Role of the epididymis in sperm competition

Although it is generally understood that the testes recruited kidney ducts for reproductive function during the evolution of vertebrates, little is understood of the biological significance of the adaptation. In the context of the evolution of the mammalian epididymis, this report provides evidence that a major role of the epididymis is to enhance a male＇s chance of achieving paternity in a competitive mating system. A unique example of sperm cooperation in monotremes is used as evidence that the epididymis produces sperm competition proteins to form groups of 100 sperm into bundles that have a forward motility nearly thrice that of individual spermatozoa. As it required 3-h incubation in vitro under capacitation conditions to release motile sperm from the bundles, it is suggested that the monotremes provide an example of capacitation that is quite different from capacitation in higher mammals. It is suggested that variation between species in the intensity of sperm competition could explain the variation that occurs between species in the amount of post-testicular sperm maturation and storage in the epididymis, an explanation of why the human epididymis does not play as important a role in reproduction as the epididymis of most mammals. （Asian J Androl 2007 July; 9： 493-499）

Male sexual signaling and expected effects of hatchery-induced sperm competition vary with water depth at which whitefish are caught

Salmonids like whitefish(Coregonus spp.)are often propagated in supportive breeding.Spawners are caught from their spawning locations,their gametes mixed,and the resulting offspring reared in a protected environment before being released into the wild.This procedure can affect sexual selection,for example,by enhancing the importance of sperm competition or by reducing the relevance of sexual signals.While it is often unclear how sperm competitiveness is affected by a male's overall genetic quality,there is accumulating evidence that sexual signals reveal good genes and that mate choice based on such signals can increase offspring viability(Auld et al.2019).Therefore,supportive breeding may affect the genetic variance and the mean genetic quality of next generations.

A novel method to collect sperm from brood parasitic cuckoos:Urodeum stimulation(UroS)method

Sperm competition has been studied in numerous species as a representative example of postcopulatory sexual selection,where sampling sperm from male is the most basic and important step.Sperm collection can be tricky in birds,however,because unlike mammals,the genitals of birds are generally latent in the cloacal region and their characteristics vary among species.Various methods to collect sperm from different birds have been tested,such as cloacal massage,feces collection,and electro-stimulation,but their applicability varies depending on species.In this study,we introduced the urodeum stimulation method(UroS method)to collect sperm from Cuculus cuckoos,such as the Common Cuckoo(C.canorus).These species are expected to have interesting patterns of pair bonding and sperm competition because of their unique breeding strategy called brood parasitism;however,it remains unexplored.In this study,we described the application of our new method to expel semen from male common cuckoos,measured the volume of semen collected,checked the presence of sperm in the semen sample,and finally estimated its success rate among 82 males.Samples were successfully collected from 76 cuckoos(approximately 93%)and the colors and volumes of the samples were very diverse.Sperm was present in 43 of these samples(57%),showing a sperm observation rate approximately twice as high as that of the conventional cloacal massage method.We believe that this novel method will contribute to a better understanding of postcopulatory sexual selection in avian brood parasites and facilitate the process of sperm collection and artificial insemination in other medium-sized birds.

Sexual selection affects the sizes of the mammalian prostate gland and seminal vesicles

The accessory reproductive glands of male mammals contribute the bulk of the secretions in which spermatozoa are transported to the female tract during copulation. Despite their morphological diversity, and the chemical complexity of their products, little is known about the possible effects of sexual selection upon these glands in mammals. Here we consider the seminal vesicles and prostate glands in a sample of 89 species and 60 genera representing 8 Orders of mammals. The sizes of the accessory glands are analysed in relation to body weight and testes weight. Both the seminal vesicles size and prostate size （corrected for body weight） correlate positively with relative testes size in this sample; this finding remains highly significant after application of procedures to correct for possible phylogenetic biases in the data set. The accessory reproductive glands are also significantly larger in those mammals which have large relative testes sizes, and in which the likelihood of sperm competition is greatest. These results support the hypothesis that sexual selection has played an important role in the evolution of the mammalian prostate gland and seminal vesicles.

Importance of sperm morphology during sperm transport and fertilization in mammals

After natural or artificial insemination, the spermatozoon starts a journey from the site of deposition to the place of fertilization. However, only a small subset of the spermatozoa deposited achieves their goal： to reach and fertilize the egg. Factors involved in controlling sperm transport and fertilization include the female reproductive tract environment, cell-cell interactions, gene expression, and phenotypic sperm traits. Some of the significant determinants of fertilization are known （i.e., motility or DNA status）, but many sperm traits are still indecipherable. One example is the influence of sperm dimensions and shape upon transport within the female genital tract towards the oocyte. Biophysical associations between sperm size and motility may influence the progression of spermatozoa through the female reproductive tract, but uncertainties remain concerning how sperm morphology influences the fertilization process, and whether only the sperm dimensions per se are involved. Moreover, such explanations do not allow the possibility that the female tract is capable of distinguishing fertile spermatozoa on the basis of their morphology, as seems to be the case with biochemical, molecular, and genetic properties. This review focuses on the influence of sperm size and shape in evolution and their putative role in sperm transport and selection within the uterus and the ability to fertilize the oocyte.

Sperm allocation strategies in a sperm heteromorphic insect

Theories predict that in polyandrous species,the focal male should increase sperm allocation per mate in the presence of rivals to gain greater share of paternity,but in the presence of additional mates,he should reduce sperm allocation per mate to save sperm for insemination of more mates.However,empirical findings are often inconsistent and reasons behind are unclear.Furthermore,many studies use copulation duration as an estimate of the number of sperm transferred.Yet,empirical evidence for such assumption is largely lacking.Here,we used a sperm heteromorphic insect Ephestia kuehniella whose males produce two types of sperm,eupyrenes(fertile)and apyrenes(nonfertile),to test these postulations.We allowed focal males to detect chemical and acoustic but no tactile cues from rivals or additional mates both before and during mating and measured copulation duration and sperm allocation in successive copulations.We demonstrate that males transfer significantly more eupyrenes per mate in the presence of rivals and that the sperm allocation pattern persists in successive copulations under this condition.However,males do not adjust apyrene allocation in response to rivals probably because apyrenes play a relatively minor role in male reproductive success.Contrary to a previous study,focal males do not respond to additional mates most likely due to the lack of tactile cues in the present study.We reveal that sperm allocation is not a function of copulation duration in this insect for spermatophore formation and delivery occupy most of copulation duration and sperm transfer is complete near the end of copulation.

Penis size and sperm quality,are all bats grey in the dark?

Penises play a key role in sperm transport and in stimulating female genitals.This should impact post-copulatory competition,and expose penis characteristics to sexual selective pressures.Studies of male genitalia have repeatedly reported negative static allometries,which mean that,within species,large males have disproportionally small genitals when compared with smaller individuals.Males of some sperm-storing bat species may stand as an exception to such a pattern by arousing from hibernation to copulate with torpid females.The selection for large penises might take place,if a long organ provides advantages during post-copulatory competition and/or if females have evolved mechanisms allowing the choice of sire,relying on characters other than pre-copulatory traits(e.g.,penis size).In this study,we measured dimensions of the erected penis in 4 sperm-storing bat species.Furthermore,we collected sperm and evaluated the link between penis dimensions and sperm velocity.Our results revealed steep allometric slopes of the erected penis length in Barbastella barbastellus and an inverse allometry of penis head width in Myotis nattereri.More detailed studies of copulatory behavior are urgently needed to explain the range of observed scaling relations.Furthermore,penis head width correlates with sperm velocity in Plecotus auritus.For this last species,we propose that penis shape might act as a marker of male fertility.

Attractive males have faster sperm in three-spined sticklebacks Gasterosteus aculeatus

Recent studies have revealed that sexually selected traits may signal sperm quality and hence male fertilisation ability. There is also evidence that the expression of male sexual ornamentation and associated sperm characteristics depend on an indi- vidual＇s ability to cope with oxidative stress. Carotenoids are known for their antioxidant properties and carotenoid-based orna- ments might represent honest signals as these pigments can be traded offbetween the investment in sexual ornamentation, sperm function as well as immune response. In this study, we examined the relationship between sexual ornamentation （breeding colo- ration） and sperm characteristics （e.g., velocity and morphology） in the three-spined stickleback Gasterosteus aeuleatus, an ex- ternally fertilising fish species, in which sperm competition commonly occurs. During the breeding season males are sperm limi- ted and develop a conspicuous carotenoid-based coloration, which is under strong pre-copulatory sexual selection due to female mate choice and male-male competition. The results of the present study show that the expression of stickleback male breeding coloration is significantly positively associated with the linearity of sperm movement, whereas sperm morphology （head length to tail length ratio） is significantly negatively related to the trajectory of sperm movement. Moreover, there is some support for the phenotype-linked fertility hypothesis as the intensity of male red breeding coloration is significantly positively correlated with sperm velocity, which is supposed to be an important determinant of fertilisation success in external fertilisers, indicating the honesty of the sexually selected nuptial red coloration.

Patterns of sperm use by twice-mated female Queensland fruit flies

Multiple mating by females,polyandry,is common in insects,including in tephritid fruit flies.Female insects that remate commonly store sperm of multiple males.How the sperm of different males contribute to paternity is an important element of sexual selection.Sexual behavior and reproduction of the Queensland fruit fly(Qfly),Bactrocera tryoni,has been extensively investigated both in relation to understanding this economically important species’reproductive biology and in relation to implications for Sterile Insect Technique(SIT),whereby sterile flies are released to constrain reproduction of pest populations.Despite numerous studies of pre-and postcopulatory sexual selection in Qfly,there have been no direct studies of paternity patterns in polyandrous female Qflies.We used two morphologically distinguishable lines to investigate patterns of sperm use in Qfly.The two lines showed comparable mating performance evidenced by similar mating and remating frequency,copula duration,and proportion of second mate paternity(P2)between reciprocal crosses.The mechanism of sperm usage,with P2 close to 0.5 immediately after the second mating followed by gradual decrease of P2 as females aged,is most consistent with stratification or repositioning of sperm.Patterns observed in the present study are compared with the available information from other tephritid fruit flies,and are discussed in relation to this species’reproductive biology,known patterns of sperm storage,and SIT.

Experimental manipulation of reproductive tactics in Seba’s short-tailed bats:consequences on sperm quality and oxidative status

To reproduce,males have to fertilize the female’s eggs,sometimes in competition with ejaculates of other males.In species where males display alternative reproductive tactics,whereby territorial males secure mating and non-territorial males have to sneak copulations,the latter might be expected to invest relatively more resources towards sperm quality compared with the territorial males.Sperm cells are especially vulnerable to oxidative stress,which reduces male fertility.Therefore,antioxidant resources are expected to modulate sperm quality,and might be allocated differently between reproductive tactics.To test the link between reproductive tactics,redox profile and sperm quality,we experimentally induced changes in the reproductive tactics of 39 captive males Seba's short-tailed bats Carollia perspicillata.We monitored the blood and ejaculate oxidative balance,and the sperm quality before,7 days and 21 days after the manipulation of reproductive tactic.Although ejaculates'oxidative damage was negatively related to sperm velocity,males exhibited similar blood and ejaculates redox profiles and similar sperm quality,regardless of their reproductive tactic.Possibly,these results arise as a consequence of some constraints having been lifted during the experiment.Our results also suggest that,in Seba's short-tailed bats,the expression of alternative reproductive tactics is not subjected to strong oxidative constraints.Furthermore,our results could reflect an absence of trade-off between pre-and post-copulatory traits in harem males,as they could be selected to invest both in female attraction and sperm quality,as a consequence of their inability to fully monopolize females.

Mate-choice copying： Status quo and where to go

Mate-choice copying is a fascinating and widespread mate-choice strategy. Individuals gather public information about potential mates by observing others during sexual interactions and choose or reject the same individual as a mate as the observed individual did before. The influence of copying behavior on an individual＇s mate choice can be so strong that socially acquired information can override genetically based preferences for certain phenotypes. Thus, mate-choice copying enforces dy- namic processes in sexual selection. Here, we review the current state of research on mate-choice copying and focus on sex-specific aspects. We present evidence that mate-choice copying can support the evolution of novel sexual ornaments, and we discuss potential costs of mate-choice copying when public information is not reliable. Moreover, we discuss the conflict faced by males that copy since mate-choice copying increases sperm competition. In conclusion we suggest interesting topics for future research in mate-choice copying .

Evolution of deceit by worthless donations in a nuptial gift-giving spider

Males of the nursery web spider Pisaura mirabil usually offer an insect prey wrapped in white silk as a nuptial gift to facilitate copulation. Males exploit female foraging preferences in a sexual context as females feed on the gift during copula- tion. It is possible for males to copulate without a gift, however strong female preference for the gift leads to dramatically higher mating success for gift-giving males. Females are polyandrous, and gift-giving males achieve higher mating success, longer copulations, and increased sperm transfer that confer advantages in sperm competition. Intriguingly, field studies show that ap- proximately one third of males carry a worthless gift consisting of dry and empty insect exoskeletons or plant fragments wrapped in white silk. Silk wrapping disguises gift content and females are able to disclose gift content only after accepting and feeding on the gift, meanwhile males succeed in transferring sperm. The evolution of deceit by worthless gift donation may be favoured by strong intra-sexual competition and costs of gift-construction including prey capture, lost foraging opportunities and investment in silk wrapping. Females that receive empty worthless gifts terminate copulation sooner, which reduces sperm transfer and likely disadvantages males in sperm competition. The gift-giving trait may thus become a target of sexually antagonistic co-evolution, where deceit by worthless gifts leads to female resistance to the trait. We discuss factors such as female mating rate and intensity of sperm competition that may shape the evolution of male deception, and how ecological factors may influence the evolution and maintenance of worthless gifts as an evolutionarily stable alternative mating strategy by frequency dependent selection

Sexual audience affects male's reproduction investment without consequences on reproductive outputs

Males evolved plastic strategies to respond to male-male competition and exhibit adaptive traits and behaviors maximizing their access to the females and limiting sperm competition.Mating behaviors allow males to express quick responses to current sexual audience,that is,the number of nearby conspecifics prone to mate.In contrast,physiological responses are frequently delayed because they are constrained by the time and resources having to be mobilized to produce and export sperm and associated products.This is especially critical in species for which males produce spermatophores.Here we investigated in what extend moth males(the tortricid moth Lobesia botrana)producing spermatophores exhibit plastic behavioral and physiological responses to different sexual audiences before and during mating and the consequences for their reproductive output.We found that males adjusted their mating behaviors and spermatophore size to a potentially elevated risk of sperm competition perceived before mating.In addition,males responded to the closed presence of females during mating by reducing their mating duration.Surprisingly,the various behavioral and physiological responses we highlighted here were not fully reflected in their reproductive performance as we did not reveal any effect on fecundity and fertility of their mate.The selective pressure exerted on males experiencing male-male competition could thus be sufficient to trigger adjustment in male mating behaviors but constrains physiological responses according to the perception of competition.

Contrasting female mate preferences for red coloration in a fish

Understanding how animals select their mates requires knowing the factors that shape mate preferences.Recent theoretical and empirical considerations suggest that female mating status can influence the degree to which a female engages in mate choice,with virgin females predicted to be less choosy than mated females.In this study,we investigated mate choice in both virgin and mated females in the pygmy halfbeak Dermogenys collettei.Halfbeaks are small,live-bearing,internally fertilizing freshwater fish that live in mixed-sex groups where females have ample opportunity to engage in mate choice.Using a dichotomous choice assay,we quantified and contrasted in virgin and mated females mate preferences for differences in male body size,beak size,and area of yellow and red coloration.We also examined how mating status influenced the amount of time a female associated with the first male encountered and the relative amount of time a female associated with each male.We demonstrate that mate preferences of female halfbeaks are driven primarily by the size of red coloration present on males.Females showed contrasting preferences based on mating status,with virgin females preferentially associating with drab males whereas mated females preferentially associate with males possessing large areas of red.Contrary to expectations,female mating status did not influence how females associate with the first males encountered or how females biased their association time among males.Although the precise drivers of these effects need further studying,our finding highlights a possible explanation for how variation in male ornamentation can be maintained.

A comparison of two methods to assess audience-induced changes in male mate choice

Multidirectional communicative interactions in social networks can have a profound effect on mate choice behavior. Male Atlantic molly Poecilia mexicana exhibit weaker mating preferences when an audience male is presented. This could be a male strategy to reduce sperm competition risk： interacting more equally with different females may be advantageous because ri- vals might copy mate choice decisions. In line with this hypothesis, a previous study found males to show a strong audience effect when being observed while exercising mate choice, but not when the rival was presented only before the choice tests. Audience effects on mate choice decisions have been quantified in poeciliid fishes using association preference designs, but it remains un- known if patterns found from measuring association times translate into actual mating behavior. Thus, we created five audience treatments simulating different forms of perceived sperm competition risk and determined focal males＇ mating preferences by scoring pre-mating （nipping） and mating behavior （gonopodial thrusting）. Nipping did not reflect the pattern that was found when association preferences were measured, while a very similar pattern was uncovered in thrusting behavior. The strongest response was observed when the audience could eavesdrop on the focal male＇s behavior. A reduction in the strength of focal males＇ preferences was also seen after the rival male had an opportunity to mate with the focal male＇s preferred mate. In comparison, the reduction of mating preferences in response to an audience was greater when measuring association times than actual mating behavior. While measuring direct sexual interactions between the focal male and both slimulus females not only the male＇s motivational state is reflected but also females＇ behavior such as avoidance of male sexual harassment [Current Zoology 58 （1）： 84-94, 2012].