Risk Of Sperm Competition Research Articles

Male alternative reproductive tactics and sperm competition: a meta-analysis.

ABSTRACTIn many animal species, males may exhibit one of several discrete, alternative ways of obtaining fertilisations, known as alternative reproductive tactics (ARTs). Males exhibiting ARTs typically differ in the extent to which they invest in traits that improve their mating success, or the extent to which they face sperm competition. This has led to the widespread prediction that males exhibiting ARTs associated with a high sperm competition risk, or lower investment into traits that improve their competitiveness before mating, should invest more heavily into traits that improve their competitiveness after mating, such as large ejaculates and high‐quality sperm. However, despite many studies investigating this question since the 1990s, evidence for differences in sperm and ejaculate investment between male ARTs is mixed, and there has been no quantitative summary of this field. Following a systematic review of the literature, we performed a meta‐analysis examining how testes size, sperm number and sperm traits differ between males exhibiting ARTs that face either a high or low sperm competition risk, or high or low investment in traits that increase mating success. We obtained data from 92 studies and 67 species from across the animal kingdom. Our analyses showed that male fish exhibiting ARTs facing a high sperm competition risk had significantly larger testes (after controlling for body size) than those exhibiting tactics facing a low sperm competition risk. However, this effect appears to be due to the inappropriate use of the gonadosomatic index as a body‐size corrected measure of testes investment, which overestimates the difference in testes investment between male tactics in most cases. We found no significant difference in sperm number between males exhibiting different ARTs, regardless of whether sperm were measured from the male sperm stores or following ejaculation. We also found no significant difference in sperm traits between males exhibiting different ARTs, with the exception of sperm adenosine triphosphate (ATP) content in fish. Finally, the difference in post‐mating investment between male ARTs was not influenced by the extent to which tactics were flexible, or by the frequency of sneakers in the population. Overall, our results suggest that, despite clear theoretical predictions, there is little evidence that male ARTs differ substantially in investment into sperm and ejaculates across species. The incongruence between theoretical and empirical results could be explained if (i) theoretical models fail to account for differences in overall resource levels between males exhibiting different ARTs or fundamental trade‐offs between investment into different ejaculate and sperm traits, and (ii) studies often use sperm or ejaculate traits that do not reflect overall post‐mating investment accurately or affect fertilisation success.

The Effects of a Male Audience on Male and Female Mating Behaviour in the Blood-Sucking Bug Rhodnius prolixus

Social context is a key factor affecting sexual behaviour and cannot be neglected in gregarious species, such as triatomine blood-sucking bugs. Here we study the influence of the social context on the sexual reproductive behaviour of males and females of Rhodnius prolixus Stål. Specifically, we identify and compare the frequencies and sequence of sexual behaviours exhibited by a focal pair in presence or absence of a male audience. We expect that in presence of a male audience females increase their selectiveness level since the risk of losing infertile eggs decreases in presence of more candidates and females can benefit from copulating with a better quality male. Besides, in presence of potential rivals, we expect changes in the sexual behaviour of focal males, associated to a reduction in the risk of sperm competition. As expected, in presence of a male audience, females significantly increased the exhibition of rejection behaviour. Moreover, focal males exhibited shorter latencies to mount the female, longer duration of copula, and differences in their stereotyped behaviour exhibited during copula. We discuss the influence of the social context on the reproductive behaviour of females and males of R. prolixus.

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.

Sperm Numbers as a Paternity Guard in a Wild Bird.

Sperm competition is thought to impose strong selection on males to produce competitive ejaculates to outcompete rival males under competitive mating conditions. Our understanding of how different sperm traits influence fertilization success, however, remains limited, especially in wild populations. Recent literature highlights the importance of incorporating multiple ejaculate traits and pre-copulatory sexually selected traits in analyses aimed at understanding how selection acts on sperm traits. However, variation in a male’s ability to gain fertilization success may also depend upon a range of social and ecological factors that determine the opportunity for mating events both within and outside of the social pair-bond. Here, we test for an effect of sperm quantity and sperm size on male reproductive success in the red-back fairy-wren (Malurus melanocephalus) while simultaneously accounting for pre-copulatory sexual selection and potential socio-ecological correlates of male mating success. We found that sperm number (i.e., cloacal protuberance volume), but not sperm morphology, was associated with reproductive success in male red-backed fairy-wrens. Most notably, males with large numbers of sperm available for copulation achieved greater within-pair paternity success. Our results suggest that males use large sperm numbers as a defensive strategy to guard within-pair paternity success in a system where there is a high risk of sperm competition and female control of copulation. Finally, our work highlights the importance of accounting for socio-ecological factors that may influence male mating opportunities when examining the role of sperm traits in determining male reproductive success.

Morph‐specific investment in testes mass in a trimorphic beetle, Proagoderus watanabei

AbstractWhen competition between males for mates is intense, it is common to find that some males will adopt alternative tactics for acquiring fertilizations, often involving the use of ‘sneak’ tactics whereby males avoid precopulatory contests. These alternative tactics are sometimes associated with discrete differences in male morphology, with sneak males investing less in weaponry but more in traits such as testes which may give an advantage in sperm competition. In some cases, it appears that males develop into more than two morphs, with a number of examples of tri‐ and even tetramorphic arthropod species being described. Here, we analyse the scaling relations of the dung beetle species Proagoderus watanabei, which expresses two distinct weapon traits: paired head horns and a pronotal horn. We find that males of this species are trimorphic, with alpha males expressing long head horns and a pronotal horn, beta males with long head horns but no pronotal horn and gamma males with short head horns only. We also find that alpha males invest relatively less in testes than do beta or gamma males, indicating that beta and gamma males in this species probably experience higher risks of sperm competition than do alphas.

Mate-guarding duration is mainly influenced by the risk of sperm competition and not by female quality in a golden orb-weaver spider.

Males are expected to mate with as many females as possible, but can maximize their reproductive success through strategic mating decisions. For instance, males can increase their own fitness by mating with high quality females that produce more offspring. Additionally, males can adjust mating effort based on the relative distribution of females and male competitors. To test factors that influence male mate choice, we assessed male mating decisions in the golden silk orb-weaver spider, Trichonephila clavipes (Nephilidae), a species in which females are polyandrous, males guard females before and after copulation occurs and large males are the most successful at guarding mates. We tested the hypothesis that males spend more time guarding high quality females that are spatially isolated, and when the risk of sperm competition is higher. We also hypothesized that this effect increases with male body size. We assessed solitary and aggregated female webs in the field and quantified female quality (i.e., female body condition), male size (i.e., male body size), the risk of sperm competition (i.e., number of males in each female web), and mate-guarding duration (i.e., number of days each male spent in each web). We found that mate-guarding behaviour is largely influenced by the presence of male competitors. In addition, male body size seems to moderately influence male guarding decisions, with larger males guarding for a longer time. Finally, female body condition and type of web (i.e., solitary or aggregated) seem to play small roles in mate-guarding behaviour. As mate-guarding duration increased by 0.718 day per each additional male competitor in the web, and guarding behaviour prevents males from seeking additional mates, it seems that guarding females can be considerably costly. We conclude that failing to guard a sexual partner promotes high costs derived from sperm competition, and a male cannot recover his relative loss in fertilization success by seeking and fertilizing more females. In addition, the search for more sexual partners can be constrained by possible high costs imposed by weight loss and fights against other males, which may explain why the type of web only moderately influenced male mate choice. Following the same rationale, if high-quality females are not easy to find and/or mating with a high-quality female demands much effort, males may search females and guard them regardless of female quality. In conclusion, the factor that most influences male mate-guarding behaviour among T. clavipes in the field is the risk of sperm competition.

Postcopulatory sexual selection is associated with sperm aggregate quality in Peromyscus mice.

In some species, sperm form coordinated groups that are hypothesized to improve their swimming performance in competitive contexts or to navigate through the viscous fluids of the female reproductive tract. Here we investigate sperm aggregation across closely related species of Peromyscus mice that naturally vary by mating system to test the predictions that sperm aggregates 1) are faster than solitary sperm in species that females mate multiply to aid cells in sperm competition, and 2) outperform solitary sperm cells in viscous environments. We find significant variation in the size of sperm aggregates, which negatively associates with relative testis mass, a proxy for sperm competition risk, suggesting that postcopulatory sexual selection has a stabilizing effect on sperm group size. Moreover, our results show that sperm aggregates are faster than solitary sperm in some, but not all, species, and this can vary by fluid viscosity. Of the two species that produce the largest and most frequent groups, we find that sperm aggregates from the promiscuous P. maniculatus are faster than solitary sperm in every experimentally viscous environment but aggregation provides no such kinematic advantage under these same conditions for the monogamous P. polionotus. The reduced performance of P. polionotus aggregates is associated with less efficient aggregate geometry and the inclusion of immotile or morphological abnormal sperm. Our cross-species comparison yields insight into the evolution of sperm social behaviors, provides evidence of extensive variation in the Peromyscus lineage, and reveals that differences in sperm aggregate quality associate with postcopulatory sexual selection.

Free‐ranging male northern map turtles use public information when interacting with potential mates

AbstractSperm competition is prevalent in animals, and many adaptations have evolved to reduce its risk. Males can reduce the risk of sperm competition by using public information when interacting with potential mates. Specifically, males can reduce sperm competition by avoiding females already affiliated with rival males. We tested this hypothesis in a population of wild northern map turtles (Graptemys geographica), a gregarious species with seemingly prevalent sperm competition. We used 3D‐printed decoys and underwater videography to record the response of free‐ranging males to female decoys affiliated or not with rivals. More visits were made by males to female decoys when rivals were present, suggesting a form of eavesdropping during mate selection. Males were more likely to interact with the female decoy, however, when rivals were absent, suggesting that they behave to reduce sperm competition. Moreover, the types of interactions differed between the accompanied and the unaccompanied female decoys, indicating an audience effect during male–female interactions. Finally, males interacted more with the male decoys than with the female decoys in the treatment with rivals, indicating a yet unclear form of male–male interaction. Collectively, our results suggest that free‐ranging male northern map turtles use public information for mate selection and to reduce sperm competition.

Pupal Cues Increase Sperm Production but Not Testis Size in an Insect.

Simple SummaryAnimals adjust their resource allocation strategies to maximize their reproductive benefit under dynamic socio-sexual environments. For example, male insect adults increase their testicular investment with the perceived increase of rivals to gain a competitive advantage in fathering offspring. To date, it is not clear whether insect pupae, which do not feed and crawl, can fine-tune their investment in sperm and testis size according to their social-sexual settings. This knowledge is vital to understanding how male insects respond to their surroundings experienced at different life stages. Using a moth which produces both fertile and unfertile sperm, we demonstrated for the first time that after detecting cues from conspecific pupae regardless of sex, male pupae increased production of both types of sperm at the same rate but kept testis size unchanged. Because most morphological traits are formed during the larval stage in insects, testis size may be fixed after pupation, allowing little room for the pupae to adjust testis size with social changes. Like adults, male pupae with fully grown testes have sufficient resources to produce more sperm of both types according to the perceived increase of sperm competition risk. Theoretic and empirical studies show that social surroundings experienced by male insects during their larval or adult stage can influence their testicular investment in diverse ways. Although insect pupae do not feed and crawl, they can communicate using sex-specific and/or non-sex specific cues. Yet, it is unknown, in any insect, whether and how male pupae can fine-tune their resource allocation to sperm production and testis size in response to socio-sexual environments. We investigated this question using a moth, Ephestia kuehniella, which produces fertile eupyrene sperm and unfertile apyrene sperm. We held male pupae individually or in groups with different sex ratios, and dissected adults upon eclosion, measured their testis size, and counted both types of sperm. We demonstrated that after exposure to conspecific pupal cues regardless of sex, male pupae increased production of eupyrenes and apyrenes at the same rate but kept testis size unchanged. We suggest that testis size is fixed after pupation because most morphological traits are formed during the larval stage, allowing little room for pupae to adjust testis size. Like adults, male pupae with fully grown testes have sufficient resources to produce more sperm of both types according to the perceived increase in sperm competition risk.

Sperm competition risk affects ejaculate strategy in terms of sperm number but not sperm size in squid.

In polygamous species, the mode of sperm storage in females influences evolution of sperm quantitative and qualitative traits because it provides the arena for sperm competition, cryptic female choice and fertilization processes. In this study, we compared ejaculate traits of two squid species, Heterololigo bleekeri and Loligo reynaudii. Both species show dimorphic sperm traits associated with alternative reproductive tactics where consort and sneaker males transfer sperm to different storage sites within a female (on the oviduct and near the mouth, respectively). Due to differences in reproductive behaviours and sperm placement, sperm competition risk is expected to be higher in sneakers than in consorts of both species and higher overall in L.reynaudii. Our results demonstrate that the instantaneous number of released sperm is adjusted to the expected sperm competition risk via an elaborate sperm package. Consort sperm are similar in size; however, sneaker sperm have a significantly longer flagellum in H.bleekeri than in L.reynaudii, most likely due to intra-tactic conflicts associated with sperm storage conditions.From consideration of the different mating tactics, we suggest that while levels of sperm competition determine quantitative traits, sperm quality traits are determined more by the mode of sperm storage and fertilization.

Males of a Neotropical spider adjust prey‐gift construction but not mate search in response to sperm competition

AbstractIn scramble competition mating systems, males do not guard females and females usually mate polyandrously, leading to high levels of sperm competition. Mathematical models predict that males facing sperm competition can either decrease or increase their reproductive effort. To test how male reproductive responses vary according to sperm competition risk, we ran two experiments using the polyandrous gift‐giving spiderParatrechalea ornata. First, we tested whether males prefer substrates containing silk of unmated females than substrates containing silk of once‐mated females, competitor males or without silk. Males spent more time on substrates containing silk of females but showed no preference for unmated females, which indicates that males may optimize mate search by prioritizing substrates containing cues of potential mating partners. The lack of preference for unmated females suggests that males cannot detect female mating status from silk or that the costs of searching preferentially for unmated females are high in the field. Second, we tested whether males adjust prey‐gift construction when facing sperm competition risk. Experimental groups (substrate containing silk of an unmated female and silk of another malevs. substrate containing only silk of an unmated female) did not differ in the frequency of gift construction, latency to gift construction and number of flies added to the gift. However, males facing sperm competition risk constructed gifts with less silk, suggesting the occurrence of a less evident form of male mate choice based on differential allocation of resources to prey‐gift. Additionally, given the trade‐off between investing in prey‐gifts quantity or quality, a lower silk investment should enable males to increase the number of gifts constructed and the number of courted females. Taken together, our results indicate that males detect female and male pheromones on substrate and adjust the construction of prey‐gifts in response to apparent sperm competition risk.

Sperm and alternative reproductive tactics: a review of existing theory and empirical data.

Males that exhibit alternative reproductive tactics (ARTs) often differ in the risk of sperm competition and the energetic trade-offs they experience. The resulting patterns of selection could lead to between-tactic differences in ejaculate traits. Despite extensive research on male ARTs, there is no comprehensive review of whether and what differences in sperm traits exist between male ARTs. We review existing theory on ejaculate evolution relevant to ARTs and then conduct a comprehensive vote-counting review of the empirical data comparing sperm traits between males adopting ARTs. Despite the general expectation that sneaker males should produce sperm that are more competitive (e.g. higher quality or performance), we find that existing theory does not predict explicitly how males adopting ARTs should differ in sperm traits. The majority of studies find no significant difference in sperm performance traits between dominant and sneaker males. However, when there is a difference, sneaker males tend to have higher sperm performance trait values than dominant males. We propose ways that future theoretical and empirical research can improve our understanding of the evolution of ejaculate traits in ARTs. We then highlight how studying ejaculate traits in species with ARTs will improve our broader knowledge of ejaculate evolution. This article is part of the theme issue 'Fifty years of sperm competition'.

Spatially clustered resources increase male aggregation and mating duration in Drosophila melanogaster

In environments where females mate multiply, males should adjust their behaviour and physiology in response to the perceived level of sperm competition to maximize their fitness. Evidence of such plasticity has been found in several laboratory and field studies, but little is known about the cues stimulating these responses in natural populations. One way in which males appear to assess sperm competition risk is through encounter rates with conspecific males. Such encounter rates may be driven by the spatial distribution of resources required by males (i.e. food patches or potential mates), which in turn affects local density. However, explicit links between resource distribution, male encounter rates and shifts in behaviour related to sperm competition have not been demonstrated. We found that when group size of Drosophila melanogaster males was held constant, a small decrease in the distance between patches of food resources had striking effects on male behaviour. Compared to those from dispersed resources, males on clustered resources had a significantly reduced intermale distance (and hence encounter rate) and subsequently a longer noncompetitive copulation duration, previously shown to be a reliable indicator of male perception of sperm competition risk. The aggregation of resources, operating via increased encounter rate, can stimulate shifts in behaviour affecting male sperm competition performance. Given that the spatial distribution of resources is typically variable in natural populations (and often unpredictable), selection is likely to favour the evolution of plasticity in sexual behaviour where resource aggregation increases the probability of sperm competition.

Genotype-by-environment interactions for precopulatory mate guarding in a lek-mating insect.

In sexually reproducing species, males often experience strong pre‐ and postcopulatory sexual selection leading to a wide variety of male adaptations. One example is mate guarding, where males prevent females from mating with other males either before or after they (will) have mated themselves. In case social conditions vary short term and in an unpredictable manner and if there is genetic variation in plasticity of mate guarding (i.e., genotype‐by‐environment interaction, G × E), adaptive behavioral plasticity in mate guarding may evolve.Here, we test for genetic variation in the plasticity of precopulatory mate‐guarding behavior in the lek‐mating lesser wax moth Achroia grisella. When offered two females in rapid succession, virgin males of this species usually copulate around 10–20 min with the first female. With the second female, however, they engage in copulation posture for many hours until they have produced another spermatophore, an unusual behavior among insects possibly functioning as precopulatory mate guarding. Previous studies showed the mating latency with the second female to be shorter under higher perceived sperm competition risk. We accordingly measured the mate‐guarding behavior of males from six inbred lines under either elevated perceived male–male competition risk or under control conditions allowing us to test for G × E interactions.We found significant inbred line‐by‐competitor treatment interactions on mating latency and copulation duration with the second female suggesting genetic variation in the degree of behavioral plasticity. However, we found no significant G × E interaction on the sum of mating latency and copulation duration. Our results suggest a potential for adaptive evolution of mate‐guarding plasticity in natural populations of lek‐mating species. Future studies using selection experiments and experimental evolution approaches in laboratory populations, or comparisons of multiple natural populations will be helpful to study under which conditions plasticity in male mate‐guarding behavior evolves.

Males adjust their manipulation of female remating in response to sperm competition risk.

To reduce the potential for sperm competition, male insects are thought to inhibit the post-mating reproductive behaviour of females through receptivity-inhibiting compounds transferred in the ejaculate. Selection is expected to favour phenotypic plasticity in male post-copulatory expenditure, with males investing strategically in response to their perceived risk of sperm competition. However, the impact that socially cued strategic allocation might have on female post-mating behaviour has rarely been assessed. Here, we varied male perception of sperm competition risk, both prior to and during mating, to determine if a male's competitive environment impacts the extent to which he manipulates female remating behaviour. We found that female Australian field crickets (Teleogryllus oceanicus) mated to males that were reared under sperm competition risk emerged from a shelter in search of male song sooner than did females mated to males reared without risk, but only when mating occurred in a risk-free environment. We also found that females reared in a silent environment where potential mates were scarce emerged from the shelter sooner than females exposed to male calls during development. Collectively, our findings suggest complex interacting effects of male and female sociosexual environments on female post-mating sexual receptivity.

Males adjust their manipulation of female remating in response to sperm competition risk: Plasticity in female remating inhibition

Males adjust their manipulation of female remating in response to sperm competition risk: Plasticity in female remating inhibition

Male body size does not affect the refractory period of females in the West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) and the seed bug Togo hemipterus (Scott) (Heteroptera: Lygaeidae)

Multiple mating by females increases both the risk and intensity of sperm competition for males. The ability to temporarily or permanently inhibit female receptivity to further mating represents a highly adaptive strategy to reduce the risk of sperm competition. Males of the West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) and the seed bug Togo hemipterus (Scott) (Heteroptera: Lygaeidae) use accessory gland substances to inhibit remating in females. It is known that there is variance in the length of the female refractory period (i.e., the effectiveness of male ejaculate substances that inhibit female remating) within a species. However, it is unclear what types of male traits affect the length of the female refractory period. The mechanism of the female refractory period is not sufficiently revealed. We focused on male body size, which is often a target of sexual selection, and investigated the relationship between male body size and both the refractory period of females and mating duration. Contrary to our thought, male body size did not significantly influence the refractory period of females or mating duration in either species. Smaller male body size was not particularly favored by post-copulatory sexual selection in either species.

Anthropogenic noise reduces male reproductive investment in an acoustically signaling insect

Rapidly changing environments impose novel selection pressures on organisms, and sometimes adaptive phenotypic plasticity allows organisms to survive and reproduce in the face of environmental change. However, plastic responses can also be maladaptive. In this study, we investigate whether male reproductive investment responds plastically to varied experience with traffic noise. We exposed male crickets chronically to one of three noise treatments from the 2nd-3rd instar until their natural death: masking traffic noise (including noise that overlaps in frequency with the male crickets’ mating calls), non-masking traffic noise (an identical traffic noise track from which we digitally removed the frequencies that mask the crickets’ mating call), and silence. We dissected and weighed their testes and spermatophore molds. Controlling for body mass, we found that the spermatophore molds of crickets reared in masking and non-masking noise were 29% and 24% lighter, respectively, than those of crickets reared in silence. There were no differences in body mass-adjusted testes mass among treatments. If spermatophore mold mass is positively associated with male reproductive output, this reduction in size could have negative fitness consequences for animals exposed to traffic noise. We encourage future work to investigate impacts of noise on reproductive investment in other study systems that are likely sensitive to anthropogenic noise (e.g., birds, frogs, singing insects). Anthropogenic noise is a pervasive pollutant and chronic noise can negatively affect fitness. How does anthropogenic noise influence reproductive investment? Phenotypically plastic responses to noise may increase survival and reproduction in noisy environments. Traffic noise masks the sounds crickets make, potentially changing conspecifics’ perception of population density, mate availability, and the risk of sperm competition. We found that male crickets reared in silence developed significantly larger spermatophore molds (a reproductive structure that delivers the sperm containing packet to the female) than those reared in traffic noise. If the spermatophore mold influences male reproductive output, this reduction in size may have negative impacts on reproductive success of animals exposed to masking traffic noise.

Combined cues of male competition influence spermatozoal investment in a moth

Abstract Male animals usually raise their sperm allocation after detecting sperm competition risk. To date, only a few studies have investigated the cues used by males to sense and respond to rivals. Yet, it is still largely unknown whether males respond to single or combined cues and whether they can increase their lifetime spermatozoal investment after a perception of rival cue(s). Here we postulate that males increase ejaculation and production of sperm after detecting combined cues from rivals, but such response quickly diminishes after the cues are removed. We exposed newly emerged and virgin focal males of the moth Ephestia kuehniella to various rival cues and then permanently removed the cues. We introduced a virgin female to an exposed focal male and an unexposed focal male, respectively, per day and counted the number of sperm transferred by the focal males in each mating and recovered in their body after death. We demonstrate that males significantly increased their lifetime sperm allocation and production after premating detection of either single (acoustic or chemical) or combined cues (acoustic + chemical or acoustic + chemical + tactile) from their rivals with combined cues (acoustic + chemical + tactile) somewhat strengthening such response in eupyrene production. The number of sperm ejaculated by males significantly decreased over successive matings in their lifetime regardless of whether they were exposed to rival cues, but the decline was significantly faster in rival‐cue‐exposed males than in unexposed ones. This suggests that the increase in spermatogenesis cannot fully compensate for that of sperm expenditure in response to rival cues. We show that 10‐hr premating rival exposure was sufficient to maximize males' response in sperm ejaculation and production. The impact of the rival perception on sperm transfer persisted for most of males' reproductive life, suggesting that the moth males have a long‐term memory of sperm competition risk experienced in the early adulthood. A free Plain Language Summary can be found within the Supporting Information of this article.

Effects of high CO2 on mating success, mating duration, and sperm transfer in the West Indian sweet potato weevil, Euscepes postfasciatus

AbstractMost terrestrial animals live in natural atmospheric conditions, but some are also adapted to low oxygen (hypoxic) or high‐carbon dioxide (hypercapnic) conditions, such as in social insect nests, soil, caves, wood, and decaying material. Although it is possible that individuals adapt their behaviour to the environmental condition of their habitats, mating behaviour under ecologically possible ranges of CO2 has not been well studied. We compared walking activity, duration of mating behaviour, and sperm transfer ability in the West Indian sweet potato weevil, Euscepes postfasciatus Fairmaire (Coleoptera: Curculionidae), in high CO2 (10 000 p.p.m.) vs. normal atmospheric conditions (laboratory air, ca. 800 p.p.m.). We found that high CO2 enhanced walking activity and mounting frequency. Under such circumstances, we predicted that enhanced activities under a high‐CO2 environment would increase the risk of sperm competition, which induces an extended copulation period and an increase in sperm transfer. However, weevils shortened the mating period and did not alter their sperm transfer ability under high CO2. These findings are, as far as we are aware, the first report of the effect of ecologically relevant high CO2 on insect mating behaviour under mass‐rearing conditions. The effect of ambient conditions on mating behaviour and sperm transfer is discussed in relation to the intensity of female refusal behaviour directed against males.