female choice for colourful males playing a central role in the mating system. In addition, male guppies use forced copulations to circumvent female choice. Both methods of copulation are used interchangeably by individual males, but the degree to which either is used may depend on the social environment into which males are born. Here we show that male mating behaviour varies according to the rearing sex ratio: when reared in male-biased groups, males performed more forced copulations and fewer courtship displays but showed the opposite pattern of behaviour when reared in female-biased groups. Our prediction, based on sperm competition theory, that stripped sperm number would reflect social structure was not supported by our results. Instead, the overall level of sexual activity (gonopodial thrusts+sigmoid displays) was a better predictor of sperm number in the different groups of males. Rearing density, where sex ratio was controlled, did not significantly affect male mating behaviour or sperm traits. Males reared under the different sex ratios continued to show their characteristic behaviour patterns when placed in equal sex ratio tanks. We conclude, therefore, that males adopt mating strategies to suit their social environment, and that these strategies remain fixed, for short periods at least, if population structure changes. Copyright 1999 The Association for the Study of Animal Behaviour.     

Ovoviviparity and genital evolution: a lesson from an earwig species with coercive traumatic mating and accidental breakage of elongated intromittent organs

Ovoviviparity or viviparity has evolved independently in animals and involves adaptations in females to accommodate developing embryos for a prolonged duration in their bodies, a condition which has likely to have influenced the evolution of the male genitalia. We aimed to ascertain whether the elongated male genitalia of the ovoviviparous free‐living earwig species Marava arachidis (Dermaptera: Spongiphoridae) delivers sperm directly to the female ovaries where fertilization occurs. Males mated coercively with females by grabbing the female antenna with their mouth parts. Although females resisted the mating attempts, pairs mated 3.3 times on average over 15 h. The elongated intromittent organ, known as a virga, was inserted into the long‐tubed spermatheca during insemination. Surgical ectomy of the spermatheca confirmed that sperm migrated from here to the ovaries with a variable delay. A pair of sclerites in the male genitalia frequently inflicted wounds near the spermathecal opening, while the single, thin virga sometimes broke off during mating. However, unlike earwigs bearing a ‘spare’ virga, damage was restricted to the tip of the virga, without which the males could still inseminate the females. We discuss the evolution of the genitalia in this insect in the light of sexual selection and sexual conflict over mating and fertilization.     

The evolution of sperm length in moths

Sperm form and function remain poorly understood despite being of fundamental biological importance. An instructive approach has been to examine evolutionary associations across comparable taxa between sperm characters and other, potentially selective reproductive traits. We adopt this approach here in a comparative study examining how sperm lengths are associated with male and female reproductive characters across moths. Primary data have revealed Lepidoptera to be an ideal order for examination: there is profound variation in the dimensions (but not organization) of the reproductive traits between closely related species which all share a monophyletic ancestry, for example, eupyrene sperm length varies from 110 to 12,675 microm. Eupyrene (normal fertilizing) and apyrene (anucleate and non-fertile) sperm lengths are positively correlated across taxa and both sperm types show positive associations with mating pattern (as measured by the residual testis size). At fertilization, eupyrene sperm must migrate down the often elongated female spermathecal duct from storage to unite with the ovum. Across taxa, the elongation of this duct is associated with increased eupyrene sperm length, suggesting a positive female influence on sperm size since longer, more powerful sperm may be selected to migrate and/or compete successfully down greater ductal lengths. Apyrene sperm length is not associated with female reproductive tract dimensions. However, we found a positive relationship between the residual testis volume and spermathecal volume, suggesting coevolution between male investment in spermatogenesis and the extent of the female sperm storage capacity. Within males, there is a positive association between the two organs which form the ejaculate-containing spermatophore: the testes and the accessory gland. The 'trade-up' in investment to these components is discussed in relation to paternal investment and mating patterns.