Risk Of Sperm Competition Research Articles

Ejaculate Adjustment in Response to Sperm Competition Risk in Humans.

Previous research suggests that human males, like males of many mammalian and avian species, adjust their ejaculate quality in accordance with sperm competition risk. Men who spend less time with their regular female partner since the couple's last copulation produce ejaculates with more sperm at the couple's next copulation (Baker & Bellis, 1993). We conducted a conceptual replication of this research to investigate whether sperm competition risk predicts ejaculate adjustment in human males using additional measures of sperm competition risk (e.g., perceptions of partner infidelity, presence of potential sexual rivals) and updated laser-optic semen analysis technology. We collected data from 34 heterosexual couples (age range 18-32years) from a university population who completed self-report surveys on their relationship dynamics and provided six ejaculate samples (three copulatory and three masturbatory) across a 45-day period. Time spent together since the couple's last copulation was not significantly associated with ejaculate quality. However, sperm concentration for copulatory ejaculates was higher for men who perceived more potential sexual rivals. Discussion situates the current results within the literature on human sperm competition and suggests several directions for future research.

Transcriptomic changes in the posterior pallium of male zebra finches associated with social niche conformance

Animals plastically adjust their physiological and behavioural phenotypes to conform to their social environment—social niche conformance. The degree of sexual competition is a critical part of the social environment to which animals adjust their phenotypes, but the underlying genetic mechanisms are poorly understood. We conducted a study to investigate how differences in sperm competition risk affect the gene expression profiles of the testes and two brain areas (posterior pallium and optic tectum) in breeding male zebra finches (Taeniopygia castanotis). In this pre-registered study, we investigated a large sample of 59 individual transcriptomes. We compared two experimental groups: males held in single breeding pairs (low sexual competition) versus those held in two pairs (elevated sexual competition) per breeding cage. Using weighted gene co-expression network analysis (WGCNA), we observed significant effects of the social treatment in all three tissues. However, only the treatment effects found in the pallium were confirmed by an additional randomisation test for statistical robustness. Likewise, the differential gene expression analysis revealed treatment effects only in the posterior pallium (ten genes) and optic tectum (six genes). No treatment effects were found in the testis at the single gene level. Thus, our experiments do not provide strong evidence for transcriptomic adjustment specific to manipulated sperm competition risk. However, we did observe transcriptomic adjustments to the manipulated social environment in the posterior pallium. These effects were polygenic rather than based on few individual genes with strong effects. Our findings are discussed in relation to an accompanying paper using the same animals, which reports behavioural results consistent with the results presented here.

Ejaculate Adjustment in Response to Sperm Competition Risk in Humans [Conference Presentation Abstract

Ejaculate Adjustment in Response to Sperm Competition Risk in Humans [Conference Presentation Abstract

The transfer of male cuticular hydrocarbons provides a reliable cue of the risk and intensity of sperm competition in decorated crickets.

Theoretically, males should increase their ejaculate expenditure when the probability of sperm competition occurring (or risk) is high but decrease ejaculate expenditure as the number of competing ejaculates (or intensity) increases. Here we examine whether male decorated crickets (Gryllodes sigillatus) use cuticular hydrocarbons (CHCs) transferred to females by rival males at mating to assess the risk and intensity of sperm competition and adjust their ejaculate accordingly. Unmated females and those perfumed with CHCs extracted from one, three, or five males could be distinguished chemically, providing a reliable cue of the risk and intensity of sperm competition. In agreement with theory, males mating with these females increased sperm number with the risk of sperm competition and decreased sperm number with the intensity of sperm competition. Similarly, as the risk of sperm competition increased, males produced a larger and more attractive spermatophylax (an important non-sperm component of the ejaculate) but these traits did not vary with the intensity of sperm competition. Our results therefore demonstrate that both sperm and non-sperm components of the male ejaculate respond to the risk and intensity of sperm competition in different ways and that CHCs provide males with an important cue to strategically tailor their ejaculate.

Evolutionary trade-offs between testes size and parenting in Neotropical glassfrogs.

In males, large testes size signifies high sperm production and is commonly linked to heightened sperm competition levels. It may also evolve as a response to an elevated risk of sperm depletion due to multiple mating or large clutch sizes. Conversely, weapons, mate or clutch guarding may allow individuals to monopolize mating events and preclude sperm competition, thereby reducing the selection of large testes. Herein, we examined how paternal care, sexual size dimorphism (SSD), weaponry and female fecundity are linked to testes size in glassfrogs. We found that paternal care was associated with a reduction in relative testes size, suggesting an evolutionary trade-off between testes size and parenting. Although females were slightly larger than males and species with paternal care tended to have larger clutches, there was no significant relationship between SSD, clutch size and relative testes size. These findings suggest that the evolution of testes size in glassfrogs is influenced by sperm competition risk, rather than sperm depletion risk. We infer that clutch guarding precludes the risk of fertilization by other males and consequently diminishes selective pressure for larger testes. Our study highlights the prominent role of paternal care in the evolution of testes size in species with external fertilization.

Elevated sperm production and faster transfer: plastic responses to the risk of sperm competition in males of the black sodier fly *Hermetia illuce*

Elevated sperm production and faster transfer: plastic responses to the risk of sperm competition in males of the black sodier fly *Hermetia illuce*

Meta-analytical evidence that males prefer virgin females.

Males are often predicted to prefer virgin over non-virgin females because of the reduced risk of sperm competition. Does this prediction hold across studies? Our systematic meta-analysis of 138 studies, mainly conducted in invertebrates, confirms that males generally prefer virgin females. However, males preferred virgin females even in species with last male sperm precedence, suggesting that sperm competition alone does not drive male preferences. Furthermore, our results suggest that males may reject mated females even when no alternative exists. Preference for virgins is unlikely to influence female reproductive success since virginity cannot be selected for, but strong preference for virgin females could swamp or reinforce selection on other traits. Our results add to growing evidence that males are not indiscriminate in mating. However, given the unexplained heterogeneity in effect sizes, we urge caution in assuming that males will prefer virgins and recommend considering the natural context of mating decisions.

Male responses to sperm competition risk associated with increased macronutrient intake and reduced lifespan.

Increased expenditure on the ejaculate is a taxonomically widespread male response to sperm competition. Increased ejaculate expenditure is assumed to come at a cost to future reproduction, otherwise males should always invest maximally. However, the life-history costs of strategic ejaculation are not well documented. Macronutrient intake is known to affect the trade-off between reproduction and lifespan. Intakes of protein and carbohydrate that maximize reproduction often differ from those that maximize lifespan. Here, we asked whether strategic expenditure on the ejaculate by male crickets, Teleogryllus oceanicus, is mediated by macronutrient intake, and whether it comes at a cost of reduced lifespan. Males were exposed to rival song throughout their lifespan or were held in a silent non-competitive environment. Males exposed to song had a higher intake of both protein and carbohydrate, they reached adulthood sooner, produced ejaculates of higher quality, and died sooner than males living in a silent environment. Our findings provide a rare example of both the mechanisms and life-history costs associated with strategic ejaculation.

Mating in the cold. Prolonged sperm storage provides opportunities for forced copulation by male bats during winter.

In a wide range of heterothermic mammals, hibernation interrupts the reproductive cycle by forcing reproductive delays. In hibernating bats with delayed fertilization, an opportunity for sperm competition is enhanced by extending a time-window between copulations and fertilization. In order to achieve greater fertilization success, males are expected to show adaptations for sperm competition by increasing their opportunities for mating over an extended period. We aimed to clarify the physiological and behavioral characteristics of male bats experiencing increased risks of sperm competition. We investigated the characteristics of the reproductive cycle of the little horseshoe bat (Rhinolophus cornutus), and examined whether males retain reproductive physiology related to sexual behavior, and attempt to copulate with females even during the hibernation period. Field observations and histological examinations of the reproductive cycle confirmed that females, having mated in the autumn, store spermatozoa in the uterus during hibernation and give birth in the early summer to just one offspring per year, thus males face a low certainty of successful fertilization. Although their testes regressed rapidly and their testosterone levels were lower during winter than in autumn, males stored motile spermatozoa in their cauda epididymides from autumn throughout the winter. During hibernation, we found that males occasionally aroused from torpor and attempted to mate forcibly with torpid females. Forced copulations appear to increase a male's chances of obtaining a mate while avoiding pre-copulatory female choice. Epididymal sperm storage could be advantageous for males in allowing them to extend their potential mating period even though their testes have regressed. We also found that some hibernating nulliparous females were ready for fertilization in spring after hibernation, whereas few parous females appeared in the same roost. In contrast to males, forced copulations would be maladaptive for females because they cannot opt for higher-quality males while in torpor. Females that have experienced sexual coercion when young may subsequently avoid hibernacula where adult males are present.

The coevolutionary dynamics of cryptic female choice.

In contrast to sexual selection on traits that affect interactions between the sexes before mating, little theoretical research has focused on the coevolution of postmating traits via cryptic female choice (when females bias fertilization toward specific males). We used simulation models to ask (a) whether and, if so, how nondirectional cryptic female choice (female-by-male interactions in fertilization success) causes deviations from models that focus exclusively on male-mediated postmating processes, and (b) how the risk of sperm competition, the strength of cryptic female choice, and tradeoffs between sperm number and sperm traits interact to influence the coevolutionary dynamics between cryptic female choice and sperm traits. We found that incorporating cryptic female choice can result in males investing much less in their ejaculates than predicted by models with sperm competition only. We also found that cryptic female choice resulted in the evolution of genetic correlations between cryptic female choice and sperm traits, even when the strength of cryptic female choice was weak, and the risk of sperm competition was low. This suggests that cryptic female choice may be important even in systems with low multiple mating. These genetic correlations increased with the risk of sperm competition and as the strength of cryptic female choice increased. When the strength of cryptic female choice and risk of sperm competition was high, extreme codivergence of sperm traits and cryptic female choice preference occurred even when the sperm trait traded off with sperm number. We also found that male traits lagged behind the evolution of female traits; this lag decreased with increasing strength of cryptic female choice and risk of sperm competition. Overall, our results suggest that cryptic female choice deserves more attention theoretically and may be driving trait evolution in ways just beginning to be explored.

Male larval experience of cues from adult rivals alters lifetime sperm investment patterns in a sperm heteromorphic moth, Ephestia kuehniella.

Male animals may adjust their resource allocations for reproduction and other fitness functions in response to cues from rivals. For instance, adult males increase their investment in sperm for a higher paternity share when they perceive sperm competition risk in their surroundings. In nature, both juveniles and adults may coexist spatially and temporally. Yet, it is not clear how juvenile males of different ages respond to cues from adult rivals and fine-tune their lifetime investment in sperm production and ejaculation in any insect. Here we used the Mediterranean flour moth, Ephestia kuehniella, which produces both fertile eupyrene and infertile apyrene sperm, to explore this question. We demonstrate that the late, but not early, instar larvae are sensitive to adult male cues. As a response, they produce more sperm before emergence and their resultant adults have shorter mating latency and ejaculate more sperm in the first few matings. When the juvenile stage produces more eupyrenes, the adult stops making these sperm, but regardless of the number of apyrenes produced during the juvenile stage, the adult continues to make them. These findings suggest that the number of spermatogonia for eupyrenes may be limited and that for apyrenes may be flexible. Our results show that the insect does not trade off survival, mating frequency, body size, or testis size for sperm production in response to adult males during the larval stage. Knowledge created in the present study offers insight into the stage-dependent sensitivity of juvenile males to cues from adult rivals and subsequent lifetime resource allocations.

Intergenerational response to sperm competition risk in an invasive mammal.

Studies of socially mediated phenotypic plasticity have demonstrated adaptive male responses to the 'competitive' environment. Despite this, whether variation in the paternal social environment also influences offspring reproductive potential in an intergenerational context has not yet been examined. Here, we studied the descendants of wild-caught house mice, a destructive pest species worldwide, to address this knowledge gap. We analysed traits that define a 'competitive' phenotype in the sons of males (sires) that had been exposed to either a high-male density (competitive) or high-female density (non-competitive) environment. We report disparate reproductive strategies among the sires: high-male density led to a phenotype geared for competition, while high-female density led to a phenotype that would facilitate elevated mating frequency. Moreover, we found that the competitive responses of sires persisted in the subsequent generation, with the sons of males reared under competition having elevated sperm quality. As all sons were reared under common-garden conditions, variation in their reproductive phenotypes could only have arisen via nongenetic inheritance. We discuss our results in relation to the adaptive advantage of preparing sons for sperm competition and suggest that intergenerational plasticity is a previously unconsidered aspect in invasive mammal fertility control.

Sperm Competition Risk: The Connections That Partner Attractiveness and Infidelity Risk Have with Mate Retention Behaviors and Semen-Displacing Behaviors.

The present studies investigated the relationships between men's perceived risk of experiencing sperm competition (i.e., when the ejaculates of two or more men simultaneously occupy the reproductive tract of a single woman), and their use of strategies to detect, prevent, and correct their partner's sexual infidelity. We investigated these associations using self-reports provided by men (Study 1, n = 113), partner-reports provided by women (Study 2, n = 136), and dyadic reports (Study 3, n = 103 couples). The results of these studies indicated that the attractiveness of women was consistently associated with men's use of benefit-provisioning mate retention behaviors (e.g., buying expensive gifts for one's partner, showing signs of physical affection) and semen-displacing behaviors (e.g., deeper copulatory thrusting, more thrusts during copulation), whereas the infidelity risk of women was often associated with men's use of cost-inflicting mate retention behaviors (e.g., threatening to end the relationship, monopolization of partner's free time). Discussion addresses the evolutionary implications of these results, including the possibility that men use both benefit-provisioning mate retention behaviors and semen-displacing behaviors when they perceive their partner to be more attractive, ostensibly as a way to mitigate their risk of sperm competition. Discussion also explores the extent to which these results extend those of previous studies concerning sperm competition risk.

Does diet influence ejaculate expenditure under experimentally altered risk of sperm competition in guppies?

According to the sperm competition risk (SCR) model, males should increase their investment in ejaculates when there is a high chance that their sperm will compete for fertilization with those from another male. However, as ejaculates are energetically costly to produce, the extent to which males can adjust ejaculate investment may depend on the availability of metabolic resources. We tested these ideas using the guppy, Poecilia reticulata, a livebearing fish known to exhibit strong condition dependence in ejaculate traits and among the highest recorded polyandry rates in any vertebrate. Specifically, we manipulated SCR by visually exposing mature males to females either in the presence (high SCR) or absence (low SCR) of a rival male over 1 month, which comprises a full spermatogenic cycle. To test for condition dependence in male response to SCR, both groups were further subdivided into two diet quantity treatments (high and low), enabling us to test fixed and interactive effects of SCR and diet treatment on ejaculate expenditure (sperm production, sperm length and sperm velocity). Contrary to our predictions, we found no evidence that males increase ejaculate expenditure under high SCR, and indeed report the opposite pattern (higher expenditure under low SCR) for sperm velocity. Furthermore, we found no evidence that dietary restriction (when administered to mature adult males over 1 month) affected any ejaculate traits, leading us to suspect that ejaculates may only be susceptible to dietary restriction effects during a critical ontogenetic stage of development. We discuss these unexpected findings in the context of guppy biology, which may render the species less likely to exhibit long-term adjustments in ejaculates when perceiving short-term changes in SCR. Moreover, the high polyandry rates in this species may mean that the visual presence of a rival is a cue of altered sperm competition intensity, rather than risk.

Male social niche conformance? Effects of manipulated opportunity for extra-pair mating on behavior and hormones of male zebra finches

Success in sperm competition is an important determinant of male fitness in mating systems with female multiple mating. Thus, sperm competition risk represents a key dimension of the male social environment to which individual males are expected to adaptively adjust their reproductive phenotype. Such adaptive phenotypic adjustment we here refer to as male social niche conformance. In this pre-registered study, we investigated how male zebra finches, Taeniopygia guttata, adjust their behavior to sperm competition risk. We experimentally manipulated the opportunity for extra-pair mating to create two levels of sperm competition risk: 1) Single-pair, no sperm competition risk; 2) Double-pair, sperm competition risk. We compared male courtship, mate guarding, copulation rates, and aggression between the treatment groups. To identify hormonal correlates of male behavioral adjustment, we measured plasma testosterone and corticosterone levels before and after the social treatment started. Contrary to our pre-registered predictions, males from the Double-pair treatment group decreased courtship rates compared to those from the Single-pair group, and Double-pair males responded less aggressively towards intruders than Single-pair males. Testosterone levels decreased over the breeding cycle, but social treatment had no effect on either testosterone or corticosterone levels. Our results indicate that male zebra finches do not intensify courtship or competitive reproductive behaviors, or upregulate key hormones when another breeding pair is present. Although we found no evidence for the predicted adaptive behavioral responses to sperm competition risk, we show that male zebra finches plastically adjust their behavior to their social environment.

Erectile Dysfunction and Sexual Coercion: The Role of Sperm Competition Risk.

Erectile dysfunction (ED) has been shown to be associated with several negative aspects of intimate relationships. Our goal for the present research was to examine whether ED was associated with men's use of sexual coercion in their intimate relationships (which may include the use of physical coercion, psychological manipulation, or emotional manipulation to gain sexual access) and if perceived sperm competition risk (i.e., perceived risk of partner infidelity, which may place a man's sperm into competition with sperm from another man) played a role in this association. These associations were examined in Study 1 using self-reports provided by men (N = 202) who had a mean age of 30.48years (SD = 5.03) and were recruited through Amazon's Mechanical Turk (MTurk). ED was found to have a large positive association with sexual coercion. However, men's self-reports did not provide support for sperm competition risk moderating the association between ED and sexual coercion, but an exploratory analysis revealed that sperm competition risk mediated this association. We attempted to replicate and extend these results in Study 2 by using partner-reports provided by women (N = 151) who had a mean age of 30.41years (SD = 4.77) and were recruited through MTurk. Women's partner-reports provided support for sperm competition risk moderating the association between ED and sexual coercion. In addition, an exploratory analysis found that sperm competition risk also mediated the association between ED and sexual coercion, similar to Study 1. Discussion explores the implications of these results for understanding the role that sperm competition risk may play in the connection between ED and sexual coercion.

The effect of genital stimulation on competitive fertilization success in house mice

In species with internal fertilization, copulation is characterized by the mechanical interaction of male and female genitalia. Genital interaction during copulation stimulates the neuroendocrine system, promoting the physiological changes necessary for the initiation of pregnancy. Previous studies on mice have shown that the baculum (penis bone) influences male reproductive success under a competitive fertilization scenario and that males perform less mating behaviour and ejaculate sooner under an elevated risk of sperm competition. Here we explore how mating behaviour and baculum morphology interact to influence a male's competitive fertilization success. We selected male and female house mice, Mus musculus domesticus , from families with breeding values at the extreme of baculum shape (narrow/wide) and recorded polyandrous matings to quantify mating behaviour. Our analyses revealed that competitive fertilization success was dependent on the relative baculum shape of competing males and the amount of mating behaviour that each male delivered. Our data thereby provide support for the stimulatory hypothesis for the evolution of the mammalian baculum. • We tested how genital stimulation influences male competitive fertilization success. • Males' baculum shape and mating behaviour influenced paternity outcome. • Males' competitive fertilization success may be subject to female sensory preference. • Our data offer support for the stimulatory role of the mammalian baculum.

Sperm oxidative status varies with the level of sperm competition and affects male reproductive success

Sperm competition theory predicts that males should strategically adjust the allocation of sperm in response to the level of sperm competition (i.e. the number of competing ejaculates). Predictions of sperm competition models have mostly focused on sperm numbers. However, whether males strategically adjust other important traits of their ejaculate such as sperm antioxidants in response to potential rivals and the consequences of such allocation strategies on sperm integrity (e.g. oxidative damage accumulation) and offspring viability is poorly investigated. Using an experimental design in which I manipulated the perceived level of sperm competition experienced by male field crickets, Gryllus bimaculatus , I show that besides sperm numbers, sperm oxidative status can also vary in response to the level of sperm competition. Male crickets increased the antioxidant content of their ejaculates in response to sperm competition risk, an effect that presumably led to less sperm oxidative damage, at least when the level of competition was low. Moreover, I provide evidence suggesting that such strategic adjustments can have fitness consequences for males, as eggs sired by males with increased sperm damage had lower viability. Overall, my results suggest that sperm competition drives plasticity in nonsperm components affecting sperm oxidative status and support the idea that oxidative stress may play a key role in postcopulatory sexual selection. • Male crickets adjusted sperm numbers as expected by sperm competition theory. • Males also increased sperm antioxidants in response to other potential rivals. • Males under a low level of sperm competition had reduced sperm oxidative damage. • Eggs fertilized by males with reduced sperm oxidative damage were more viable.

On the role of eviction in group living sex changers

In most sex-changing fishes in coral reefs, a dominant male and multiple females form a mating group (harem). In a few species, the subordinates are simultaneous hermaphrodites that may act as sneakers. In this paper, we ask whether the subordinates in most sex changers choose to be female or whether they are forced to give up their male function to avoid eviction by the harem holder. We consider a game model in which (1) the dominant male evicts some hermaphroditic subordinates if the risk of sperm competition in regard to fertilizing eggs is high, and (2) each subordinate individual chooses its own sex allocation considering the risk of being evicted. In the evolutionarily stable state, the dominant male evicts subordinates only when the subordinates vary greatly in their reproductive resources. All the subordinate individuals are female if the summed male function of the subordinates is smaller than that of the dominant male. Otherwise, all the subordinates are hermaphrodites, and the large individuals have the same male investment but a greatly different female investment, while small individuals have a reduced male investment to avoid eviction risk. We conclude that situations in which the sex allocation of subordinates is affected by the possibility of eviction by the harem holder are rather limitedSignificance statementWe studied the role of eviction in social evolution. In most sex-changing fishes in coral reefs, a dominant male and multiple females form a mating group. In a few species, subordinates are simultaneous hermaphrodites. We asked whether the subordinates are forced to give up their male function to avoid eviction by the harem holder. We examined a game model in which the dominant male evicts hermaphroditic subordinates with a high risk of sperm competition, and each subordinate chooses its own sex allocation considering the eviction risk. We derived mathematical conditions for when subordinates are females or hermaphrodites in the ESS. The model demonstrated that the control by the dominant over subordinate reproductive decisions is rather limited.

Do male meadow voles (Microtus pennsylvanicus) always prefer and signal to 'popular' females? The role of social information context.

For males, the behaviour of rival conspecifics can provide vital information about finding reproductively available females, and which have high-quality phenotypes. While valuable, this information represents sperm competition risk and even the potential for rejection by females. Thus, males should show flexibility in signalling behaviours towards females based on social information acquired with emphasis on the context of that information. We examined male meadow vole scent-marking behaviours and preferences for females based on previously obtained social information across different contexts. Social information context resulted in increased scent marking depending on the social odour pairing. Males scent marked more near a female whose odour had associated with three older rivals than one younger rival. But males marked equally toward a female whose odour had associated with three younger rivals and a female whose odour had associated with one older rival. This demonstrates that social odours are not of all equal value and that males can distinguish differences. Males then use these differences to tailor their signalling towards potentially high-quality phenotype females thereby maximizing their reproductive opportunities.