Cryptic female preference for colorful males in guppies

Cryptic female choice (CFC) refers to female-mediated processes occurring during or after copulation that result in biased sperm use in favor of preferred or compatible males. Despite recent empirical support for this hypothesis, evidence that CFC contributes towards the evolution of male body ornaments, in the same way that precopulatory female choice does, is currently lacking. Here, we tested the possibility that CFC selects for increased male attractiveness in the guppy Poecilia reticulata, a freshwater fish exhibiting internal fertilization. Specifically, we examined whether females are able to manipulate the number of sperm transferred or retained at copulation in favor of relatively attractive males. In support of this prediction, we found that following solicited copulations the number of sperm inseminated is influenced exclusively by the female's perception of relative male coloration, independent of any direct manipulation of males themselves. Because females prefer brightly colored males during precopulatory mate choice, our finding that colorful males are also favored as a consequence of enhanced insemination success indicates that cryptic female choice can reinforce precopulatory preferences for extravagant male ornaments.     

Female crickets assess relatedness during mate guarding and bias storage of sperm towards unrelated males

Recent evidence shows that females exert a post‐copulatory fertilization bias in favour of unrelated males to avoid the genetic incompatibilities derived from inbreeding. One of the mechanisms suggested for fertilization biases in insects is female control over transport of sperm to the sperm‐storage organs. We investigated post‐copulatory inbreeding‐avoidance mechanisms in females of the cricket Teleogryllus oceanicus. We assessed the relative contribution of related and unrelated males to the sperm stores of double‐mated females. To demonstrate unequivocally that biased sperm storage results from female control rather than cryptic male choice, we manipulated the relatedness of mated males and of males performing post‐copulatory mate guarding. Our results show that when guarded by a related male, females store less sperm from their actual mate, irrespective of the relatedness of the mating male. Our data support the notion that inhibition of sperm storage by female crickets can act as a form of cryptic female choice to avoid the severe negative effects of inbreeding.     

Mate guarding, male attractiveness, and paternity under social monogamy

Socially monogamous species vary widely in the frequency ofextrapair offspring, but this is usually discussed assumingthat females are free to express mate choice. Using game-theorymodeling, we investigate the evolution of male mate guarding,and the relationship between paternity and mate-guarding intensity.We show that the relationship between evolutionarily stablemate-guarding behavior and the risk of cuckoldry can be complicatedand nonlinear. Because male fitness accumulates both throughpaternity at his own nest and through his paternity elsewhere,males evolve to guard little either if females are very faithfulor if they are very unfaithful. Attractive males are usuallyexpected to guard less than unattractive males, but within-pairpaternity may correlate either positively or negatively withthe number of extrapair offspring fertilized by a male. Negativecorrelations, whereby attractive males are cuckolded more, becomemore likely if the reason behind female extrapair behavior appliesto most females (e.g., fertility insurance) rather than thesubset mated to unattractive males (e.g., when females seek"good genes") and if mate guarding is efficient in controllingfemale behavior. We discuss the current state of empirical knowledgewith respect to these findings.     

An integrative view of sexual selection in Tribolium flour beetles

Sexual selection is a major force driving the evolution of diverse reproductive traits. This evolutionary process is based on individual reproductive advantages that arise either through intrasexual competition or through intersexual choice and conflict. While classical studies of sexual selection focused mainly on differences in male mating success, more recent work has focused on the differences in paternity share that may arise through sperm competition or cryptic female choice whenever females mate with multiple males. Thus, an integrative view of sexual selection needs to encompass processes that occur not only before copulation (pre-mating), but also during copulation (peri-mating), as well as after copulation (post-mating), all of which can generate differences in reproductive success. By encompassing mechanisms of sexual selection across all of these sequential reproductive stages this review takes an integrative approach to sexual selection in Tribolium flour beetles (Coleoptera: Tenebrionidae), a particularly well-studied and economically important model organism. Tribolium flour beetles colonize patchily distributed grain stores, and juvenile and adult stages share the same food resources. Adults are highly promiscuous and female reproduction is distributed across an adult lifespan lasting approximately 1 year. While Tribolium males produce an aggregation pheromone that attracts both sexes, there appears to be little pre-mating discrimination among potential mates by either sex. However, recent work has revealed several peri-mating and post-mating mechanisms that determine how offspring paternity is apportioned among a female's mates. During mating, Tribolium females reject spermatophore transfer and limit sperm numbers transferred by males with low phenotypic quality. Although there is some conflicting evidence, male copulatory leg-rubbing appears to be associated with overcoming female resistance to insemination and does not influence a male's subsequent paternity share. Evidence suggests that Tribolium beetles have several possible post-mating mechanisms that they may use to bias paternity. Male sperm precedence has been extensively studied in Tribolium spp. and the related Tenebrio molitor, and several factors influencing male paternity share among a female's progeny have been identified. These include oviposition time, inter-mating interval, male strain/genotype, the mating regimen of a male's mother, male starvation, and tapeworm infection. Females exert muscular control over sperm storage, although there is no evidence to date that females use this to differentiate among mates. Females could also influence offspring paternity by re-mating with additional males, and T. castaneum females more readily accept spermatophores when they are re-mating with more attractive males. Additional work is needed to examine the possible roles played by both male and female accessory gland products in determining male paternity share. Sexual selection during pre-mating episodes may be reinforced or counteracted by peri- and post-copulatory selection, and antagonistic coevolution between the sexes may be played out across reproductive stages. In Tribolium, males' olfactory attractiveness is positively correlated with both insemination success and paternity share, suggesting consistent selection across different reproductive stages. Similar studies across sequential reproductive stages are needed in other taxa to provide a more integrative view of sexual selection.     

Pairing and insemination patterns in a giant weta (Deinacrida rugosa: Orthoptera; Anostostomatidae)

Positive size assortative mating can arise if either one or both sexes prefer bigger mates or if the success of larger males in contests for larger females leaves smaller males to mate with smaller females. Moreover, body size could not only influence pairing patterns before copulation but also the covariance between female size and size of ejaculate (number of spermatophores) transferred to a mate. In this field study, we examine the pre-copulatory mate choice, as well as insemination, patterns in the Cook Strait giant weta (Deinacrida rugosa). D. rugosa is a large orthopteran insect that exhibits strong female-biased sexual dimorphism, with females being nearly twice as heavy as males. Contrary to the general expectation of male preference for large females in insects with female-biased size dimorphism, we found only weak support for positive size assortative mating based on size (tibia length). Interestingly, although there was no correlation between male body size and the number of spermatophores transferred, we did find that males pass more spermatophores to lighter females. This pattern of sperm transfer does not appear to be a consequence of those males that mate heavier females being sperm depleted. Instead, males may provide lighter females with more spermatophores perhaps because these females pose less of a sperm competition risk to mates.     

Correlated responses to selection on female egg size in male reproductive traits in a butterfly

Genetic correlations between male and female traits can act as evolutionary constraints and, if involving reproductive traits, potentially influence sexual selection. Artificial selection on egg size in the tropical butterfly Bicyclus anynana has yielded highly divergent lines. Here we report evidence for correlated evolution in male traits. Males from the large-egg selected lines produced significantly heavier spermatophores independent of body size and tended to have more fertile sperm stored in their reproductive tracts than those from the small-egg selected lines. This may be due to an underlying genetic correlation in reproductive effort between the sexes. However, non-fertile sperm number and testis size remained unaffected by selection on egg size. Phenotypic correlations within an unselected population revealed that spermatophore mass and fertile sperm number, but not testis size and non-fertile sperm number, were positively related to male body size, and that larger spermatophores contained more fertile, but not non-fertile sperm. In addition, males provided larger females with bigger spermatophores and more fertile sperm, indicating males may be exercising mate choice during copulation.     

The assessment of insemination success in yellow dung flies using competitive PCR

In spite of considerable interest in postcopulatory sexual selection, separating the effects of sperm competition from cryptic female choice remains difficult because mechanisms underlying postcopulatory processes are poorly understood. One methodological challenge is to quantify insemination success for individual males within the sperm stores of multiply mated females to discover how insemination translates into eventual paternity. Any proposed method must be applicable in organisms without extensive DNA sequence information (which include the majority of model species for sexual selection). Here, we describe the development and application of microsatellite competitive-multiplex-PCR for quantifying relative contributions to a small number of sperm in storage. We studied how DNA template characteristics affect PCR amplification of known concentrations of mixed DNA and generated regressions for correcting observations of allelic signal strength based on such characteristics. We used these methods to examine patterns of sperm storage in twice-mated female yellow dung flies, Scathophaga stercoraria. We confirm previous findings supporting sperm displacement and demonstrate that average paternity for the last mate accords with the mean proportion of sperm stored. We further find consistent skew in storage across spermathecae, with more last male sperm stored in the singlet spermatheca on one side of the body than in the doublet on the opposite side. We also show that the time between copulations may be important for effectively sorting sperm. Finally, we demonstrate that male size may influence the opportunity for sperm choice, suggesting future work to disentangle the roles of male competition and cryptic female choice.     

Sperm precedence in the deer tick Ixodes dammini

ABSTRACT We determined whether female deer ticks Ixodes dammini Spielman, Clifford, Piesman & Corwin (Acari: Ixodidae) can be inseminated repeatedly and whether sperm from first or second matings take precedence in fertilizing eggs. Such information is essential to the design of attempts to reduce the fertility of these vectors of Lyme disease. Although spermatophores are present in about half of questing female ticks, they are present in virtually all those found on deer; the abundance of males on deer exceeds that of females and copulation is common. Females must be inseminated before commencing the rapid engorgement phase of feeding. Males need not be in attendance during feeding, provided that the female has been inseminated preprandially. Thus, preprandial insemination suffices to stimulate rapid engorgement, but less blood is taken than when the female is perprandially inseminated. Both types of insemination effectively fertilize eggs. Eggs from females sequentially inseminated by irradiated and non irradiated males, were fertilized mainly by sperm from the last male. Cobalt-irradiated males mate effectively and their sperm compete with those of non-irradiated males. Sperm from the second two sequential inseminations fertilize most of the eggs. By infesting deer with such irradiated male I.dammini , the abundance of these vector ticks may effectively be reduced.     

Competitive PCR reveals the complexity of postcopulatory sexual selection in Teleogryllus commodus

The outcome of mate choice depends on complex interactions between males and females both before and after copulation. Although the competition between males for access to mates and premating choice by females are relatively well understood, the nature of interactions between cryptic female choice and male sperm competition within the female reproductive tract is less clear. Understanding the complexity of postcopulatory sexual selection requires an understanding of how anatomy, physiology and behaviour mediate sperm transfer and storage within multiply mated females. Here we use a newly developed molecular technique to directly quantify mixed sperm stores in multiple mating females of the black field cricket, Teleogryllus commodus. In this species, female postcopulatory choice is easily observed and manipulated as females delay the removal of the spermatophore in favour of preferred males. Using twice‐mated females, we find that the proportion of sperm in the spermatheca attributed to the second male to mate with a female (S₂) increases linearly with the time of spermatophore attachment. Moreover, we show that the insemination success of a male increases with its attractiveness and decreases with the size of the female. The effect of male attractiveness in this context suggests a previously unknown episode of mate choice in this species that reinforces the sexual selection imposed by premating choice and conflicts with the outcome of postmating male harassment. Our results provide some of the clearest evidence yet for how sperm transfer and displacement in multiply mated females can lead directly to cryptic female choice, and that three distinct periods of sexual selection operate in black field crickets.     

Determinants of Sperm Transfer by Males of the Noctuid Moth Heliothis virescens

Ejaculate composition can be an important determinant of male reproductive success in the face of sperm competition, which varies with the mating history of the female. Here we examine the effect of various male and female mating histories and morphological traits on ejaculate sperm numbers in the polyandrous moth Heliothis virescens. We show that when mating with nonvirgin females, males passed larger sperm packages (spermatophores) but did not alter either the sperm count or the ratio of nucleated-to-nonnucleated sperm. Males also passed fewer sperm in their second ejaculates. Finally, older males passed more sperm than did younger males. Earlier research found that females store more sperm from older males and that older males are more likely to gain sperm precedence over younger rivals. These earlier results, taken together with the present results, indicate that the advantage enjoyed by older males is due to an increased sperm count.     

Male Condition, Female Choice, and Extreme Variation in Repeated Mating in a Scaly Cricket, Ornebius aperta (Orthoptera: Gryllidae: Mogoplistinae)

Mating in the scaly cricket Ornebius aperta often includes the transfer of many spermatophores to individual females during extended copulatory interactions. We manipulated male condition in staged matings to determine whether this could explain variation in the number of repeated copulations seen across pairs. Males on a high nutrient diet were in good condition, were more likely to mate repeatedly, and transferred more spermatophores on average than low-diet males (in poor condition). High-diet males were more likely to produce a vibratory signal that increased female receptivity to repeated mating attempts. Courtship and copulatory interactions were always terminated by females, and in every case males had already formed a spermatophore when deserted by females. We conclude that variation in male repeated mating success may be due to female choice rather than an inability or unwillingness of low-diet males to produce spermatophores.     

Male courtship attractiveness and paternity success in Photinus greeni fireflies

Although female mate choice and male sperm competition have separately attracted much attention, few studies have addressed how precopulatory and postcopulatory episodes of sexual selection might interact to drive the evolution of male traits. In Photinus fireflies, females preferentially respond to males based on their bioluminescent courtship signals, and females gain direct benefits through male nuptial gifts acquired during multiple matings over several nights. We experimentally manipulated matings of P. greeni fireflies to test the hypothesis that postcopulatory paternity success might be biased toward males that are more attractive during courtship interactions. We first measured male courtship attractiveness to individual females using field behavioral assays. Females were then assigned to two double-mating treatments: (1) least attractive second male-females were first mated with their most attractive male, followed by their least attractive male, or (2) most attractive second male-females mated with males in reverse order. Larval offspring produced by each female following these double matings were genotyped using random amplified polymorphic DNA (RAPD) markers, and male paternity was determined. Contrary to prediction, firefly males that were more attractive to females based on their bioluminescent courtship displays subsequently showed significantly lower paternity, reflecting possible male trade-offs or sexual conflict. Differences in male paternity were not related to male body condition, testes or accessory gland mass, or to variation in female spermathecal size. Additionally, this study suggests that changes in phenotypic selection gradients may occur during different reproductive stages. These results indicate that it is crucial for future studies on sexual selection in polyandrous species to integrate both precopulatory and postcopulatory episodes to fully understand the evolution of male traits.     

Differential Sperm Priming by Male Sailfin Mollies (Poecilia latipinna): Effects of Female and Male Size

Recent interest in sperm competition has led to a re‐evaluation of the ‘cheap sperm’ assumption inherent in many studies of sexual selection. In particular, mounting evidence suggests that male sperm availability can be increased by the presence of females. However, there is little information on how this interacts with male traits presumably affected by female mate choice, such as larger size. This study examines the effects on male sperm availability of female presence, male body size, and female body size in the sailfin molly, Poecilia latipinna. Individual males of variable body sizes were isolated in divided tanks for 3 d, after which time either a female or no female was added to the other side of the tank. Prior to the treatments, larger males had more stripped sperm than smaller males. Female presence significantly increased the amount of sperm males primed, but this effect was strongest in small males. Furthermore, males showed a greater priming response in the presence of larger females than in the presence of smaller females. These results demonstrate that the presence of sexually mature females increases the amount of sperm males have for insemination. Furthermore, traits that indicate female fecundity may be used by males as cues in male mate choice.     

Spawning behavior of Arctic charr (Salvelinus alpinus): Spawning synchrony,vibrational communication,and mate guarding

A mismatch in synchrony between male and female gamete release in external fertilizers can result in reduced or failed fertilization, sperm competition, and reduced paternity. In Arctic charr (Salvelinus alpinus), males can adopt either a guard or sneak tactic resulting in both pre‐ and postcopulatory competition between males with alternative reproduction tactics. Here, spawning behavior of free‐living Arctic charr was video‐recorded, and their reproductive behavior was analyzed. From evaluating 157 spawning events, we observed that females mainly spawned with a guarding male and that the female and the guarding male synchronized timing of gamete release under sperm competition. Although sneakers spawned with higher synchrony than the guarding male in single‐male spawning events, the average sneaker released his milt less synchronized with the female than the guarding male under sperm competition. Approximately 50% of the recorded spawning events occurred under sperm competition, where each event included an average of 2.7 males. Additionally, sneakers were more exposed to sperm competition than guarding males. An influx of males, in close proximity to the female, occurred during the behavioral sequences leading up to egg release, but this influx seemed not dependent on egg release, suggesting that something else than gonadal product attracts sneaker males to the spawning female. Just before and during the actual release of gametes, the spawning couple vibrates their bodies in close contact and it seems likely that this vibrational communication between the spawning couple, which results in a larger amplitude sound wave than seen under regular courting, reveals time of gamete release to sneaker males. Thus, vibrational communication may enable synchrony between the guarding male and the female, and this might be traded against the cost of higher detectability from surrounding sneaker males, eavesdropping in close proximity.     

Female mate choice and male–male competition in Tonkean macaques: Who decides?

The theory of sexual selection predicts that females should be discriminatory in the choice of sexual partners. Females can express their choice in two ways. In direct mate choice, they show preferences for certain partners. In indirect mate choice, they select partners by displaying sexually attractive traits, thus eliciting contest competition between males. We focused on a primate species in which females advertise the timing of their ovulation and studied the balance between these two choice strategies. We tested predictions related to three hypotheses about direct and indirect female choice, namely the best‐male, graded‐signal and weak‐selectivity hypotheses. We investigated the sexual and agonistic interactions occurring during oestrous periods in five captive groups of Tonkean macaques (Macaca tonkeana). The results showed that dominant males used mate guarding to monopolise sexual access to parous females that were in the fertile stage of their reproductive cycle, while lower‐ranking males monitored only nulliparous females. The distribution of sexual presentations indicated that females accepted different types of partners, supporting the weak‐selectivity hypothesis regarding direct mate choice. The analysis of behavioural sequences revealed that mate‐guarding males used mild coercive behaviours to prevent females from mating with other males at conception time. The distribution of mounts showed that females mainly mated with dominant males, which leads us to argue that the best‐male hypothesis provides the most parsimonious explanation regarding indirect mate choice in Tonkean macaques. At the individual level, it may be concluded that male competitive strategies prevented females from exercising direct mate choice. At the evolutionary level, however, female sexual advertising and thus indirect choice promoted competition between males. The outcome is that indirect mate choice appears more important than direct mate choice in female Tonkean macaques.     

Impact of cryptic female choice on insemination success: Larger sized and longer copulating male squid ejaculate more,but females influence insemination success by removing spermatangia

In polyandrous mating systems, sperm competition and cryptic female choice (CFC) are well recognized as postcopulatory evolutionary forces. However, it remains challenging to separate CFC from sperm competition and to estimate how much CFC influences insemination success because those processes usually occur inside the female's body. The Japanese pygmy squid, Idiosepius paradoxus, is an ideal species in which to separate CFC from sperm competition because sperm transfer by the male and sperm displacement by the female can be observed directly at an external location on the female's body. Here, we counted the number of spermatangia transferred to, removed from, and remaining on the female body during single copulation episodes. We measured behavioral and morphological characteristics of the male, such as duration of copulation and body size. Although males with larger body size and longer copulation time were capable of transferring larger amounts of sperm, females preferentially eliminated sperm from males with larger body size and shorter copulation time by spermatangia removal; thus, CFC could attenuate sperm precedence by larger males, whereas it reinforces sperm precedence by males with longer copulation time. Genetic paternity analysis revealed that fertilisation success for each male was correlated with remaining sperm volume that is adjusted by females after copulation.     

Pre‐ and postcopulatory sexual selection act in concert to determine male reproductive success in Tribolium castaneum

To capture how sexual selection shapes male reproductive success across different stages of reproduction in Tribolium castaneum (Coleoptera, Tenebrionidae), we combined sequential sperm defence (P1) and sperm offence (P2) trials with additional trials where both males were added simultaneously to the female. We found a positive correlation between the relative paternity share in simultaneous male–male competition trials and the P2 trial. This suggests that males preferred by females as sires achieve superior fertilization success during sperm competition in the second male position. In simultaneous male–male competition trials, where pre‐, peri‐ and postcopulatory sexual selection were all allowed to act, the relative paternity share of preferred males was more than 20% higher than in P2 sperm competition trials where precopulatory female choice was disabled. Additional behavioural observations revealed that mating with more attractive males resulted significantly more frequently in offspring production than mating with less attractive males. Thus, by comparing male fertilization success in trials where precopulatory choice was turned off with more inclusive estimates of fertilization success where pre‐ and pericopulatory choice could occur, we show that female mate choice may effectively inhibit sperm competition. Female mate choice and sperm competition (P2) are positively correlated, which is consistent with directional sexual selection in this species. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 67–75.     

Ejaculate investment and attractiveness in the stalk‐eyed fly,Diasemopsis meigenii

The phenotype‐linked fertility hypothesis proposes that male fertility is advertised via phenotypic signals, explaining female preference for highly sexually ornamented males. An alternative view is that highly attractive males constrain their ejaculate allocation per mating so as to participate in a greater number of matings. Males are also expected to bias their ejaculate allocation to the most fecund females. We test these hypotheses in the African stalk‐eyed fly, Diasemopsis meigenii. We ask how male ejaculate allocation strategy is influenced by male eyespan and female size. Despite large eyespan males having larger internal reproductive organs, we found no association between male eyespan and spermatophore size or sperm number, lending no support to the phenotype‐linked fertility hypothesis. However, males mated for longer and transferred more sperm to large females. As female size was positively correlated with fecundity, this suggests that males gain a selective advantage by investing more in large females. Given these findings, we consider how female mate preference for large male eyespan can be adaptive despite the lack of obvious direct benefits.     

Pattern of sperm storage and migration in the reproductive tract of the swallowtail butterfly Papilio xuthus: cryptic female choice after second mating

Females of the swallowtail butterfly Papilio xuthus L. (Lepidoptera: Papilionidae) mate multiply during their life span and use the spermatophores transferred to increase their longevity as well as fecundity. Sperm from different males may be stored in the sperm storage organs (bursa copulatrix and spermatheca). To clarify the pattern of sperm storage and migration in the reproductive tract, mated females are dissected after various intervals subsequent to the first mating, and the type and activity of sperm in the spermatheca are observed. When virgin females are mated with virgin males, the females store sperm in the spermatheca for more than 10 days. Sperm displacement is found in females that are remated 7 days after the first mating. Immediately after remating, these females flush out the sperm of the first male from the spermatheca before sperm migration of the second male has started. However, females receiving a small spermatophore at the second mating show little sperm displacement, and the sperm derived from the small spermatophore might not be able to enter the spermatheca. Females appear to use spermatophore size to monitor male quality.     

Competing for last place: Mating behaviour in a pill-box crab, Halicarcinus cookii (Brachyura: Hymenosomatidae)

The mating strategy of Halicarcinus cookii was investigated to ascertain how males maximised their fitness through mate choice. An intertidal population at Kaikoura, New Zealand, was dominated by mature crabs of both sexes in summer and by immature crabs in the colder months. More than 95% of mature females were ovigerous with early stage and late stage broods found in almost every month, indicating that egg production and larval release is continuous. The operational sex ratio was less than 1 male/female in summer, but often more than 1.0 in the colder months. The gonosomatic index increased along with brood development so that as soon as zoeae were released, the next clutch of eggs was ready to be fertilised. Males searched for receptive females and began pre-copulatory mate guarding without any courtship display. They mated preferentially with late stage or non-ovigerous females: copulation duration was longest for stage 5 females as was post-copulatory guarding (mean 18.3 h). Late stage females were up to 14% of the female population. Mate attraction seems to be the result of an ovarian signal rather than from the developing brood. Manipulation of the sex ratio had effects upon copulation duration and post-copulatory guarding: presence of a rival male increased duration of guarding. Females showed precocious mating in the penultimate instar and were able to lay fertilised eggs after their pubertal moult in the absence of males. H. cookii females have many mates, but males attempt to ensure paternity by preferentially pursuing mature females close to egg laying and by guarding these females after copulation. These behaviours are all elements of a competitive strategy to ensure that a male loses (not wins) the race to copulate because females have a ventral seminal receptacle, giving sperm precedence to the last male to mate. Male mating behaviour is a consequence and evolutionary response to female morphology.