Adjustment of copula duration and ejaculate size according to the risk of sperm competition in the golden egg bug (Phyllomorpha laciniata)

Several hypotheses have been proposed to explain the adaptivesignificance of prolonged copulations in insects, which includemate guarding and sperm loading functions. We have exploredthe adaptive significance of the prolonged copulations in thegolden egg bug (copulations up to 50 h) and the effect of anincreased risk of sperm competition on ejaculate investment.Our data support predictions derived from sperm competitiontheory, which posits that males are expected to increase ejaculateexpenditure in response to an increased risk of sperm competition.Results show a combined response by males that has not beenpreviously described: males in the presence of rivals increasecopulation duration and the rate of sperm transfer. No relationshipwas found between male or female size and copulation durationor ejaculate size. Golden egg bug males transfer sperm slowlyand gradually throughout copulation; thus an increase in theamount of sperm transferred and the corresponding increase inthe male's numerical representation in the female's storageorgans could be particularly important in a system in whichso few sperm are transferred and in which so few sperm are storedby females. In addition, copulation duration may not only serveto increase the total amount of sperm transferred, but it mayalso increase the chances that the female will lay an egg soonafter copulation has ended. This could explain why males tendto accept eggs after copulation, since they could be maximizingthe chances that such eggs are fathered by them, and in thisway they would substantially increase the survival rates oftheir offspring because eggs laid on plants suffer high mortalityrates.     

Male crickets adjust ejaculate quality with both risk and intensity of sperm competition

Sperm competition theory predicts that males should increase their expenditure on the ejaculate with increasing risk of sperm competition, but decrease their expenditure with increasing intensity. There is accumulating evidence for sperm competition theory, based on examinations of testes size and/or the numbers of sperm ejaculated. However, recent studies suggest that ejaculate quality can also be subject to selection by sperm competition. We used experimental manipulations of the risk and intensity of sperm competition in the cricket, Teleogryllus oceanicus. We found that males produced ejaculates with a greater percentage of live sperm when they had encountered a rival male prior to mating. However, when mating with a female that presented a high intensity of sperm competition, males did not respond to risk, but produced ejaculates with a reduced percentage of live sperm. Our data suggest that males exhibit a fine-tuned hierarchy of responses to these cues of sperm competition.     

Body size of virtual rivals affects ejaculate size in sticklebacks

Sperm competition occurs when sperm of two or more males competeto fertilize a given set of eggs. Theories on sperm competitionexpect males under high risk of sperm competition to increaseejaculate size. Here we confirm this prediction experimentallyin the three-spined stickleback (Gasterosteus aculeatus). Inthis species, sneaking (i.e., stealing of fertilizations byneighboring males) can lead to sperm competition. Sneaking malesinvade foreign nests, and the owners vigorously try to preventthis intrusion. In such fights, male body size is assumed tobe an important predictor of success. Consequently, the riskof sperm competition may depend on the size of a potential competitor.We experimentally confronted males before spawning with eithera large or a small computer-animated rival. We show that malesejaculated significantly more sperm after the presentation ofthe larger virtual rival than after the small stimulus. In addition,the time between the initiation of courting and the spawningwas shorter in the large virtual male treatment. The resultssuggest that stickleback males tailor ejaculate size relativeto the risk of sperm competition perceived by the size of apotential competitor.     

Pitfalls in experiments testing predictions from sperm competition theory

As females of many species mate with more than one male, ejaculates often face competition from the sperm of other males. In recent years, numerous papers have been published on theoretical predictions of evolutionary, behavioural and physiological responses to variation in the strength of sperm competition (SC). These theoretical predictions have also been extensively tested. However, although predictions from SC theory are relatively straightforward, extra caution has to be paid in the design of experiments testing them. One difficulty is for example to disentangle immediate and mean SC risk and intensity. Without carefully designed experiments, it is also very easy to simultaneously increase SC risk and the probability of intense SC--a situation for which we currently have no clear predictions, as the theoretical models to date only assume variation in either SC risk or intensity. In this paper, we discuss these and some other pitfalls related to manipulations of SC risk and intensity and suggest how to avoid them.     

Evolution of ejaculates: patterns of phenotypic and genotypic variation and condition dependence in sperm competition traits

Sperm competition is widely recognized as a potent force in evolution, influencing male behavior, morphology, and physiology. Recent game theory analyses have examined how sperm competition can influence the evolution of ejaculate expenditure by males and the morphology of sperm contained within ejaculates. Theoretical analyses rest on the assumption that there is sufficient genetic variance in traits important in sperm competition to allow evolving populations to move to the evolutionarily stable equilibrium. Moreover, patterns of genotypic variation can provide valuable insight into the nature of selection currently acting on traits. However, our knowledge of genetic variance underlying traits important in sperm competition is limited. Here we examine patterns of phenotypic and genotypic variation in four sperm competition traits in the dung beetle Onthophagus taurus. Testis weight, ejaculate volume, and copula duration were found to have high coefficients of additive genetic variation (CV(A)S), which is characteristic of fitness traits and traits subject to sexual selection. Heritabilities were high, and there was some evidence for Y-linked inheritance in testis weight. In contrast, sperm length had a low CV(A), which is characteristic of traits subject to stabilizing selection. Nevertheless, there was little residual variance so that the heritability of sperm length exceeded 1.0. Such a pattern is consistent with Y-linked inheritance in sperm length. Interestingly, we found that testis weight and sperm length were genetically correlated with heritable male condition. This finding holds important implications for potential indirect benefits associated with the evolution of polyandry.     

Sperm number trumps sperm size in mammalian ejaculate evolution

Postcopulatory sexual selection is widely accepted to underlie the extraordinary diversification of sperm morphology. However, why does it favour longer sperm in some taxa but shorter in others? Two recent hypotheses addressing this discrepancy offered contradictory explanations. Under the sperm dilution hypothesis, selection via sperm density in the female reproductive tract favours more but smaller sperm in large, but the reverse in small, species. Conversely, the metabolic constraint hypothesis maintains that ejaculates respond positively to selection in small endothermic animals with high metabolic rates, whereas low metabolic rates constrain their evolution in large species. Here, we resolve this debate by capitalizing on the substantial variation in mammalian body size and reproductive physiology. Evolutionary responses shifted from sperm length to number with increasing mammalian body size, thus supporting the sperm dilution hypothesis. Our findings demonstrate that body-size-mediated trade-offs between sperm size and number can explain the extreme diversification in sperm phenotypes.     

Testis size,epididymis weight,and sperm competition in rhesus macaques

The intensity of sperm competition is often measured using the gonadosomatic index (testes/body weight). But sperm competition could be mediated more by size of the epididymis than by size of the testicles, and little information is available on the relationship between testicular and epididymal size. We found that both organs were positively correlated in size among male rhesus macaques. Body weight accounted for over 70% of the variance in testicle size and volumetric estimates of testicle size accurately reflected testicle weight. We conclude that methods for ascertaining testicle size are accurate, but the covariation in size between testicles and epididymis will hamper understanding of the physiological mechanisms involved in sperm competition in primates. © 1993 Wiley-Liss, Inc.     

Determinants of male ejaculate investment in the cotton bollworm Helicoverpa armigera: mating history,female body size and male age

Many studies demonstrate that ejaculate size may be influenced by male condition, female quality and the risk or intensity of sperm competition. In the present study, the effect of male and female conditions, male mating history and female mating status on ejaculate sperm numbers in the polyandrous moth Helicoverpa armigera is examined. A large variation in ejaculate size is found and, although female body size and male age influence ejaculate size, female age and copula duration do not. Both male and female mating histories have significant effects on ejaculate sperm numbers. Males reduce ejaculate expenditure in successive matings but deliver significantly more apyrene and eupyrene sperm to nonvirgin than to virgin females.     

Numbers and size of sperm storage tubules and the duration of sperm storage in birds: a comparative study

Estimates of the numbers of sperm storage tubules (SSTs) in the utero-vaginal junction of 11 bird species are presented. Numbers of SSTs varied by a factor of 40 between species, and ranged from 500 to 20000. Body mass accounted for over 50% of the variation in SST mumbers. SST length was positively correlated with the length of spermatozoa across species. The duration of sperm storage was not correlated with the number of SSTs or the volume of sperm storage tissue. However, the number of 'active' SSTs appears to vary between species and it was not possible to make allowance for this. Sperm storage duration was weakly, positively correlated wth clutch size, but showed a significant positive relationship with the number of days over which laying occurred. The number of SSTs was also positively correlated with the number of sperm per ejaculate. The best predictor of sperm storage duration was a multiple regression equation using the spread of laying and the length of sperm storage tubules. The duration of sperm storage in birds which remain together during the pre-laying period is such that a single insemination immediately before the start of laying could fertilize the entire clutch.     

Sperm competition and ejaculate economics

Sperm competition was identified in 1970 as a pervasive selective force in post‐copulatory sexual selection that occurs when the ejaculates of different males compete to fertilise a given set of ova. Since then, sperm competition has been much studied both empirically and theoretically. Because sperm competition often favours large ejaculates, an important challenge has been to understand the evolution of strategies through which males invest in sperm production and economise sperm allocation to maximise reproductive success under competitive conditions. Sperm competition mechanisms vary greatly, depending on many factors including the level of sperm competition, space constraints in the sperm competition arena, male mating roles, and female influences on sperm utilisation. Consequently, theoretical models of ejaculate economics are complex and varied, often with apparently conflicting predictions. The goal of this review is to synthesise the theoretical basis of ejaculate economics under sperm competition, aiming to provide empiricists with categorised model assumptions and predictions. We show that apparent contradictions between older and newer models can often be reconciled and there is considerable consensus in the predictions generated by different models. We also discuss qualitative empirical support for some of these predictions, and detail quantitative matches between predictions and observations that exist in the yellow dung fly. We argue that ejaculate economic theory represents a powerful heuristic to explain the diversity in ejaculate traits at multiple levels: across species, across males and within individual males. Future progress requires greater understanding of sperm competition mechanisms, quantification of trade‐offs between ejaculate allocation and numbers of matings gained, further knowledge of mechanisms of female sperm selection and their associated costs, further investigation of non‐sperm ejaculate effects, and theoretical integration of pre‐ and post‐copulatory episodes of sexual selection.     

Male sperm expenditure under sperm competition risk and intensity in quacking frogs

In the frog Crinia georgiana, reproductive behavior comprisesa "guarding tactic," in which males defend spawn sites and attractfemales by calling, and a "sneak tactic," in which males joinspawning pairs. The aims of the present study were to (1) relateejaculate expenditure by "guarding" and "sneak" males to theirprobability of mating with other males present (sperm-competitionrisk), and (2) determine if males adjust their ejaculate expenditureaccording to the number of males involved in a spawning (sperm-competitionintensity). Theory predicts that because sneak males alwaysmate with other males present, they will experience a highersperm-competition risk and should release larger ejaculatesrelative to that of guarding males. However, as the proportionof sneaks in a population increases so does the risk of spermcompetition to guarders, so expenditure by each tactic shouldmove toward equality. Given that the incidence of sneak behavioris high in C. georgiana, guarders and sneaks were expected toexperience similar risks of sperm competition and show similarinvestment in spermatogenesis. Comparison of testes size andejaculate size showed no difference between tactics. Modelsof sperm-competition intensity predict that males should increasetheir ejaculate size when spawning in the presence of one othermale but decrease their ejaculate size when spawning in thepresence of multiple males. Here, males maintained a constantsperm number irrespective of whether a mating involved one,two, or three males. This result suggests that male C. georgianado not facultatively adjust ejaculate investment in responseto fluctuating intensities of sperm competition.     

Sperm competition and male ejaculate investment in Nauphoeta cinerea: effects of social environment during development

Selective pressure arising from sperm competition has been predicted to influence evolutionary and behavioural adjustment of ejaculate investment, but also may influence developmental adjustment of ejaculate investment. Immature males able to target resources strategically based on the competitive environment they will experience when they become sexually mature should be at a selective advantage. In our study we investigated how the presence of potential competitors or mates affects ejaculate and testes investment during development in the cockroach Nauphoeta cinerea, a species where males control female remating via their ejaculate size (large spermatophores prevent females from remating and therefore function to avoid sperm competition for males) and females store sperm. Our aim was to determine whether the social environment influences developmental adjustment of ejaculate investment and the relative importance of ejaculate components with different functions; avoidance of or engagement in sperm competition. We conclude that the social environment can influence developmental and behavioural flexibility in specific ejaculate components that may function to avoid or engage in sperm competition.     

Larger testes are associated with a higher level of polyandry, but a smaller ejaculate volume, across bushcricket species (Tettigoniidae)

While early models of ejaculate allocation predicted that both relative testes and ejaculate size should increase with sperm competition intensity across species, recent models predict that ejaculate size may actually decrease as testes size and sperm competition intensity increase, owing to the confounding effect of potential male mating rate. A recent study demonstrated that ejaculate volume decreased in relation to increased polyandry across bushcricket species, but testes mass was not measured. Here, we recorded testis mass for 21 bushcricket species, while ejaculate (ampulla) mass, nuptial gift mass, sperm number and polyandry data were largely obtained from the literature. Using phylogenetic-comparative analyses, we found that testis mass increased with the degree of polyandry, but decreased with increasing ejaculate mass. We found no significant relationship between testis mass and either sperm number or nuptial gift mass. While these results are consistent with recent models of ejaculate allocation, they could alternatively be driven by substances in the ejaculate that affect the degree of polyandry and/or by a trade-off between resources spent on testes mass versus non-sperm components of the ejaculate.     

Variation in sperm precedence in Chorthippus grasshoppers (Caelifera: Gomphocerinae)

Variation in the proportions of offspring fathered by a second male to mate (the P₂ value) has been studied in two species of grasshoppers, Chorthippus parallelus (Zetterstedt) and Ch. biguttulus (Linnaeus), by means of the sterile‐male technique. In both species the P₂‐values of the first egg pods laid were in the range of 50–100%, and the temporal variation of P₂‐values appeared to follow two modes. In one, the P₂‐value steadily declined with time, in the other it remained constant at a high level. It is concluded that sperm is passively lost between two matings. The remaining sperm then either mixes within the spermatheca or is stratified. Further variability in sperm precedence can possibly be explained by spermatophore ejection or differential use of stored sperm.     

Reduced heterozygosity impairs sperm quality in endangered mammals

Inbreeding causes increases in homozygosity and is commonly associated with reductions in fertility and embryogenesis. Although the mechanisms underlying such effects are unknown, recent work has suggested that inbred males may suffer impaired ejaculate quality, thus providing a functional explanation for reductions in reproductive function in inbred populations. However, the relationship between inbreeding and sperm quality remains controversial, particularly in wild populations where the level of inbreeding is typically estimated using neutral molecular markers. Such markers are thought to reflect genome-wide levels of heterozygosity only under restricted conditions, and rarely in outbred populations. Here we employ a comparative approach that takes account of these criticisms and evaluates the evidence linking inbreeding to reductions in sperm quality in 20 mammal species. We focus on sperm abnormalities and sperm motility, which are key determinants of male fertility in many species. We show that species with reduced mean heterozygosity have impaired ejaculated quality, although subsequent analyses revealed that these effects were confined to endangered populations. Our findings therefore support the notion that inbreeding can severely impair sperm quality while concomitantly addressing criticisms surrounding the use of heterozygosity estimates to estimate the level of inbreeding.     

Male mating behavior and ejaculate expenditure under sperm competition risk in the eastern mosquitofish

Theory predicts that males should tailor the size of their ejaculatesaccording to temporal changes in the risk of sperm competition.Specifically, males are predicted to allocate more sperm toeach mating event with increasing risk (i.e., the probabilitythat the sperm from two males will compete for fertilization).We tested this hypothesis by using the eastern mosquitofish,a freshwater species of fish exhibiting a coercive mating systemand internal fertilization. We manipulated the perception ofsperm competition risk by adjusting the sex ratio under whichmales were maintained over 8 days. Males were housed eitherwith three females and one male (simulating high sperm competitionrisk) or with four females (low risk). After the treatment,we presented each test male individually to an unfamiliar male-deprivedfemale for 30 minutes and observed his mating behavior. We thenartificially stripped the test males of sperm and recoveredthe ejaculates from the females. Our results revealed that malesin the high-risk group performed higher levels of mating activityand sperm expenditure (i.e., used up more of their sperm reserves)than did low-risk males. A control experiment, in which testmales were treated but did not participate in the mating trials,revealed no significant difference in the number of sperm strippedfrom high- and low-risk males, indicating that sperm productionwas not affected by the treatment. We did not detect a differencein the number of sperm retrieved from females among the groups,raising the possibility that some sperm are lost during matingactivity, either through ejaculation with incomplete or interruptedpenetration, or via female ejection.     

Male-derived cuticular hydrocarbons signal sperm competition intensity and affect ejaculate expenditure in crickets

Female sexual promiscuity can have significant effects on male mating decisions because it increases the intensity of competition between ejaculates for fertilization. Because sperm production is costly, males that can detect multiple matings by females and allocate sperm strategically will have an obvious fitness advantage. The presence of rival males is widely recognized as a cue used by males to assess sperm competition. However, for species in which males neither congregate around nor guard females, other more cryptic cues might be involved. Here, we demonstrate unprecedented levels of sperm competition assessment by males, which is mediated via the use of chemical cues. Using the cricket Teleogryllus oceanicus, we manipulated male perception of sperm competition by experimentally coating live unmated females with cuticular compounds extracted from males. We found that males adjusted their ejaculate allocation in response to these compounds: the viability of sperm contained within a male's ejaculate decreased as the number of male extracts applied to his virgin female partner was increased. We further show that males do not respond to the relative concentration of male compounds present on females, but rather to the number of distinct signature odours of individual males. Our results conform to sperm competition theory, and show for the first time, to our knowledge, that males can detect different intensities of sperm competition by using distinct chemical cues of individual males present on females.