INTERSEXUAL ARMS RACE? GENITAL COEVOLUTION IN NEPHILID SPIDERS (ARANEAE,NEPHILIDAE)

Genital morphology is informative phylogenetically and strongly selected sexually. We use a recent species-level phylogeny of nephilid spiders to synthesize phylogenetic patterns in nephilid genital evolution that document generalized conflict between male and female interests. Specifically, we test the intersexual coevolution hypothesis by defining gender-specific indices of genital complexity that summarize all relevant and phylogenetically informative traits. We then use independent contrasts to show that male and female genital complexity indices correlate significantly and positively across the phylogeny rather than among sympatric sister species, as predicted by reproductive character displacement. In effect, as females respond to selection for fecundity-driven fitness via giantism and polyandry (perhaps responding to male-biased effective sex ratios), male mechanisms evolve to monopolize females (male monogamy) via opportunistic mating, pre- and postcopulatory mate guarding, and/or plugging of female genitalia to exclude subsequent suitors. In males morphological symptoms of these phenomena range from self-mutilated genitalia to total castration. Although the results are compatible with both recently favored sexual selection hypotheses, sexually antagonistic coevolution, and cryptic female choice, the evidence of strong intersexual conflict and genitalic damage in both sexes is more easily explained as sexually antagonistic coevolution due to an evolutionary arms race.     

Visual acuity in larval zebrafish: behavior and histology

Background

Mating plugs that males place onto the female genital tract are generally assumed to prevent remating with other males. Mating plugs are usually explained as a consequence of male-male competition in multiply mating species. Here, we investigated whether mating plugs also have collateral effects on female fitness. These effects are negative when plugging reduces female mating rate below an optimum. However, plugging may also be positive when plugging prevents excessive forced mating and keeps mating rate closer to a females' optimum. Here, we studied these consequences in the gonochoristic nematode Caenorhabditis remanei. We employed a new CO₂-sedation technique to interrupt matings before or after the production of a plug. We then measured mating rate, attractiveness and offspring number.

Results

The presence of a mating plug did not affect mating rate or attractiveness to roving males. Instead, females with mating plugs produced more offspring than females without copulatory plugs.

Conclusions

Our experiment suggests that plugging might have evolved under male-male competition but represents a poor protection against competing males in our experiment. Even if plugging does not reduce mating rate, our results indicate that females may benefit from being plugged in a different sense than remating prevention.     

Securing paternity: mating plugs in the dwarf spider Oedothorax retusus (Araneae: Erigoninae)

One of the various male strategies to prevent or impede female remating is the production of a mating plug that covers the female genital opening or remains inside of the female genital tract after mating. Such structures have been described for many species in many animal taxa; however, in most cases, we know little or nothing about their specific adaptive value. Our investigations demonstrate that females of the dwarf spider species Oedothorax retusus (Westring, 1851) (Linyphiidae, Erigoninae) exhibit a substance on one or both of her paired genital openings only after copulation. We performed double-mating trials and forced the second male to mate into the previously used or unused spermathecal duct of the female by amputating one of his paired male gonopods (pedipalps). Furthermore, to investigate whether the duration of the first mating has an effect on the size and efficiency of the mating plug, we interrupted first matings after either 1 or 3 min, categorized plug size and recorded mating behaviour of subsequent males. The amount of secretion transferred was larger in long compared to short copulations. A long first copulation successfully prevented subsequent males from mating into the used ducts, whereas mating success after short first matings was similar to matings into unused copulatory ducts of the females. The present study demonstrates that a male O. retusus can prevent a rival from transferring sperm into the same spermatheca by applying a mating plug, but only if he mates for long enough.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 574–583.     

Females of the Cellar Spider Discriminate Against Previous Mates

Mate choice for novel partners should evolve when remating with males of varying genetic quality provides females with fitness‐enhancing benefits. We investigated sequential mate choice for same or novel mating partners in females of the cellar spider Pholcus phalangioides (Pholcidae) to understand what drives female remating in this system. Pholcus phalangioides females are moderately polyandrous and show reluctance to remating, but double‐mated females benefit from a higher oviposition probability compared to single‐mated females. We exposed mated females to either their former (same male) or a novel mating partner and assessed mating success together with courtship and copulatory behaviours in both sexes. We found clear evidence for mate discrimination: females experienced three‐fold higher remating probabilities with novel males, being more often aggressive towards former males and accepting novel males faster in the second than in the first mating trial. The preference for novel males suggests that remating is driven by benefits derived from multiple partners. The low remating rates and the strong last male sperm precedence in this system suggest that mating with novel partners that represent alternative genotypes may be a means for selecting against a former mate of lower quality.     

Transition from monogyny to polygyny in Nephila senegalensis (Araneae: Nephilidae) is not accompanied by increased investment in sperm

A common male adaptation to prevent sperm competition is the placement of a mating plug. Such plugs are considered as an extreme investment if they comprise parts of the genital systems and render the male sterile. Genital mutilation occurs in monogynous spiders of several families and may co‐occur with permanent sperm depletion, meaning that sperm production is terminated once males become mature. Within the orb‐web spider genus Nephila, monogynous mating strategies are considered ancestral, although some species have reverted to a polygynous mating strategy. Although genital mutilation does not occur in these species, permanent sperm depletion (PSD) remained. We compared investment in sperm between an effectively plugging (Nephila fenestrata Thorell, 1859) and a closely‐related nonplugging species [Nephila senegalensis (Walckenaer, 1841)]. Sperm investment should be higher in N. senegalensis because males are able to mate with several females, whereas N. fenestrata males can only achieve a maximum of two copulations, generally performed with the same female. The absence of a plugging mechanism in N. senegalensis and the inability to monopolize females by means of mating plugs results in a higher risk of sperm competition. Thus, we predicted higher investment in sperm producing tissue and larger sperm storage organs in males of N. senegalensis compared to N. fenestrata. We examined the testes and deferent ducts of both species for size and cell‐quality differences using light and transmission electron microscopy and analyzed the volume of the sperm reservoir in the male copulatory organ (i.e. spermophor) using X‐ray microcomputed tomography. In contrast to our prediction, the lumen of testes, deferent ducts, and spermophor of N. senegalensis males were significantly smaller than in N. fenestrata.     

No Effect of Male Courtship Intensity on Female Remating in the Butterfly <Emphasis Type="Italic">Pieris napi</Emphasis>

In Pieris napi, female fitness increases with number of matings, but wild females mate at an unexpectedly low rate. From a sexual conflict perspective this could be because males manipulate female remating, or alternatively, because wild females experience costs associated with remating which are not applicable under laboratory conditions. To get an indication which sex controls remating and/or the different sexes’ relative costs and benefits of remating, we here test whether female mating frequency is affected by male courtship intensity. We found no effect on female mating frequency or lifespan. This indicates that (i) females control remating and their optimal mating frequency is lower compared to males, or (ii) males can manipulate female remating. We argue that both these alternatives may apply simultaneously to P. napiand that they are inseparable.     

The copulatory plug delays ejaculation by rival males and affects sperm competition outcome in house mice

Females of many species mate with multiple males (polyandry), resulting in male–male competition extending to post‐copulation (sperm competition). Males adapt to such post‐copulatory sexual selection by altering features of their ejaculate that increase its competitiveness and/or by decreasing the risk of sperm competition through female manipulation or interference with rival male behaviour. At ejaculation, males of many species deposit copulatory plugs, which are commonly interpreted as a male adaptation to post‐copulatory competition and are thought to reduce or delay female remating. Here, we used a vertebrate model species, the house mouse, to study the consequences of copulatory plugs for post‐copulatory competition. We experimentally manipulated plugs after a female's first mating and investigated the consequences for rival male behaviour and paternity outcome. We found that even intact copulatory plugs were ineffective at preventing female remating, but that plugs influenced the rival male copulatory behaviour. Rivals facing intact copulatory plugs performed more but shorter copulations and ejaculated later than when the plug had been fully or partially removed. This suggests that the copulatory plug represents a considerable physical barrier to rival males. The paternity share of first males increased with a longer delay between the first and second males' ejaculations, indicative of fitness consequences of copulatory plugs. However, when males provided little copulatory stimulation, the incidence of pregnancy failure increased, representing a potential benefit of intense and repeated copulation besides plug removal. We discuss the potential mechanisms of how plugs influence sperm competition outcome and consequences for male copulatory behaviour.     

Female Remating Behavior in a Lekking Moth

In polyandrous species, male reproductive success will at least partly be determined by males' success in sperm competition. To understand the potential for post‐mating sexual selection, it is therefore important to assess the extent of female remating. In the lekking moth Achroia grisella, male mating success is strongly determined by female choice based on the attractiveness of male ultrasonic songs. Although observations have indicated that some females will remate, only little is known about the level of sperm competition. In many species, females are more likely to remate if their first mating involved an already mated male than if the first male was virgin. Potentially, this is because mated males are less well able to provide an adequate sperm supply, nutrients, or substances inhibiting female remating. This phenomenon will effectively reduce the strength of pre‐copulatory sexual selection because attractive males with high mating success will be more susceptible to sperm competition. We therefore performed an experiment designed both to provide a more precise estimate of female remating probability and simultaneously to test the hypothesis that female remating is influenced by male mating history. Overall, approximately one of five females remated with a second male. Yet, although females mated to non‐virgin males were somewhat more prone to remate, the effect of male mating history was not significant. The results revealed, however, that heavier females were more likely to remate. Furthermore, we found that females' second copulations were longer, suggesting that, in accordance with theory, males may invest more sperm in situations with an elevated risk of sperm competition.     

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.     

Pair-clinging behavior by males ofAtrophaneura alcinous (Lepidoptera: Papilionidae)

Females ofAtrophaneura alcinous usually mate soon after eclosion. Their ostium bursae becomes plugged with male secretion which reduces chances of remating. Males frequently cling to a copulating pair and wait for completion of copulation. This was observed in 66% of 198 copulating pairs, with a maximum of 5 males clinging at one time during the course of a copulation. Males clinging for longer periods were more successful in copulation with the freshly mated female than those clinging for shorter periods. Despite the plugging effect, females may mate more than once. Clinging males were responsible for 61% of re-copulations and 53% of re-inseminations. Clinging behavior may be regarded as an effective male mating strategy to exploit freshly mated females, and an alternative to finding virgin females.     

Queens remate despite traumatic mating in stingless bees

Males can control female reproduction using genital plugs to impede access by rivals. In social bees, ants, and wasps, plugging may involve traumatic mating, with females being harmed. In stingless bees, chances are that plugs may promote ovarian activan, and are thought to ensure single mating—a general tendency among the social Hymenoptera. However, understanding on relationships between mating plugs, traumatic mating, and mating systems in stingless bees remains limited. To address this, we (1) compared mated queens of 7 Neotropical species to understand the patterns of copulatory marks in females and (2) compared pre- and post-mating genitalia of males and females in Melipona fasciculata to depict plug functional morphology. Data revealed an unprecedented consequence of mating in stingless bees: the characteristic marks left by mating plugs on female abdomens and the inferences that can be made from them. To our surprise, in 1 species M. fasciculata we found that queens retain the plug long after mating, and may carry it for the rest of their lives. All the other 6 species retained the plug for only a short period. Remated queens were only found in M. seminigra, whose multiple copulatory marks match previous findings of polyandry in this species. Our study shows that queens can remate, and suggests that male genital morphology may determine in part the time persistence of plugs. We conclude that traumatic mating plugs do not fully prevent remating in stingless bees and that mating systems are not uniform in this group. Nonetheless, exceptional cases of facultative polyandry in social insects—for example, when mating plugs fail—may confirm a general tendency for single mating in close link with efficient mating plugs.     

Female wolf spiders exert cryptic control drastically reducing ejaculate size

Sexes' roles in post‐copulatory processes have important effects on individual fitness and are promising to study in species showing complex mating behaviours. In the spider Schizocosa malitiosa, males perform two different copulatory patterns, pattern 1 includes 80% of total pedipalp insertions and pattern 2 includes 20%. Both patterns produce similar number of offspring, but pattern 1 induces higher female reluctance to remating than pattern 2. We hypothesised that the complex copulatory patterns are linked to post‐copulatory sexual selection, affecting males' sperm transfer and the resulting sperm storage by females. First, we examined amounts of sperm in males and live females from uninterrupted (pattern 1 + 2) and interrupted matings (pattern 1, pattern 2). Second, in order to disentangle male and female actions, we induced males to mate with dead females and examined amounts of sperm. Males transfer in total 71% of the sperm available in their pedipalps, being higher but not significant in pattern 1 than in pattern 2. Females drastically reduced the amount of sperm stored in their spermathecae and such control is stronger in pattern 1 compared to pattern 2 matings. We propose that cryptic female control is a main factor driving males to strengthen sperm transfer. Active female reduction in ejaculate most probably diminished her reluctance to remate.     

Selection on female remating interval is influenced by male sperm competition strategies and ejaculate characteristics

Female remating rate dictates the level of sperm competition in a population, and extensive research has focused on how sperm competition generates selection on male ejaculate allocation. Yet the way ejaculate allocation strategies in turn generate selection on female remating rates, which ultimately influence levels of sperm competition, has received much less consideration despite increasing evidence that both mating itself and ejaculate traits affect multiple components of female fitness. Here, we develop theory to examine how the effects of mating on female fertility, fecundity and mortality interact to generate selection on female remating rate. When males produce more fertile ejaculates, females are selected to mate less frequently, thus decreasing levels of sperm competition. This could in turn favour decreased male ejaculate allocation, which could subsequently lead to higher female remating. When remating simultaneously increases female fecundity and mortality, females are selected to mate more frequently, thus exacerbating sperm competition and favouring male traits that convey a competitive advantage even when harmful to female survival. While intuitive when considered separately, these predictions demonstrate the potential for complex coevolutionary dynamics between male ejaculate expenditure and female remating rate, and the correlated evolution of multiple male and female reproductive traits affecting mating, fertility and fecundity.     

Double insurance of paternity by a novel type of mating plug in a monandrous solitary mason bee Osmia bicornis (Hymenoptera: Megachilidae)

Males of some hymenopteran species are able to behaviourally induce unreceptivity to mating in females by a post‐copulatory display (post‐copulatory courtship). This method of preventing further inseminations requires a high degree of female cooperation by the female and is especially susceptible to manipulations. If mating chances are low and sexual competition between males high, males can be expected to evolve additional mechanisms to protect their fertilization opportunities. This hypothesis was checked in the red mason bee Osmia bicornis (Linnaeus, 1758; syn. Osmia rufa). Males of this species were found to use a mating plug as a reinsurance to protect their paternity, although they induce females' monandry by a post‐copulatory display. Male accessory gland secretions transferred during ejaculation form a plastic, spongy plug in the females' vagina that also contains the spermatozoa. This mating plug does not hinder rivals to mate with the female but prevents intermixing of the sperm. Thus, the sperm precedence of the first male is secured. The plug is ejected by the female after approximately 1 day when the spermatheca has been completely filled with sperm. The male accessory gland supply is sufficient for the formation of two to three plugs only and is not replenished. This pattern fits well with the low mating opportunities of O. bicornis males. The evolution of a mating plug in addition to the behavioural induction of unreceptivity to mate in females is discussed. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 28–37.     

Sequential mate encounters: female but not male body size influences female remating behavior

Whether and how individuals choose sequentially among matesis an important but largely neglected aspect in sexual selectionstudies. Here, we explore female remating behavior in the cellarspider Pholcus phalangioides. We focus on body size as one ofthe most important traits involved in mate choice. Large andsmall females (n = 216) were double mated with large or smallmales in all eight possible combinations. All females copulatedwhen virgin, but only 82% accepted a second male. The chanceof a female remating was not significantly predicted by thebody size of the second or first male or by the size differencebetween the two. In contrast, a previous study demonstrateda male size effect in that larger males monopolized femalesuntil egg laying when two males of different sizes were present.We suggest that sequential encounters are more common undernatural conditions than male monopolization of females becauseestimates of concurrent multiple paternity together with observationsin a natural population do not favor mate guarding as the predominantmating strategy in this species. It follows from our study thatthe intensity of sexual selection on male size may be greatlyoverestimated when using a competitive laboratory setting fora species in which females generally encounter mates in a sequentialfashion. Female remating probability was significantly predictedby female size, with large females remating with higher probabilitythan small females. Thus, when mating with large females, malesmay gain higher fertilization success through increased femalefecundity but also face a higher sperm competition risk.     

Evidence that Mating Plug is Related to Null Female Mating Activity in the Scorpion Vaejovis punctatus

We document a case in which the mating plug of the scorpion Vaejovis punctatus seems correlated with a reduction in female sexual receptivity. We used two morphological and behavioral analyses. The mating plug was dissected and described from both males and females: it bears a set of spines and two enlarged terminations which possibly allow anchoring to the female inside and impede removal. Behavioral observations support this view: after sperm transfer ended, the female tried to remove the plug by rubbing the ventral side of her body using her second and third pair of legs. This pattern was also performed after mate separation. All females seemed unable to displace the plug and rejected male mating attempts. The mating plug suffered a progressive degradation into the female. This degradation was completed 2 or 3 mo prior to parturition, so that the plug did not seem a barrier for the emergence of embryos. Furthermore, females bearing mating plugs in different degradation stages did not accept another mating during the reproductive season, and sexual acceptance was reinitiated only after parturition. The mating plug therefore seems to correlate with a lost in mating activity. The comparison of this structure with that of other scorpion species allows us to hypothesize a sexual coevolutionary scenario in which the effectiveness of the plug to inhibit female remating may lie at the center of such interaction.     

Sperm competition and small size advantage for males of the golden orb‐web spider Nephila edulis

Sexual selection, through female choice and/or male–male competition, has influenced the nature and direction of sexual size dimorphism in numerous species. However, few studies have examined the influence of sperm competition on size dimorphism. The orb‐web spider Nephila edulis has a polygamous mating system and extreme size dimorphism. Additionally, the frequency distribution of male body size is extremely skewed with most males being small and few large. The duration of copulation, male size and sexual cannibalism have been identified as the significant factors determining patterns of sperm precedence in spiders. In double mating trials, females were assigned to three treatments: either they mated once with both males or the first or the second male was allowed to mate twice. Paternity was strongly associated with the duration of copulation, independent of mating order. Males that were allowed to mate twice not only doubled the duration of copulation but also their paternity. Small males had a clear mating advantage, they copulated longer than large males and fertilized more eggs. Males of different sizes used different tactics to mate. Large males were more likely to mate through a hole they cut into the web, whereas small males approached the female directly. Furthermore, small males usually mated at their first attempt but large males required several attempts before mating took place. There was no obvious female reaction towards males of different sizes.     

Silk‐ and volatile‐based male mate choice in the genital plug‐producing spider

Female mating history can have a strong effect on male fertilization success. Although males often prefer to mate with virgin females, they often also engage with mated females. As the intensity of sperm competition can differ among mated females, males are expected to evolve means to identify their status. In spiders, males often use female silk to gather information about female quality. Males of many spider species deposit mating plugs into female genitalia to hinder further copulations. We tested whether males of the foliage‐dwelling, plug‐producing spider Philodromus cespitum, which is an important natural enemy of pests, discriminate between females of different mating status and whether they can determine the extent of genital plugging in mated females solely on the basis of cues gained from deposited female silk. We presented males with draglines of females that varied in either mating status (virgin vs. mated), the extent of plugging (small vs. big plug), or the age of the plug (fresh vs. old plug) and examined their mate preferences. Additionally, we tested whether males were attracted to volatile cues produced by female bodies. Our experiments revealed that males preferred draglines of virgin females to those of mated females, and mated females with small plugs to those with large plugs. They were also attracted to female volatile cues. This study suggests that males are able to extract fine‐scale information on mating status from female draglines.     

Radiation effects on Drosophila suzukii (Diptera: Drosophilidae) reproductive behaviour

Female remating is a widespread behaviour, reported in several insect species. This behaviour can affect the efficiency of sterile insect technique (SIT); however, little is known about the postcopulatory behaviour of some pest species considered as candidates to be controlled by this technique, such as Drosophila suzukii (Matsumura, 1931) (Diptera: Drosophilidae). In this study, we investigated the effects of male and female sterilization on mating and remating behaviour of D. suzukii. First, we tested the occurrence of multiple mating in different combinations between sterile and fertile males and females. Then, we tested the effects of male and female sterility on female propensity to mate and remate. We found an overall low remating rate by D. suzukii females. Male sterility did not influence mating and remating likelihood; however, copula duration of sterile males was shorter compared to fertile males. On the other hand, sterile females were less likely to mate. Our findings encourage further research regarding the use of SIT to control D. suzukii.     

Copulatory Plug Displacement Evidences Sperm Competition in Lemur catta

Male displacement of copulatory (sperm) plugs from female vaginas provides further evidence for sperm competition in ring-tailed lemurs (Lemur catta), a gregarious prosimian species with a multimale, multifemale mating system. During two mating seasons, I studied two groups of free-ranging ring-tailed lemurs on St. Catherines Island, GA, USA. I observed 22 mating pairs in which males achieved penile intromission. Copulatory plug displacement by males occurred in 9 cases. Plugs were displaced during copulation by male penes upon withdrawl following deep vaginal thrusting. In every case of copulatory plug displacement, the male displacing a plug mated to ejaculation with the estrous female. In a mating system in which females typically mate with more than one male during estrous, often in succession, copulatory plug displacement may function to disrupt or preclude other males' successful insemination of estrous females. The effects of sperm plug displacement on paternity in Lemur catta are unknown, as no study had heretofore documented copulatory plug displacement in this species. The first-male mating advantage suggested for Lemur catta should be re-evaluated where mating order is known, and copulatory plug displacement during mating, or lack thereof, is identified. Because there is a tendency for first-mating males to mate-guard for longer periods of time in Lemur catta, the latency period between the first mate's ejaculation and that of subsequent mates may be an important determinant of male fertilization success.