Strategic Ejaculation in the Black Scavenger Fly Sepsis cynipsea Revisited: Copula Duration as a Function of Sperm Depletion and Body Size

The massive numbers of sperm males transfer during a single mating are physiologically costly and the amount of sperm that can be stored is limited. Therefore, males can perform only a finite number of successive copulations without loss of fertility, and males should allocate sperm prudently. We investigated sperm availability and depletion in male black scavenger flies, Sepsis cynipsea (Diptera: Sepsidae), asking whether males adjust copula duration according to nutrition, their sperm stores, their own and their partner’s body size, as predicted by theory. We created a gradient of sperm limitation by restricting dung (their protein resource as adults) and subjecting males to a varying number of copulations. While male fertility did not depend on access to fresh dung (contrary to females), it did decline after three copulations, and more so when males were small. Larger females tended to lay more unfertilized eggs after copulating with previously mated males. However, copula duration was not influenced by a male’s number of previous copulations, and therefore apparently not by his current sperm stores. Nevertheless, copula duration varied with male size, with small males copulating longer, and with female size, as copulations lasted longer with larger females, suggesting that males are investing more sperm in larger, more fecund females. While male copula adjustments to their own nutrition and body size may be simple (proximate) physiological responses, responses to female size indicate more strategic and sophisticated sperm‐allocation strategies than previously thought.     

Age-dependent insemination success of sterile Mediterranean fruit flies

The Sterile Insect Technique (SIT) is used in many regions worldwide to manage wild populations of the Mediterranean fruit fly (`medfly'), an important pest species. This technique relies on released sterile males outcompeting their wild counterparts in fertilizing ova of wild females. Numerous studies have investigated the ability of sterile males to secure copulations, an essential step toward overall success. Here we progress further along the mating sequence by studying reproductive barriers that may remain ahead of sterile males that manage to secure copulations in field cage experiments and whether ability to pass these barriers is influenced by a male's age, diet and size, or the size of his mate. Amongst those virgin males that succeeded in copulating, both the number of sperm stored by mates and the chances of having any sperm stored at all decreased with age. Sperm tended to be stored asymmetrically between the females' two spermathecae, and this tendency was more apparent when few sperm were stored. In accord with effects of male age on number of sperm stored, sperm of older males were stored more asymmetrically than that of young males. We found no evidence that male size, male diet or female size influenced copula duration, number of sperm stored or allocation of sperm between the female's two spermathecae. The decline in number of sperm stored as males aged was not accompanied by age-dependent changes in copula duration, indicating that copula duration and insemination success are not deterministically linked. We discuss these results in light of their relevance to SIT and the medfly mating system.     

INDIVIDUAL VARIATION IN SPERM COMPETITION SUCCESS OF YELLOW DUNG FLIES,SCATOPHAGA STERCORARIA

Intraspecific variation in the proportion of offspring sired by the second male to mate with a female (P₂) is an aspect of sperm competition that has received little attention. We examined variation in the sperm competition success of individual male dung flies, Scatophaga stercoraria. In unmanipulated matings, copula duration was dependent on male size with smaller males copulating for longer. A principal component analysis was used to generate uncorrelated scores based on a male's size and copula duration. Using these scores demonstrated that P₂ values were dependent both on the relative size and copula durations of competing males. When copula duration was held constant, the success of an individual male increased as his body size, relative to the first male, increased. We interrupted copulations of “large” and “small” second males and fitted the resultant P₂ values to a linear model of sperm competition with unequal ejaculates. The data fit well to a model of sperm displacement in which sperm mix quickly on introduction to the sperm stores. Furthermore, they show that “large” males have a greater rate of sperm displacement than “small” males. The levels of prey availability during testis maturation may influence a male's success in sperm competition although his immediate mating history does not. We show why an understanding of variation in sperm competition success is important for understanding the mechanisms and evolutionary significance of sperm competition.     

Male Age,Copulation Duration and Insemination Success in Mystacides azurea (Leptoceridae,Trichoptera)

Time in copula in the swarming caddis fly Mystacides azurea L. (Trichoptera: Leptoceridae) ranged from 0.33 to 44.5 min. It increased with male age (wing wear) and male dry weight, but was independent of male and female size (forewing length), female wing wear and number of eggs in the females. Older males failed to transfer sperm during copulation more frequently than did younger ones. It is suggested that females benefit by interrupting prolonged copulations if sperm is not transferred rapidly, since being in copula might increase the risk of predation. Alternatively, young and old males follow different mating tactics; old males have a lower chance to acquire new mates and they do better by monopolizing a female, once they get one, by prolonging the copulation.     

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.     

Female mate choice and mating costs in the polyandrous butterflyPieris napi (Lepidoptera: Pieridae)

Males of the green-veined white butterfly (Pieris napi L.) transfer large ejaculates that represent on average 15% of their body mass when mating for a first time. Shortly after mating a male is able to transfer only a small ejaculate when mating a second time. Male ejaculate production plays a crucial role in the mating system ofP. napi because females use male-derived nutrients for egg production and somatic maintenance. Here we study how timing of female rematings and copulation duration are influenced by the mating history of their mates and, also, study if females exert mate choice to minimize their mating costs. Mating with a recently mated male increased female mating costs by increasing time in copula and mating frequency. Virgin females that mated with virgin males remated after an average of 6 days, whereas virgin females that mated with recently mated males remated after an average of 2 days. Moreover, copulations involving recently mated males lasted on average almost 7 h, whereas copulations involving virgin males lasted on average 2 h. Recently mated males were eager to remate, in spite of the fact that the size of the ejaculate they transfer is small and that they remain in copula for a long time. Hence it seems that males are more successful in the sexual conflict over mating decisions and that females do not minimize mating costs by choosing to mate preferentially with virgin males.     

Sperm competition games played by dimorphic male beetles

Reproductive strategies often consist of two alternative tactics whereby males either compete for and guard females, or sneak copulations. By their nature, alternative tactics expose males to differing risks of sperm competition; sneaks will always be subject to sperm competition but guards will be subject to sperm competition with low probability, dependent on the number of sneaks. Recent game-theoretical models predict that males in the sneak role should have the greater gametic expenditure but that the disparity in expenditure should decrease with increasing numbers of sneaks. Male dung beetles in the genus Onthophagus can be separated into two morphs: major males have horns and guard females whereas minor males are hornless and sneak copulations. Here we compare testis size and ejaculate characteristics between these alternative morphs. We find that in O. binodis 30% of males are sneaks, and sneaks have larger testes, ejaculate volumes, and longer sperm than guards. In O. taurus 60% of males are sneaks and there are no differences in gametic traits. Our data thus provide empirical support for game-theoretical models of sperm competition.     

Mating status regulates post‐mating refractory period in Striacosta albicosta females

In polyandrous insect species, males may transfer substances to reduce sperm competition by affecting female sexual receptivity. In this study, we determined the incidence of polyandry in females of Western bean cutworm (WBC), Striacosta albicosta (Smith) (Lepidoptera: Noctuidae), and investigated the influence of both previous female and male mating history on the duration of mating, the female refractory period, and subsequent calling behavior of females under controlled laboratory conditions. The mating status of WBC males influenced mating duration, with copulations involving previously mated males taking longer, possibly related to the time required to produce an ejaculate. The duration of the female refractory period and the onset time of recalling during the scotophase were both affected by female mating history, but not by that of the males. Females had a shorter refractory period and resumed calling activity earlier after their second and third matings than after their first mating. The earlier onset of calling by previously mated females could reduce competition with virgin females and their shorter refractory period could explain the high incidence of polyandry observed in nature.     

Investment in Mate Guarding May Compensate for Constraints on Ejaculate Production in the Cricket Gryllodes sigillatus

Although there is a corpus of evidence that females of many taxa are choosy about males, there is less information on how males may react to females of different 'quality' (i.e. potential fecundity). The cricket Gryllodes sigillatus shows distinct mate guarding behaviour. We examined how long males mate guard females of different sizes (reflecting egg load and potential fecundity). We also examined the sperm number in ampullae donated to females of different sizes to see if males make a concomitant difference in investment in ejaculate. We also examined mate-guarding behaviour and ejaculate size of males mated to virgin and nonvirgin females of the same size to see if males equate size with increased age and increased likelihood of mating (increased sperm competition). The results showed that males mate guard larger females for longer but make no difference in ejaculate investment between sizes of female. Males make no significant difference in mate guarding investment or ejaculate investment between virgins and nonvirgins of the same size. There is evidence that other species of crickets do alter their ejaculate depending on the female size and mating history, but have less distinct guarding behaviour. We suggest that mate-guarding investment in G. sigillatus may serve a similar function to that of ejaculate investment in other crickets.     

The effects of reproduction on courtship, fertility and longevity within and between alternative male mating tactics of the horned beetle, Onthophagus binodis

Life history theory provides a powerful tool to study an organism's biology within an evolutionary framework. The notion that males face a longevity cost of competing for and displaying to females lies at the core of sexual selection theory. Likewise, recent game theory models of the evolution of ejaculation strategies assume that males face a trade-off between expenditure on the ejaculate and expenditure on gaining additional matings. Males of the dung beetle Onthophagus binodis adopt alternative reproductive tactics in which major males fight for and help provision females, and minor males sneak copulations with females that are guarded by major males. Minor males are always subject to sperm competition, and consistent with theoretical expectation, minor males have a greater expenditure on their ejaculate than major males. We used this model system to seek evidence that mating comes at a cost for future fertility and/or male expenditure on courtship and attractiveness, and to establish whether these traits vary between alternative mating tactics. We monitored the lifespan of males exposed to females and nonmating populations, and sampled males throughout their lives to assess their fertility and courtship behaviour. We found a significant longevity cost of reproduction, but no fertility cost. On average, males from mating populations had a lower courtship rate than those from nonmating populations. This small effect, although statistically nonsignificant, was associated with significant increases in the time males required to achieve mating. Minor males had lower courtship rates than major males, and took longer to achieve mating. Although we did not measure ejaculate expenditure in this study, the correlation between lower courtship rate and longer mating speed of minor males documented here with their greater expenditure on the ejaculate found in previous studies, is consistent with game theory models of ejaculate expenditure which assume that males trade expenditure on gaining matings for expenditure on gaining fertilizations.     

Control of copula duration and sperm storage by female Queensland fruit flies

Copula duration and sperm storage patterns can directly or indirectly affect fitness of male and female insects. Although both sexes have an interest in the outcome, research has tended to focus on males. To investigate female influences, we compared copula duration and sperm storage of Queensland fruit fly females that were intact, or had been incapacitated through decapitation or abdomen isolation. We found that copulations were far longer when females had been incapacitated, indicating that constraints imposed on copula duration by intact females had been relaxed. Repeatability of copula duration for males was very low regardless of female treatment, and this is also consistent with strong female influence. Number of sperm in the spermathecae was not influenced by female treatment, suggesting that female abdominal ganglia control the transport of sperm to these long-term storage organs. However, more sperm were found in the ventral receptacles of incapacitated females compared to intact females. Overall, results implicate cephalic ganglia in regulation of copula duration and short-term sperm storage in the ventral receptacle and abdominal ganglia in regulation of long-term sperm storage in the spermathecae.     

Copula in yellow dung flies (Scathophaga stercoraria): investigating sperm competition models by histological observation

While sperm competition has been extensively studied, the mechanisms involved are typically not well understood. Nevertheless, awareness of sperm competition mechanisms is currently recognised as being of fundamental importance for an understanding of many behavioural strategies. In the yellow dung fly, a model system for studies of sperm competition, second male sperm precedence appears to result from a combination of sperm displacement and sperm mixing. Displacement was until recently thought to be directly from the female's sperm stores, the spermathecae (i.e. males were thought to ejaculate directly into these stores), and under male control. However, recent work indicates displacement is indirect (i.e. males do not ejaculate directly into the sperm stores) and that it is female-aided, although the evidence was not based on direct observation. Here, we used histological techniques to directly determine interactions during copula and sperm transfer. Our results are consistent with inference and clearly show that males ejaculate into the bursa copulatrix. Our data are also consistent with active female involvement in sperm displacement, which is indirect, and indicate the aedeagus may remove some spermatozoa from the bursa at the end of copula. In addition, evidence suggests females aid sperm transport to and from the spermathecae, possibly by muscular movement of a spermathecal invagination.     

Domestication-related changes in sexual performance of Queensland fruit fly

In Sterile Insect Technique (SIT) programs, massive numbers of insects are reared, sterilized, and released in the field to impede reproduction of pest populations. The domestication and rearing processes used to produce insects for SIT programs may have significant evolutionary impacts on life history and reproductive biology. We assessed the effects of domestication on sexual performance of laboratory reared Queensland fruit fly, Bactrocera tryoni, by comparing an old (49 generations) and a young colony (5 generations). We evaluated mating propensity, mating latency, copula duration, sperm transfer, and ability to induce sexual inhibition in mates. Overall, both males and females from the old colony had greater mating propensity than those from the young colony. Copula duration was longer when females were from the old colony. There was no evidence of sexual isolation between the colonies as males and females from the two colonies had similar propensity to mate with flies from either colony. Males from the old colony transferred more sperm regardless of which colony their mate was from. Finally, males from both colonies were similarly able to induce sexual inhibition in their mates and were also similarly able to secure copulations with already-mated females. Positive effects of domestication on sperm transfer, coupled with maintained ability to induce sexual inhibition in mates and to secure copulations with previously mated females, highlights that domestication may have little effect, or even positive effects, on some aspects of sexual performance that may advantage mass-reared B. tryoni in SIT programs.     

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.     

Butterflies tailor their ejaculate in response to sperm competition risk and intensity

Males of many insects eclose with their entire lifetime sperm supply and have to allocate their ejaculates at mating prudently. In polyandrous species, ejaculates of rival males overlap, creating sperm competition. Recent models suggest that males should increase their ejaculate expenditure when experiencing a high risk of sperm competition. Ejaculate expenditure is also predicted to vary in relation to sperm competition intensity. During high intensity, where several ejaculates compete for fertilization of the female''s eggs, ejaculate expenditure is expected to be reduced. This is because there are diminishing returns of providing more sperm. Additionally, sperm numbers will depend on males'' ability to assess female mating status. We investigate ejaculate allocation in the polyandrous small white butterfly Pieris rapae (Lepidoptera). Males have previously been found to ejaculate more sperm on their second mating when experiencing increased risk of sperm competition. Here we show that males also adjust the number of sperm ejaculated in relation to direct sperm competition. Mated males provide more sperm to females previously mated with mated males (i.e. when competing with many sperm) than to females previously mated to virgin males (competing with few sperm). Virgin males, on the other hand, do not adjust their ejaculate in relation to female mating history, but provide heavier females with more sperm. Although virgin males induce longer non-receptive periods in females than mated males, heavier females remate sooner. Virgin males may be responding to the higher risk of sperm competition by providing more sperm to heavier females. It is clear from this study that males are sensitive to factors affecting sperm competition risk, tailoring their ejaculates as predicted by recent theoretical models.     

Differential sperm expenditure reveals a possible role for post‐copulatory sexual selection in a lekking moth

Male reproductive success in the lesser wax moth Achroia grisella is strongly determined by pre‐copulatory mate choice, during which females choose among males aggregated in small leks based on the attractiveness of ultrasonic songs. Nothing is known about the potential of post‐copulatory mechanisms to affect male reproductive success. However, there is evidence that females at least occasionally remate with a second male and that males are unable to produce ejaculates quickly after a previous copulation. Here we investigated the effects of mating history on ejaculate size and demonstrate that the number of transferred sperm significantly decreased from first (i.e., virgin) to second (i.e., nonvirgin) copulation within individual males. For males of identical age, the number of sperm transferred was higher in virgin than in nonvirgin copulations, too, demonstrating that mating history, is responsible for the decrease in sperm numbers transferred and not the concomitant age difference. Furthermore, the number of transferred sperm was significantly repeatable within males. The demonstrated variation in ejaculate size both between subsequent copulations as well as among individuals suggests that there is allocation of a possibly limited amount of sperm. Because female fecundity is not limited by sperm availability in this system, post‐copulatory mechanisms, in particular sperm competition, may play a previously underappreciated role in the lesser wax moth mating system.     

Female American mink, Mustela vison, mate multiply in a free-choice environment

Like most solitary carnivores, the home ranges of male American mink overlap with those of several females, but each female typically shares its range with only a single male. Nevertheless, female mink produce multiply sired litters. Unusually among mammals, the physiological characteristics of female American mink make them highly suited to multiple paternity, suggesting that polyandry in the species may result from female behaviour rather than from coercion by males. In a free-choice experiment on captive-bred animals, all female mink mated multiply, with seven of the eight mating with all three available partners. Since females sometimes resisted copulations, multiple mating in this species may result from their behaviour rather than male coercion. Females visited males nonrandomly, but visiting patterns did not predict mating patterns, suggesting disparate social and mating preferences. There was no evidence for precopulatory female choice, since the allocation of copulations among available males did not deviate significantly from the expected distribution. Females mated multiply only around the predicted times of ovulation, contrary to the expectation that they should be more selective at times of peak fertility. Larger males copulated for longer in total: copulation duration is probably under male control, suggesting that larger males could influence mating duration, perhaps increasing their share of paternity by greater sperm transfer. Our observations suggest that in wild populations, mating patterns could be influenced by female preference for multiple partners.     

Optimal copula duration in yellow dung flies: effects of female size and egg content

We examine data on copula duration in dung flies, Scatophaga stercoraria, in relation to female phenotype. We use a marginal value theorem approach based on the plausible mechanisms of sperm competition to predict the effect of female variation on optimal copula duration, t *, from the male perspective. Future fertilizations are expected to have a trivial effect on t * with fully gravid females, but an increasing relative effect on t * towards completion of oviposition. t * is expected to be affected by female size because of variation in (1) a female's egg content, which increases the maximum egg gain available from a mating, and (2) the female reproductive tract, which affects the rate at which sperm are displaced. In fully gravid females, t * was not dependent on egg number variation, but showed a positive relation with egg content in females that had laid a varying proportion of their mature egg load at the time of mating, and were therefore not fully gravid. Our models predict that if a male can estimate egg content only by the distension of a female's abdomen, t * should increase in a similar way to that seen with 'take-over' females. We predict t * for fully gravid females by assuming that males can monitor female size. The data showed that sperm displacement rate decreased, and average egg content increased, with female size. Under two models for a sperm displacement mechanism, one (which assumes indirect displacement at a rate proportional to the increase in spermathecal volume) predicts the observed relation between t * and female size almost exactly. Small males copulated for longer than large males (as predicted and reported previously). Copyright 1999 The Association for the Study of Animal Behaviour.     

Size-Dependent Mating Success at Various Nutritional States in the Yellow Dung Fly

Mating success not only depends on genetic quality, but also equally on environmental factors, most prominently food availability. We investigated the interactive effects of nutritional state and body size on mating success and copula duration in yellow dung fly males ( Scathophaga stercoraria ; Diptera: Scathophagidae) of three body size selection lines in the laboratory in both non-competitive (single) and competitive (group) situations. Adults require protein and lipids from prey to reproduce, as well as sugars as an energy source for reproductive activity. We expected mating success to decrease with time because of sperm depletion (sugar treatment) and/or energy shortage (water treatment) relative to the control, prey plus sugar treatment. Based on physiological scaling, we also expected large-line males to become depleted either sooner because of their higher energy and sperm demands, or later because of their more efficient energy use. Average mating success indeed declined over a period of 5–7 d (or 5–15 potential copulations per male), but equally for all food treatments and body size classes. Surprisingly, water-fed and small-line males had the highest mating probability in the non-competitive setting, while in the competitive setting large-line males had the highest success. Energy-depleted males showed apparent terminal investment. Small males acquired females more readily but eventually lost them to larger males in the competitive situation. As shown before, copula duration was inversely related to body size and increased with copulation number, independent of food treatment. We conclude that sugar or prey shortage has little effect on mating success in the short term, and does not differentially affect males of different sizes.     

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.