Effect Of Operational Sex Ratio Research Articles

Effects of operational sex ratio and male density on size-dependent mating in Minshan’s toads, Bufo minshanicus, on the Tibetan Plateau of China

Abstract In many animal species, an increase in the operational sex ratio (OSR), density or a combination of both should lead to more intensive competition among individuals of the more abundant sex. To test this, we examined pairing patterns of Minshan’s toad (Bufo minshanicus) from six populations between 2008 and 2015 along the eastern Tibetan Plateau in south-west China. OSRs in breeding aggregations of Minshan’s toad are normally male biased and males actively compete with each other for acquisition and retention of mates. We found evidence that deviations from random mating by size varied between populations and between years according to the magnitude of the OSR and male density. Larger males were generally more successful in pairing than smaller males when the OSR was slightly male biased and male density was high. However, the resulting size-disproportionate mating was more evident when OSR was closer to 1.99, indicating a positive correlation with the intensity of aggressive scramble competition. Thus, the intensity of male-male competition may partly explain variation in size-disproportionate mating among populations.

Effects of spawning group sex ratio and stocking density on the outcome of captive reproduction in golden cuttlefish Sepia esculenta

The supplemental release of hatchery-reared animals is important for maintaining or enhancing fishery resources. The golden cuttlefish, Sepia esculenta, is a commercially important cephalopod utilized in stock enhancement activities throughout East Asia. The long-term success of such activities requires proper breeding regimes to ensure the efficient use of resources and the preservation of genetic diversity in the offspring. For captive-bred cephalopods, information concerning the effects of different mating regimes on the paternity of offspring is scarce. However, it could be of particular importance for polyandric species such as golden cuttlefish. As previous studies have suggested that both the number and mating order of males are related to female fecundity, we investigated the effects of stocking density and sex ratio on female fecundity and determined the paternity of the offspring in golden cuttlefish. We used two experimental approaches. First, four different female (F): male (M) sex ratios (1F:1M, 1F:2.5 M, 1F:3M, 1F:6M) were set up to investigate the effects of operational sex ratio on mating behavior, female fecundity, and hatchling paternity. Next, we investigated female fecundity and hatchling paternity patterns under low and high stocking densities (5 or 10 individuals ∙ m−3), following a standard sex ratio setting that is widely employed in cuttlefish hatcheries (1F:2M). With an increasing proportion of males, the average fecundity of females and the participation rate of males in paternity decreased. With increasing stocking density, the females had lower average daily egg production, and the males had a high variance in reproductive success and decreased participation rate in spawning. The estimated effective number of breeders (Ne) was 19.29 (Ne/N ratio = 0.54) at high-density and 10.17 (Ne/N ratio = 0.57) at low density. To achieve high reproductive performance and assure genetic diversity of golden cuttlefish offspring, our study suggests that equal (1F:1M) or slightly male-biased (1F:2.5 M) sex ratios and a low stocking density may be useful when breeding golden cuttlefish in captivity.

Effects of operational sex ratio, mating age, and male mating history on mating and reproductive behavior in Grapholita molesta.

The short-lived polygamous moth Grapholita molesta (Busck) is an important fruit pest worldwide. Trapping males by synthetic female sex pheromones is not an effective reproductive control strategy. It is important to improve this technology by understanding the mating system of G. molesta. This study investigated mating opportunities and fertile egg production by altering the operational sex ratio, mating age, and male mating history in repeated single mating and multiple mating in the two sexes. Our results showed that the mating and reproductive parameters of virgin males were affected by the number and age of virgin females. Males preferred a female number ≤three-fifths of the male number or ≤2-day-old females, while they discriminated against a female number ≥three times of the male number or ≥5-day-old females. On the other hand, the mating and reproductive parameters of virgin females were affected by repeated single mating and especially multiple mating under different male mating histories. Females preferred once-mated males and discriminated against virgin males. These results indicated that mating systems including more and older virgin females for virgin males and different virgin males for virgin females may be suitable for suppressing G. molesta populations. Hence, these results revealed that preventing mating of virgin adults by synthetic female sex pheromones should be most effective in controlling G. molesta.

Operational sex ratio and paternal age sway mating and reproductive performance in Menochilus sexmaculatus (Coleoptera: Coccinellidae)

AbstractOperational sex ratio and paternal age are known to independently affect mating behaviour and reproductive performance. We conducted experiments to assess the effects of operational sex ratio and paternal age on mating, reproduction, and total development duration of Menochilus sexmaculatus (Fabricius) (Coleoptera: Coccinellidae). Three male-biased sex ratios, no (1:1), modest (1:2), and extreme (1:5) competition treatments, were formed using young, middle-aged, and old males. The reproductive performance of M. sexmaculatus at all ages under extreme competition (1:5) surpassed the other two treatments. More competition among males decreased the time of commencement of mating and increased the duration of copulation. Females laid a greater number of eggs in extreme competition treatments. The operational sex ratio (irrespective of age) had no significant effect on the total developmental duration of offspring. Among different age groups, older males took more time to commence mating and mated for longer durations in all the sex ratio treatments. The number of eggs in the first clutch was lowest when females were mated with older males with increased offspring development duration. Thus, it can be concluded that male-biased sex ratio elevates the performance of M. sexmaculatus. Among different age groups, older males were poor performers than young and middle-aged males.

Reproductive behaviour of female rosy bitterling Rhodeus ocellatus in response to a female-biased operational sex ratio

While the effect of Operational Sex Ratio (OSR) on reproductive behaviour of males has been studied extensively, little is known of the response of females facing a female-biased OSR. We investigated the effect of different OSRs on female reproductive behaviour using the rosy bitterling, Rhodeus ocellatus, a freshwater fish that lays its eggs inside the gills of living freshwater mussels. Three levels of OSR (male/female ratio 1:1, 1:3 and 1:5) were tested. We demonstrated that inspection of the mussel (spawning substrate) by individual females increased with increasingly female-biased OSR, but that the rate of following territorial male decreased. Aggression towards other females was not affected by the OSR. Interestingly, when a male bitterling led a non-dominant female towards the mussel, the dominant female would become aggressive to the male and chase the non-dominant female away. Aggression towards male followed a bell-shaped pattern and was highest at an OSR of 1:3. In both the female-biased OSRs examined, almost 50% of dominant females tended to chase away other females and defend the mussel, showing territoriality in a similar manner to males. These observations suggest that female reproductive behaviour is strongly affected by the OSR, and their reproductive tactics during courtship change from a passive role in courtship (following a male) to an active role in courtship (approaching a male), with presence of female territorial behaviour as the OSR becomes increasingly female-biased. This study provides strong evidence that a female-biased OSR has an important effect on female reproductive behaviour.

Operational sex ratio, reproductive costs, and the potential for intrasexual competition

Even though numerous metrics exist, we still appear to be far from a consensual view on the best way of measuring or predicting the potential strength of sexual selection. One of the earliest and simplest metrics devised was the operational sex ratio (OSR) (i.e. the ratio between sexually active males and females in a population), and even, if heavily criticized, the OSR can still be viewed as a valuable measure of the potential levels of intrasexual competition. Because this ratio is influenced by the time that individuals spend in the mating pool, the OSR depends on our ability to determine who is indeed ready to mate. Moreover, because the proximate effects of OSR on mate monopolization might not be immediately apparent, we should be prepared to account for its association with the conditions making selection favour traits that reduce the time needed to acquire additional mating opportunities. Using the worm pipefish as a working model, we conducted a more stringent calculation of the OSR by eliminating individuals that, although present, do not appear to be able to reproduce, as determined by an analysis of female size classes with immature or spent ovaries and male classes without pregnancy events. Accordingly, the OSR was not only capable of correctly highlighting the potential for intrasexual competition, but also was able to translate the desertion of individuals from the mating pool as the breeding season progressed into meaningful correlations with variables associated with reproductive investment and costs (i.e. gonad investment and energy reserves). As demonstrated, the predictive power of the OSR can be broader than anticipated, depending primarily on our ability to effectively discriminate which individuals are indeed ready to mate. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 477–484.

The effect of operational sex ratio on sex allocation and neonate phenotype in a viviparous lizard Eremias multiocellata

Maternal investment in the production of male versus female neonates was approximately equal in most animal species. However, sex allocation theory predicts that under certain conditions, selection may favor the females’ ability to adjust the sex ratio of their offspring, which females tend to use more for an investment of the rare sex. The mechanism of operational sex ratio (OSR) influence on sex allocation is still unclear, and recent studies conducted on lizards have reached conflicting conclusions. Here, we selected a viviparous lizard Eremias multiocellata to test whether pregnant females could adjust the sex ratio of their offspring in response to OSRs. Our results showed that mothers did not adjust the sex ratios or phenotypes of neonates in the laboratory and field-based experiments, except tail length. However, the OSRs subsequently affected growth in both mass and SVL of the offspring in laboratory experiments; whereas only the mass was affected in the semi-nature field experiments. Our results, thus, contradict the predictions of sex allocation theory and challenge the idea that female investment in the scarcity sex might serve as a mechanism which is used for adjusting the population sex ratio.

Intrasexual competition and courtship in female and male Japanese medaka, Oryzias latipes: effects of operational sex ratio and density

Most studies of intrasexual mating competition have focused only on males, and have typically shown an increase in aggression and a decrease in courtship rate with increasing operational sex ratio (OSR), the ratio of ready-to-mate males to females. We manipulated the number of males (1, 2 or 4) and females (2, 4, 8 or 16) in a factorial design to investigate mating competition and courtship rate in Japanese medaka, a species with conventional sex roles, over a broad range of OSRs (0.0625 to 2), and in relation to the density of both sexes. As predicted, the rates of intrasexual interactions by both males and females increased with the number of competitors and decreased with the number of potential mates, whereas the rates of courtship decreased with the number of competitors and increased with the number of mates. At OSRs between 0.35 and 2, males engaged in more intrasexual interactions and courtship behaviour than females. However, females became the more competitive sex at OSRs less than 0.35, and engaged in more courtship than males at OSRs greater than 2, rather than at an OSR of 1, as predicted by conventional mating system theory. Our results indicate that OSR can predict the considerable within-sex variability in reproductive behaviour of both males and females, regardless of which sex is more competitive.

Reproductive success of male bank voles (Clethrionomys glareolus): the effect of operational sex ratio and body size

The operational sex ratio (OSR) may influence the intensity of competition for mates and mate choice and is therefore thought to be a major factor predicting the intensity and direction of sexual selection. We studied the opportunity for sexual selection, i.e., the variance in male reproductive success and the direction and intensity of sexual selection on male body mass in bank vole (Clethrionomys glareolus) enclosure populations with experimentally manipulated sex ratios. The opportunity for sexual selection was high among male-biased OSRs and decreased towards female-biased OSRs. Paradoxically, selection for large male body mass was strongest in female-biased OSRs and also considerable at intermediate OSRs, whereas at male-biased OSRs, only a weak relationship between male size and reproductive success was found. Litters in male-biased OSRs were more likely to be sired by multiple males than litters in female-biased OSRs. Our results suggest that the intensity and direction of sexual selection in males differs among different OSRs. Although the direction of sexual selection on male body mass was opposite than predicted, large body mass can be favored by sexual selection. Naturally varying OSRs may therefore contribute to maintain variation in male sexually selected traits.

Operational Sex Ratio and Alternative Reproductive Behaviours in Chinese Bushcricket, Gampsocleis gratiosa

AbstractThe effects of operational sex ratio (OSR) on male mating tactics in the Chinese bushcricket Gampsocleis gratiosa were investigated in male‐ and female‐biased environments. We measured fresh and dry spermatophore contents and copulation duration, and counted sperm numbers of each copulation. The fresh weight of spermatophore and spermatophylax was positively correlated with male body weight. The males in a strongly male‐biased environment produced significantly heavier fresh ampulla and more sperm per ejaculation, which were likely tactics for successful matings under the competition of rivals. The spermatophore might function as a structure to protect the fertilization potential of the ejaculate from rival males.

Effect of body size and larval experience on mate preference in Helicoverpa armigera (Hübner) (Lep., Noctuidae)

Abstract: We studied the mating selection in the cotton bollworm, Helicoverpa armigera (Hübner), in relation to body size and larval diet in the laboratory. When provided with an artificial diet at larval stage, weight, body and forewing length did not affect the probability of a male/female moth being selected for mating, but the abdominal width of selected female moths was significantly wider than that of non‐selected female moths. 30 female moths were dissected and number of eggs was counted after mating, and there was a correlation between the abdominal width and egg number. There was also significant difference of weight loss between selected and non‐selected male/female moths after the mating. The effect of operational sex ratio on mating latency and copulation duration were tested, and the result indicated that mating latency of male selection was significantly longer than that of female selection, but the difference of copulation duration was not significant. Cotton, corn and peanut plants were provided to larvae to test the effect of larval host plant experience on mate choice. When cotton‐ and peanut‐fed moth severed as potential partners, both female and male of cotton‐fed moths significantly preferred cotton‐ to peanut‐fed moths for mating. The possible reasons for mate preference based on larval host plant experience may account for host plants attributes on sex pheromone variation and sexual maturity. These findings may impact Bacillus thuringiensis resistance management.

Effect of operational sex ratio on female–female versus male–male competitive aggression

We compared the patterns of female–female and male–male competition in Japanese medaka (Oryzias latipes) in response to changes in the operational sex ratio (OSR), the ratio of sexually active members of one sex to sexually active members of the opposite sex. As expected, courtship behaviour and intrasexual aggression were more frequent in males than in females. However, the overall patterns of female–female and male–male aggression were similar: intrasexual aggression increased with the OSR (female/male OSR for females and male/female OSR for males), consistent with predictions of mating-systems theory. A comparative analysis of our data and those from three other studies indicated that the rate of intrasexual aggression was greater in males than in females, but the patterns of intrasexual aggression were similar: the rate increased linearly and with a common slope with increasing OSR over a range of 0.4–3.

Resource presence and operational sex ratio as determinants of copulation duration in the flyRhagoletis juglandis

The effects of the interaction between the operational sex ratio (OSR) and a resource (i.e. oviposition site) on mating dynamics have rarely been considered. We examined the effect of the resource presence and its interaction, with the effect of OSR on copulation duration inRhagoletis juglandis, a tephritid fly species characterized by a resource-defence mating system in which males defend territories on walnut fruit. In this species, copulation duration varies from 30s to over 1h and was shown previously to respond strongly to changes in OSR. In the field, short copulations tended to begin and end on fruit, whereas most long copulations generally began on fruit but ended in the foliage, suggesting a possible effect of resource presence on the copulation duration. In laboratory assays of isolated pairs, copulations were significantly shorter in the presence of a surrogate fruit, confirming the effect of resource presence. In another laboratory assay, in which we manipulated OSR independently of resource presence, resource presence and OSR were additive in their effects. Results are discussed in the context of sperm competition theory.

Operational sex ratio versus gender density as determinants of copulation duration in the walnut fly, Rhagoletis juglandis (Diptera: Tephritidae)

In laboratory and field studies of the walnut fly, Rhagoletis juglandis Cresson (Diptera: Tephritidae), we assessed the effect of operational sex ratio on copulation duration and partitioned the sex ratio effect into component effects due to male density and female density. In our first laboratory experiment, results were clearly consistent with theoretical expectation: increases in male density were associated with significant increases in copulation duration while increases in female density were associated with significant decreases in copulation duration. These component effects yielded a striking composite effect of operational sex ratio (OSR) on copulation duration in which male-biased ratios were associated with low frequencies of short copulations and female-biased ratios were associated with high frequencies of short copulations. Consistent with a priori expectations concerning costs of territorial behavior, the effect of male density on copulation duration was stronger than that of female density. There was no significant interaction between the effects of gender density on copulation duration: each gender density contributed additively to the composite OSR effect on copulation duration. In contrast to the effect of OSR, overall density had little effect. Field data corroborated these findings fully and showed additionally that OSR in the vicinity of fruit tended in nature to be male-biased. In a second laboratory experiment, we measured copulation duration for individuals exposed alternately to male-biased and female-biased ratios. Individual flies consistently copulated for longer in male-biased environments than in female-biased ones. We propose that this plasticity permits individuals to track changes in local sex ratio over space and time and respond appropriately.

The evolution of sexual size dimorphism in fish*

Life history strategy models in which female size depends on fecundity and male size on male‐male competition, are used to predict male and female size. Sperm competition alone—without any form of contest competition—cannot readily generate size ratios (male weight/female weight) greater than 1·0, and if sperm competition is very weak, small or even dwarf males are predicted. Increasing sperm competition increases male size, but only with high numbers of competing males can male size approach that of females. Size dimorphism is also affected by the ratio: growth rate/mortality rate in each sex. If this is higher in one sex, its size increases. Two trade‐offs in fish could affect this ratio but neither appears likely to yield size ratios above unity under sperm competition.Male‐male combat is capable of pushing male size beyond that of females, but male size seems never to exceed female size by more than one order of ten, whilst the reverse can proceed to many orders of ten (dwarf males), a pattern which applies generally in animals. Reasons for this asymmetry in size ratio include buffering effects of reducing growth rate (‘von Bertalanffy buffering’), and an asymmetry in the homeostatic effect of operational sex ratio (‘OSR buffering’). High adult male mortality and weak male‐male competition can generate dwarf males. A fundamental problem concerns why the sexes are often so similar in size: this may relate to sperm competition, size buffering effects, and many other features not considered in the present models.

The effects of operational sex ratio on the relative mating success of extreme male phenotypes in the water strider Gerris odontogaster (Zett.) (Heteroptera; Gerridae)

The effects of operational sex ratio on the relative mating success of extreme male phenotypes in the water strider Gerris odontogaster (Zett.) (Heteroptera; Gerridae)

The effects of operational sex ratio on the relative mating success of extreme male phenotypes in the water strider (Zett.) (Heteroptera; Gerridae)

The effects of operational sex ratio on the relative mating success of extreme male phenotypes in the water strider (Zett.) (Heteroptera; Gerridae)

The effects of operational sex ratio and food deprivation on copulation duration in the water strider (Gerris remigis Say)

A North American water strider, Gerris remigis (Hemiptera) copulates for prolonged periods of time, apparently beyond the time necessary for insemination (Rubenstein 1984; Wilcox 1984; Clark 1987), I tested predictions from two different hypotheses of the adaptive significance of this behavior. Manipulations of the operational sex ratio in artificial “streams” revealed that water striders copulate significantly longer when the sex ratio is male-biased. This result is consistent with the hypothesis that prolonged copulation functions as a type of contact mate guarding, which reduces sperm competition by preventing the female from remating prior to oviposition. The finding is also consistent with the hypothesis that copulation is prolonged to enhance female foraging efficiency. In a second experiment, sex ratio and total population density were held constant, and the period of food deprivation of females was manipulated. Contrary to the predictions of the female foraging hypothesis, “starved” females did not copulate longer than “fed” females. Of the two hypotheses considered, only the copulatory guarding hypothesis explains both results. This does not mean, however, that there is no selective benefit from the enhanced foraging efficiency resulting from prolonged copulation. Nor does it exclude the possibility that this benefit has been necessary for the evolution of prolonged copulation in water striders.

Sexual Selection and Variance in Reproductive Success

Sexual Selection and Variance in Reproductive Success