Copulation Frequency Research Articles

“Failure to launch”: is there a reproductive cost to males living at home?

Differential reproductive success commonly reflects variations in reproductive physiology, behavior, and morphology. In some species, competition among males results in the evolution of alternative reproductive tactics that confer a fitness advantage relating to social status, density, or myriad other factors. In the Cape ground squirrel (Xerus inauris), a species that is highly competitive but lacks typical mammalian aspects of intrasexual competition (e.g., territoriality and aggression), 2 alternative reproductive tactics occur relating to dispersal. While physiological and behavioral differences have been demonstrated between dispersed males and males that delay dispersal, we used microsatellite markers to quantify variations in reproductive success between tactics. We found that dispersed males are in better body condition with larger home ranges likely allowing greater encounter frequencies with estrous females. However, we found no difference in copulation frequency between tactics, and the decisio...

Dominance, access to females, and mating success among coresident male mantled howlers (Alouatta palliata) at La Pacifica, Costa Rica.

The priority-of-access (POA) model posits that high dominance rank increases male mating success by increasing access to fertile females. However, the relationship between rank, access to females, and subsequent mating success is variable in primates, and there are few studies representing Neotropical taxa. The purpose here was to test the parameters of the POA model in an asynchronously breeding Neotropical primate, Alouatta palliata, to contribute to our understanding of the relationship between dominance and reproductive strategies in platyrrhines. I collected data on adult males within two large, multimale-multifemale groups exhibiting clear dominance hierarchies at La Pacifica, Costa Rica. Females were classified as sexually receptive (SR) or potentially cycling (PC) based on behavioral and birth data. Access to mates was measured based on total time in proximity to SR/PC females, and mating success was measured based on copulation frequency. Results did not support the predictions of the POA model in that first-ranked males maintained lower than expected time in proximity to SR females, did not consistently maintain the greatest proportion of time in proximity to PC females, obtained lower than expected copulation rates, and did not obtain the highest copulation rates compared to subordinates in either group. Deviations from the POA model were significantly affected by varying operational sex ratios only when considering the lower numbers of available SR females in one group. Alternative reproductive tactics by subordinate males such as tolerance by first-ranked males appeared to be operating, allowing subordinates to obtain mating success when they would otherwise be unable to do so. The study also highlighted how factors such as operational sex ratio may limit the willingness or ability of dominant males to monopolize access to females, and can vary both within and between groups in a population.

The evolution of copulation frequency and the mechanisms of reproduction in male Anolis lizards

Abstract The evolution of many morphological structures is associated with the behavioral context of their use, particularly for structures involved in copulation. Yet, few studies have considered evolutionary relationships among the integrated suite of structures associated with male reproduction. In this study, we examined nine species of lizards in the genus Anolis to determine whether larger copulatory morphologies and higher potential for copulatory muscle performance evolved in association with higher copulation rates. In 10–12 adult males of each species, we measured the size of the hemipenes and related muscles, the seminiferous tubules in the testes, and the renal sex segments in the kidneys, and we assessed the fiber type composition of the muscles associated with copulation. In a series of phylogenetically-informed analyses, we used field behavioral data to determine whether observed rates of copulation were associated with these morphologies.We found that species with larger hemipenes had larger fibers in the RPM (the retractor penis magnus, a muscle that controls hemipenis movement), and that the evolution of larger hemipenes and RPM fibers is associated with the evolution of higher rates of copulatory behavior. However, the sizes of the seminiferous tubules and renal sex segments, and the muscle fiber composition of the RPM, were not associated with copulation rates. Further, body size was not associated with the size of any of the reproductive structures investigated. The results of this study suggest that peripheral morphologies involved in the transfer of ejaculate may be more evolutionarily labile than internal structures involved in ejaculate production.

Sexually Coercive Male Chimpanzees Sire More Offspring

In sexually reproducing animals, male and female reproductive strategies often conflict. In some species, males use aggression to overcome female choice, but debate persists over the extent to which this strategy is successful. Previous studies of male aggression toward females among wild chimpanzees have yielded contradictory results about the relationship between aggression and mating behavior. Critically, however, copulation frequency in primates is not always predictive of reproductive success. We analyzed a 17-year sample of behavioral and genetic data from the Kasekela chimpanzee (Pan troglodytes schweinfurthii) community in Gombe National Park, Tanzania, to test the hypothesis that male aggression toward females increases male reproductive success. We examined the effect of male aggression toward females during ovarian cycling, including periods when the females were sexually receptive (swollen) and periods when they were not. We found that, after controlling for confounding factors, male aggression during a female's swollen periods was positively correlated with copulation frequency. However, aggression toward swollen females was not predictive of paternity. Instead, aggression by high-ranking males toward females during their nonswollen periods was positively associated with likelihood of paternity. This indicates that long-term patterns of intimidation allow high-ranking males to increase their reproductive success, supporting the sexual coercion hypothesis. To our knowledge, this is the first study to present genetic evidence of sexual coercion as an adaptive strategy in a social mammal.

Sex ratio effects on copulation, fecundity and progeny fitness for Agasicles hygrophila, a biological control agent of alligator weed

Determining the best ratio of females to males of an insect's natural enemy is important for maximising population increase and promoting population establishment of a natural enemy. In this study, copulation behaviour, fecundity, progeny fitness and rate of population increase for the flea beetle, Agasicles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae), were compared at different female percentage treatments (i.e., 80%, 66.7%, 50%, 33.3% and 20% females). The results showed that the copulation frequency and duration in males decreased, whereas those in females increased as the number of males increased. At 20%, 33.3% and 66.7% females, the rates of population increase were 3.4-, 2.17- and 0.79-fold higher than that at 50% females. Females at 20% and 33.3% were found to be optimal for mass rearing of the beetle.

Development of Avian External Genitalia: Interspecific Differences and Sexual Differentiation of the Male and Female Phallus

Avian genitalia, particularly in waterfowl, are extremely diverse. Penis morphology varies among species, and penis length and elaboration are associated with the frequency of forced extra-pair copulations, yet the developmental mechanisms responsible for this variation are unknown. In addition, females have a small phallic structure that is homologous to the male phallus, but little is known about when or how sexual differentiation takes place. To determine whether species-specific genital morphologies and sexual differentiation occur during duck embryonic development, we characterized development from the onset of genital tubercle initiation through stages of sexual differentiation in 3 species. Pekin and Laysan ducks have long, thick penises, whereas those of Mandarin ducks are shorter and thinner. Development of the genital tubercle is similar throughout the pre-hatching period across the 3 species, suggesting that differences in penis morphology arise post-hatching. We observed that male and female phallus development is similar at early stages, but the female phallus later regresses. Then, we compared male and female genital development between ducks and chickens, which develop a non-intromittent penis. We found that external genital development in male and female chickens resembles that of female ducks, which raises the possibility that male phallus development became feminized during galliform evolution.

Heat shock suppresses mating and sperm transfer in the rice leaf folder Cnaphalocrocis medinalis

Temperature is a key environmental factor in determining the population size of Cnaphalocrocis medinalis in summer. High temperatures inhibit survival, development and fecundity of this insect. However, biological responses of female and male adults to heat shock, and physiological mechanism of high temperature suppressing population development are still ambiguous. We experimentally tested the impact of heat shock (5h day-1) on biological traits, spermatogenesis and sperm transfer of adults of C. medinalis. The result showed that heat exposure to 39 and 40°C for 5h reduced longevity and copulation frequency of adults, and hatchability of eggs. Immediate survival rate of males was lower than that of females after 3 days of exposure to 41°C. The oviposition period, copulation frequency, fecundity of adults and hatchability of eggs were significantly lower when male adults were exposed to 40 or 41°C for 3 days. Heat shock decreased frequency and success rate of mating when males were exposed, and it also resulted in postponement of mating behaviour and prolongation of mating duration as both the female and male adults were exposed. Heat shock did not affect spermatogenesis, but significantly inhibited sperms maturation. Moreover, males could not ejaculate sperm into females during copulation when these male moths received heat shock. Heat shock remarkably suppressed mating behaviour and sperm transfer, which led to a dramatic decline of rice leaf folder populations.

When males stop having sex: adaptive insect mating tactics during parental care

The theory of prudent sperm allocation predicts that males should reduce their investment in sperm transfer when their mate's propensity to produce offspring is low. Furthermore, if mating is costly for females, they would benefit from signalling this propensity to males to keep them from attempting to mate. Here we document a remarkable flexibility in insect mating tactic during the period of parental care. Extensive video observations in the biparental burying beetle Nicrophorus vespilloides revealed that pairs mate frequently before and during oviposition, temporarily discontinue their sexual activity while young larvae are present, and resume mating when males are close to leaving the brood. An experiment, in which we switched males at different time intervals and determined their P2 value (i.e. the proportion of progeny sired by the second male), confirmed the lack of mating activity during the first few days of larval care and its resumption later in parental care. Females cease laying eggs when caring for larvae, but females will resume egg laying if none of the larvae from their first clutch reach the carcass. We manipulated females into producing such replacement clutches by withholding their larvae, and we found that in this situation males did not cease to mate, but continued to engage in frequent copulations. Hence, males have the ability to assess a female's propensity for oviposition during the period of parental care. Further experiments demonstrated that males do not use the presence or absence of larvae as a cue to adjust copulation rate, but instead use female-produced cues of reproductive state. Therefore, our study reveals that female beetles express cues that display their reproductive condition and males have evolved the ability to detect these cues and respond with prudent sperm allocation. Our findings further contribute to the developing picture of the sophisticated mating tactics that insects employ.

Host-shift effects on mating behavior and incipient pre-mating isolation in seed beetle

Mating behavior is based on communication among mates and includes both sexual signaling and mating preferences. In phytophagous insects, shift to a novel host will expose associated traits to novel selection regimes and eventually cause assortative mating and sexual isolation between populations inhabiting diverse host plants. We investigated the relationship between short- and long-term changes in mating systems on Acanthoscelides obtectus. Mating preferences were evaluated using measures of copulation frequency in mating trials within and among populations and by measuring time spent in sexual interactions prior to copulation. Sexual signaling was previously analyzed by chemical detection of contact pheromones (i.e., cuticular hydrocarbons [CHCs]), and these results were used in this study. Laboratory populations evolved for 50 generations either on the optimal host (common bean) or on the suboptimal host (chickpea). To determine short-term effects, subsets of individuals from each population were exposed to the alternative host for 1 generation. We revealed higher level of indiscriminate mating in populations, which evolved on the suboptimal host. Males from these populations spent less time in assessing mates although they had the ability to discriminate between signals. Short-term larval experience on the suboptimal host also decreased selectivity of mates. The results imply that plastically induced reduction in mate discrimination, after a shift to suboptimal host, may have been canalized through long-term genetic changes underlying behavioral development. Significant reproductive isolation between the 2 sets of populations was revealed regardless of the short-term host treatment.

Effects of reproductive interference on the competitive displacement between two invasive whiteflies

Reproductive interference is one of the major factors mediating species exclusion among insects. The cryptic species Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) of the whitefly Bemisia tabaci complex have invaded many parts of the world and often exhibit niche overlap and reproductive interference. However, contrasting patterns of competitive displacement between the two invaders have been observed between regions such as those in USA and China. Understanding the roles of reproductive interference in competitive interactions between populations of the two species in different regions will help unravel other factors related to their invasion. We integrated laboratory population experiments, behavioural observations and simulation modelling to investigate the role of reproductive interference on species exclusion between MEAM1 and MED in China. In mixed cohorts of the two species MEAM1 always excluded MED in a few generations when the initial proportion of MEAM1 was ⩾0.25. Even when the initial proportion of MEAM1 was only 0.10, however, MEAM1 still had a higher probability of excluding MED than that for MED to exclude MEAM1. Importantly, we show that as MEAM1 increased in relative abundance, MED populations became increasingly male-biased. Detailed behavioural observations confirmed that MEAM1 showed a stronger reproductive interference than MED, leading to reduced frequency of copulation and female progeny production in MED. Using simulation modelling, we linked our behavioural observations with exclusion experiments to show that interspecific asymmetric reproductive interference predicts the rate of species exclusion of MED by MEAM1. These findings not only reveal the importance of reproductive interference in the competitive interactions between the two invasive whiteflies as well as the detailed behavioural mechanisms, but also provide a valuable framework against which the effects of other factors mediating species exclusion can be explored.

Mating in the presence of a competitor: audience effects may promote male social tolerance in polyandrous siamang (Symphalangus syndactylus) groups

Audience effects on sexual behavior, including changes in copulation frequency and duration in the presence of conspecifics, have been reported in multimale–multifemale groups of several primate species. We examined the interaction of male sexual behavior with group composition and within-group mating pattern in a population of siamangs (Symphalangus syndactylus) containing unimale–unifemale groups and multimale–unifemale groups using mixed models. Aggression in a sexual context was never observed. As predicted, however, copulation rates were lower and copulations were significantly shorter in duration in two-male groups than in unimale groups, even if copulations involving subordinate males were excluded. Dominant males monopolized copulations with the group female in most groups, but copulations were shared among males in three stable two-male groups. When both resident males copulated with the group’s female, there was no evidence that copulating pairs moved to secluded areas, and the duration of copulations did not differ between males. These results are consistent with the hypothesis that male–male tolerance in multimale siamang groups is facilitated by adjustments to sexual behavior in the presence of a sexual competitor.

Partner attractiveness moderates the relationship between number of sexual rivals and in-pair copulation frequency in humans (Homo sapiens).

Nonhuman males attend to the number of potential sexual rivals in the local environment to assess sperm competition risk. Males of these species sometimes perform more frequent in-pair copulations to increase the likelihood of success in sperm competition. Here, we extend this research to humans, Homo sapiens. We secured self-report data from 393 men in a committed, sexual, heterosexual relationship. The results indicate that men whose in-pair partner has more male coworkers and friends (i.e., potential sexual rivals) also perform more frequent in-pair copulations, but only among men who perceive their partner to be particularly attractive relative to assessments of partners by other men in the sample. This research is the first to empirically investigate the number of potential male rivals in the local environment as a cue to sperm competition risk in humans. Discussion addresses limitations of the current research and highlights directions for future research.

Evaluation of mating behaviour and mating compatibility methods for the Old World screwworm fly, Chrysomya bezziana

The effectiveness of the Sterile Insect Technique program (SIT) to eradicate pest insects relies on the success of mating competitiveness between irradiated male flies and wild type males for the wild type females. It has been successfully applied for the New World screwworm fly (NWSF), Cochliomyia hominivorax but remains unproven for the Old World screwworm fly (OWSF), Chrysomya bezziana. The aim of the study was to develop methods for investigating mating behaviour and mating compatibility of C. bezziana under laboratory conditions. Two methods were used for studying mating: individual mating (method 1) and group mating (method 2). The flies used in this study were 5-7 days old. Twenty four hours after emergence, adult flies were sexed and placed into different cages until studied. The female : male ratio in the group mating was 1 : 5 and the males were marked by painting a dot on the thorax using different oil colours. Observation of mating behaviour was investigated every 30 minutes through 10-20 replications for all methods depending on the availability of flies. Data were analysed using ANOVA and the Student’s t-test, with significance demonstrated at the 95% confidence level. The results demonstrated that the frequency of contacts between males and females at different ages was a significantly different (p 0.05) and method 2 (p > 0.05). Copulation was only initiated following longer periods of contact, mainly in the range of 270-449 seconds. The highest frequency of copulation occurred between 7-8 days, but the duration of mating was similar between 5-8 days old. The study demonstrated that the methods developed were suitable for a mating compatibility study of C. bezziana. Key Words: Chrysomya bezziana, Mating Behaviour, Copulation, Myiasis

Mating activity of domestic geese ganders (Anser anser f. domesticus) during breeding period in relation to age, testosterone and thyroid hormones

In breeding geese, natural mating applies. The objective of this study was to determine seasonal- and age-related changes in the sexual activity of ganders. Moreover, T, T4 and T3 plasma levels were evaluated. The study was conducted on 2/3-year-old males (n=15) and 1- or 2/3-year-old females (1♂:4♀). Sexual activity of ganders was characterized through the frequency of: courtship, attempts at and successful copulations and total mating activity (MA). Reproductive results manifested by egg production and fertility were recorded. Laying percentage (January–June) was 37.1 and 28.6% for 1-, and 2/3-year-old geese, respectively. MA was noted before the sexual maturity of females and the T level peak. MA was highest at the onset and during peak production (March) for ganders kept with 1-, and 2/3-year-old geese, respectively. From April to May a decrease in fertility with a reduction of the frequency of copulations was observed. At this time low levels of T were noted. The effect of goose age on the MA was shown, with higher frequency of copulations for ganders kept with 1-year-old geese. We suggest that in the successful seasonal mating in geese, social factors such as the presence of females and female age play an important role. The reduction in fertility during the spring period may be associated with decreases in the efficiency of successive mating sequences. The higher frequency of copulations without affecting fertility, for ganders kept with young geese, may be because of differences in sperm transfer or storage/transport efficiency.

Imposex and novel mechanisms of reproductive failure induced by tributyltin (TBT) in the freshwater snail Pomacea canaliculata

The effects of tributyltin (TBT) on mortality and reproduction were studied in the invasive snail Pomacea canaliculata. The nominal median lethal concentration (LC50) was 9 µg TBT/L, after 28 d. The nominal no-observed effect on lethality concentration (NOECL) was 6 µg TBT/L after the same period. Male-female couples and females that had been group-mated but were isolated from males during the experiment (isolated females) were exposed (for 28 d) to either 0 µg/L or 6 µg/L of TBT (nominal NOECL-exposed). Copulation and oviposition frequencies, egg clutch mass, and percentage of egg fertility were recorded. Gonads (both sexes) and the seminal receptacle (females) were studied histologically at the end of the experiment. A significant decrease in copulation frequency was observed in mated-exposed females. Exposure also decreased oviposition frequency of mated-exposed and isolated-exposed females, but only the latter reached significance. No differences in either egg clutch mass or percentage of fertility were observed at first oviposition, but both parameters were drastically reduced in subsequent egg clutches of exposed females. No histological alterations were observed in gonads of TBT-exposed animals; however, sperm storage in the seminal receptacle was drastically decreased in exposed females. Imposex but no oviductal obstruction was observed in all exposed females. It is concluded that TBT induces reproductive failure in P. canaliculata by decreasing copulation frequency and by severely affecting sperm storage by the female.

Experimentally Simulating Paternity Uncertainty: Immediate and Long-Term Responses of Male and Female Reed Warblers Acrocephalus scirpaceus

In many socially monogamous species, both sexes seek copulation outside the pair bond in order to increase their reproductive success. In response, males adopt counter-strategies to combat the risk of losing paternity. However, no study so far has tried to experimentally prove the function of behaviour for paternity assurance. Introducing a potential extra-pair partner during the female fertile period provides a standardised method to examine how pair members respond immediately (e.g. increase mate guarding or copulation frequency) or long term (e.g. later parental investment and paternity uncertainty). In this study on a socially monogamous passerine species, we experimentally confronted pairs of reed warblers with a conspecific male (caged male simulating an intruder) during egg-laying. Our results revealed that occurrence of an intruder during that period triggered aggression against the intruder, depending on the presence of the female. The male territory owner also attacked the female partner to drive her away from the intruder. Thus territory defence in reed warblers also serves to protect paternity. The increase in paternity uncertainty did not affect later paternal investment. Paternal investment was also independent of the actual paternity losses. In females, the experiment elicited both, immediate and long-term responses. E.g. female copulation solicitations during the intruder experiment were only observed for females which later turned out to have extra-pair chicks in their nest. In relation to long term response females faced with an intruder invested later less in offspring feeding, and had less extra-pair chicks in their nests. Extra-pair paternity also seems to be affected by female quality (body size). In conclusion female reed warblers seem to seek extra-pair fertilizations but we could demonstrate that males adopt paternity assurance tactics which seems to efficiently help them to reduce paternity uncertainty.

Phenotypic engineering of sperm-production rate confirms evolutionary predictions of sperm competition theory

Sperm production is a key male reproductive trait and an important parameter in sperm competition theory. Under sperm competition, paternity success is predicted to depend strongly on male allocation to sperm production. Furthermore, because sperm production is inherently costly, individuals should economize in sperm expenditure, and conditional adjustment of the copulation frequency according to their sperm availability may be expected. However, experimental studies showing effects of sperm production on mating behaviour and paternity success have so far been scarce, mainly because sperm production is difficult to manipulate directly in animals. Here, we used phenotypic engineering to manipulate sperm-production rate, by employing dose-dependent RNA interference (RNAi) of a spermatogenesis-specific gene, macbol1, in the free-living flatworm Macrostomum lignano. We demonstrate (i) that our novel dose-dependent RNAi approach allows us to induce high variability in sperm-production rate; (ii) that a reduced sperm-production rate is associated with a decreased copulation frequency, suggesting conditional adjustment of mating behaviour; and (iii) that both sperm production and copulation frequency are important determinants of paternity success in a competitive situation, as predicted by sperm competition theory. Our study clearly documents the potential of phenotypic engineering via dose-dependent RNAi to test quantitative predictions of evolutionary theory.

EFECTO DE LA COPULACIÓN DURANTE EL CELO POSTOVULATORIO EN LA MORTALIDAD EMBRIONARIA EN ALPACAS

The objetive of this research work was to study the effect of the frequency of copulation during the post-ovulatory estrous on the embryonic mortality in alpacas. Forty four female alpacas were selected having a post-partum sexual rest >20 days and bearing in the ovaries a pre-ovulatory follicle >7 mm of diameter. Animals were mated (time of coitus: >15 minutes) using a male with proven fertility and received 6 �g of busereline i.m. after copula to ensure ovulation. Three experimental groups were then ramdomly organized: the first group (S 1 , n = 14) did not receive additional matings, the second gropup (S 2 , n = 15) received an additional mating on day three post-ovulation, and the third group (S 3 , n = 15) received additional matings on days three and four postovulation. Ovulation was confirmed through ultrasound images by disappearance of the pre-ovulatory follicle 42 hours post-mating and the presence of a corpus luteum on day 5 post-ovulation. In addition, the size of the corpus luteum, morfological changes of the uterine horns and presence of an embryo were recorded on days 13-14 and 19 postmating. These observations were complemented with the observation of sexual receptivity using a male on days 6, 13-14, 19 and 26 post-mating. Ovulation rate was 91%. There was a higher tendency on embryo survival at day 19 post-mating in those alpacas that received an additional mating on days 3 (S 2 = 80%) and 3 and 4 (S 3 = 85%) post-ovulation than in those animals without additional mating (S 1 = 75%). An additional study in four inducedovulated alpacas was done to detect fertilized ova at days 2, 3 and 4 post-ovulation. Recovery of the fertilized ova from oviduct was possible only until day 3 post-ovulation.

Studies on remating behaviour in the Drosophila bipectinata species complex: Intra- and interspecific variations

Intra- and interspecific variations in female and male remating frequency, latency, and duration of copulation in first and second matings were analyzed in four species of the Drosophila bipectinata complex, employing four strains of each species i.e. D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae. Chi-square test revealed significant intraspecific variation in the number of remated females and males in D. malerkotliana and D. bipectinata, whereas D. parabipectinata showed insignificant intraspecific variations in number of remated females and males. D. pseudoananassae showed significant intraspecific variation in the number of remated females, but the frequency of remating was least. One way ANOVA depicted significant intraspecific variation in female and male remating time in D. bipectinata and D. parabipectinata. D. bipectinata took the shortest time to remate and the duration of copulation in first mating was longest. However, D. pseudoananassae exhibited the longest remating time and long duration of copulation in first mating. Results of t-test depicted that all four species exhibited shorter duration of copulation in second mating as compared to the first. Our study puts D. bipectinata and D. pseudoananassae at two extremes, and D. parabipectinata and D. malerkotliana at an intermediate position in a hierarchy of remating behaviours.

Promiscuity and elongated sperm storage organs work cooperatively as a cryptic female choice mechanism in an earwig

Female genitalia often show complex morphologies that cannot be explained by sperm reception and storage functions. However, our understanding of the forces underlying genital exaggeration in females is limited. Female earwigs, Euborellia plebeja , are promiscuous and their highly elongated sperm storage organs allow only partial removal and displacement of stored sperm by shorter male genitalia, resulting in only a 20% gain in paternity per copulation with a sperm-saturated female. This study examined the significance and optimality of restricted sperm displacement for females. A staged mating experiment with a paternity success analysis revealed that large males dominated male–male competition for burrows housing females, resulting in repeated copulations with the same female. Despite the low paternity gain per copulation, such repeated copulations resulted in a significant increase in paternity for larger males with higher resource-holding potential. A numerical simulation based on the relationship between male body size and copulation frequency showed that restricted sperm displacement (about 20% per copulation) is optimal for promiscuous females to accumulate sperm effectively from larger males. Because male body size is heritable in this species, females were estimated to benefit from a 1.4% increase in their sons' mating success. This genetic benefit disappeared when only a single copulation per male–female encounter was assumed. Since no measurable costs of mating for females have been detected in this species, the combination of promiscuity and restricted sperm displacement is best explained by the cryptic female choice hypothesis.