Intensity Of Mating Competition Research Articles

Sex ratio effects on reproductive success of male and female Daphnia.

The intensity of mating competition and the opportunity for sexual selection are thought to depend on the operational sex ratio, the ratio of sexually active males to fertilizable females. Cyclic parthenogens, organisms that alternate between sexual reproduction and female-only parthenogenesis, show particularly high variation in sex ratios in natural populations but the effects of this variation on mating competition and reproductive success of each sex are poorly understood. In a series of experiments with Daphnia magna, we experimentally imposed five sex ratio categories, varying from one male per 81 females to an even sex ratio. We found that, in males, reproductive success strongly and monotonically decreased with decreasing number of females per male. In females, in contrast, mating success and reproductive success were reduced only at the most female-biased sex ratio (1:81), when many females remained unmated and unfertilized, and then again at equal sex ratios, probably due to negative effects of high density or stress induced by numerous males. Our results suggest that females experienced male limitation at heavily female-biased sex ratios below one male to about 50 females. As this is well within the sex ratio variation observed in natural Daphnia populations, we conclude that mating competition and the opportunity for sexual selection may exist not only in males but, at least periodically, also in females.

Spatial variation in sex ratio and density explains subtle changes in the strength of size-assortative mating in Edessa contermina (Hemiptera: Pentatomidae)

Size-assortative mating (SAM) is usually explained by the mate-choice hypothesis. However, this hypothesis does not consider the effects of variations in the intensity of mating competition on mate choice and SAM. For example, in populations with male-biased operational sex ratios (OSR) and high densities, large males will have more advantages during mating competition and will have access to the most mating opportunities with large, preferred females (i.e. mating competition hypothesis). In this study, we investigated the effects of mating competition and mate choice on the strength of SAM in Edessa contermina (Hemiptera: Pentatomidae). We also described sexual size dimorphism and the mating system of this Neotropical stink bug in a conservation area of the Brazilian savanna. We used density as a proxy for mating competition intensity because it was positively correlated with OSR. Size-assortative mating was more consistent under intense mating competition. However, males copulated with relatively larger females as population density increased. Males assortatively mated based on size in the first and second copulation events observed, while both female matings were random. Thus, male mate choice may promote SAM in E. contermina, but its strength is affected by variations in the intensity of mating competition. Hence, we provided empirical support for mating competition as a potential mechanism promoting subtle changes in SAM. We discussed potential implications of this mechanism on the mating patterns of pentatomids.

Male aggressiveness as intrasexual contest competition in a cross-cultural sample

Sexual selection favors traits that increase mating and, thus, reproductive success. Some scholars have suggested that intrasexual selection driven by contest competition has shaped human male aggression. If this is the case, one testable hypothesis is that beliefs and behavior related to male aggression should be more prevalent in societies where the intensity and strength of sexual selection is higher. Measured by factors such as (a) the presence and scope of polygyny; (b) the number of same-sex competitors relative to potential mates, and (c) the amount of effort males are available to allocate to mating. Using Bayesian item response models with imputation and data from 78 societies in the Standard Cross-Cultural Sample, we found robust support for this hypothesis when using variables related to male aggression. We ruled out some potential alternative explanations by controlling for geographic region and confounding variables such as political complexity and warfare. Intersexual selection or mate attraction has been well studied in both evolutionary psychology and human behavioral ecology. Intrasexual selection or competition between members of the same sex for mates has been investigated much less. Of the current studies, there is still a divide in the literature as to whether intrasexual selection could have shaped human male aggression. For this reason, we tested the idea with data from a wide range of societies, the first systematic cross-cultural study to do so. Our results suggest that factors affecting the intensity of competition for mates led to the evolution of beliefs and behavior related to male aggression in small-scale human societies. This provides support for the hypothesis that intrasexual selection has been a driving force in shaping human male aggression.

Mate choice and the operational sex ratio: an experimental test with robotic crabs.

The operational sex ratio (OSR: sexually active males: receptive females) predicts the intensity of competition for mates. It is less clear, however, under what circumstances, the OSR predicts the strength of sexual selection - that is, the extent to which variation in mating success is attributable to traits that increase the bearer's attractiveness and/or fighting ability. To establish causality, experiments that manipulate the OSR are required. Furthermore, if it is possible to control for any OSR-dependent changes in the chosen sex (e.g. changes in male courtship), we can directly test whether the OSR affects the behaviour of the choosing sex (e.g. female choice decisions). We conducted female mate choice experiments in the field using robotic models of male fiddler crabs (Uca mjoebergi). We used a novel design with two females tested sequentially per trial. As in nature, the choice of the first female to mate therefore affected the mates available to the next female. In general, we detected significant sexual selection due to female choice for 'males' with larger claws. Importantly, the strength of sexual selection did not vary across five different OSR/density treatments. However, as the OSR decreased (hence the number of available males declined), females chose the 'males' with the largest claws available significantly more often than expected by chance. Possible reasons for this mismatch between the expected and observed effects of the OSR on the strength of sexual selection are discussed.

The effect of size and sex ratio experiences on reproductive competition in Nicrophorus vespilloides burying beetles in the wild.

Male parents face a choice: should they invest more in caring for offspring or in attempting to mate with other females? The most profitable course depends on the intensity of competition for mates, which is likely to vary with the population sex ratio. However, the balance of pay‐offs may vary among individual males depending on their competitive prowess or attractiveness. We tested the prediction that sex ratio and size of the resource holding male provide cues regarding the level of mating competition prior to breeding and therefore influence the duration of a male's biparental caring in association with a female. Male burying beetles, Nicrophorus vespilloides were reared, post‐eclosion, in groups that differed in sex ratio. Experimental males were subsequently translocated to the wild, provided with a breeding resource (carcass) and filmed. We found no evidence that sex ratio cues prior to breeding affected future parental care behaviour but males that experienced male‐biased sex ratios took longer to attract wild mating partners. Smaller males attracted a higher proportion of females than did larger males, securing significantly more monogamous breeding associations as a result. Smaller males thus avoided competitive male–male encounters more often than larger males. This has potential benefits for their female partners who avoid both intrasexual competition and direct costs of higher mating frequency associated with competing males.

Seasonal variation in female mate choice and operational sex ratio in wild populations of an annual fish, Austrolebias reicherti.

The intensity of mating competition and the potential benefits for female of mating with certain males can be influenced by several extrinsic factors, such that behavioral decisions can be highly context-dependent. Short-lived species with a single reproductive season are a unique model to study context-sensitive mating decisions. Through exhaustive sampling in the field and simultaneous choice tests in the laboratory, we evaluated operational sex ratio (OSR) and female mate choice at the beginning and end of the reproductive season in the annual killifish Austrolebias reicherti. We found seasonal change in both OSR and female mate choice. At the start of the reproductive season the OSR did not deviate from parity, and females preferred larger males. Later in the reproductive season, while the proportion of males in the ponds decreased, females became unselective with respect to male size. The particular biological cycle of annual killifish, where both life expectancy and mating opportunities decline sharply over a short timescale, could account for the seasonal change in female choice. Reduction in choosiness could arise from diminished reproductive prospects due to a decline in male availability. Moreover, as the end of the season approaches, any benefits of choosiness are presumably reduced: a female’s fitness will be higher if she mates with any male than if she forgoes reproduction and dies. Future work will disentangle the mechanisms underlying seasonal changes in mating preferences, notably direct responses to demographic factors, environmental cues, or intrinsic changes during development.

Local Scarcity of Women Predicts Higher Fertility among Married Couples and More Single Father Households

Because the reproductive strategies of men and women are somewhat divergent, influences of the sex ratio on the intensity of mating competition and selectivity for partners produce different outcomes in female biased and male biased populations. Male mating opportunities are enhanced by scarcity and incentives for long-term commitment are diminished, encouraging serial and simultaneous polygyny. Paternal investment is lower in these populations, as indicated by higher divorce rates, more out-of-wedlock births, and a greater proportion of single mother households. Scarce females are able to more effectively secure commitment from partners as well as demand higher levels of resource investment. Women marry earlier in these populations. Although single father households are relatively uncommon, we expect to see higher proportions of households with children headed by single fathers where women are scarce. We also expect to see higher fertility among married couples, both because women may have greater bargaining power in reproductive decision-making and the role of woman in childbearing may be more salient to and valued by men. Data from the 2000 U.S. Census across 318 Metropolitan Statistical Areas supported these hypotheses.

Sex Differences in Mortality Rates Have Increased in China Following the Single-Child Law

Male behavior and physiology is designed for enhanced competitiveness at the expense of longevity, resulting in higher mortality rates compared to females in most species. These differences vary across populations consistent with factors indicating the intensity of male mating competition. Reproductive dynamics are strongly influenced by the relative proportions of potentially reproductive males and females in a population. Because the reproductive strategies of men and women are somewhat divergent, market influences on the intensity of mating competition and selectivity for partners produce different outcomes in female biased and male biased populations. The single-child law implemented in China in 1979 has led to increasing proportions of men in the Chinese population. Using historical mortality data, we found a trend for increasing sex differences in Chinese mortality rates from 1982 to 2000. This increase was most prevalent in young adulthood, when male mating competition is most intense as males reach sexual maturity and seek female partners. In contrast, males exhibited more survival gains than females in infancy and early childhood.

The Influence of Operational Sex Ratio on the Intensity of Competition for Mates

The evolution and maintenance of secondary sexual characteristics and behavior are heavily influenced by the variance in mating success among individuals in a population. The operational sex ratio (OSR) is often used as a predictor of the intensity of competition for mates, as it describes the relative number of males and females who are ready to mate. We investigate changes in aggression, courtship, mate guarding, and sperm release as a function of changes in the OSR using meta-analytic techniques. As the OSR becomes increasingly biased, aggression increases as competitors attempt to defend mates, but this aggression begins to decrease at an OSR of 1.99, presumably due to the increased costs of competition as rivals become more numerous. Sperm release follows a similar but not significant trend. By contrast, courtship rate decreases as the OSR becomes increasingly biased, whereas mate guarding and copulation duration increase. Overall, predictable behavioral changes occur in response to OSR, although the nature of the change is dependent on the type of mating behavior. These results suggest considerable flexibility of mating system structure within species, which can be predicted by OSR and likely results in variation in the strength of sexual selection.

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.

Scars for war: evaluating alternative signaling explanations for cross-cultural variance in ritual costs

While males in many societies endure traumatic and painful rites, in other societies male rites are mild or completely absent. To explain these cross-cultural differences, we use data collected from the Human Relations Area Files electronic databases (eHRAF) to test two sets of hypotheses derived from signaling theory. If costly male rites serve to signal mate quality, they would be expected to correlate with the intensity of mating competition. If they serve to signal group commitments, they would be expected to be associated with the importance of overcoming problems of collective action. Our results support the latter set of hypotheses: males in societies that engage in warfare endure the costliest rites. Moreover, we show that whether wars are fought within cultural groups or against other cultural groups is an important determinant of whether or not male rites result in permanent visible marks, such as ritual scars. We argue that costly male rites signal commitment and promote solidarity among males who must organize for warfare.

The capacity for additional matings does not affect male mating competition in the sand goby

Models of sexual selection predict that mating competition within a population will increase with a biased operational sex ratio (OSR). However, these models do not consider any influence of the individuals' capacity for additional matings. Using the sand goby, Pomatoschistus minutus, we tested whether intensity of mating competition (measured as the fraction of time spent displaying) is influenced by a male's imminent capacity for additional matings (nest capacity, i.e. additional space available for egg laying in the nest). We manipulated nest size and number of females (zero to three) allowed to spawn in each male's nest, and then used display rate to estimate the male's intensity of mating competition. The nests were never filled completely, but always had room for eggs from at least one more mating. We found no significant difference in display rate between males with large or small nests, whether nest size was measured as nest capacity or area already covered by eggs. In fact, males with many eggs in their nests displayed as much as males with empty nests. However, male display decreased significantly with time. As male display behaviour was not influenced by whether the nest was empty or was partially filled, the number of males and females ready to mate (i.e. the OSR) provides a good estimate of the intensity of male mating competition. Nevertheless, many other species may adjust mating competition to their capacity for additional matings, which may thus influence the accuracy of OSR-based estimates of mating competition and sexual selection.

The Effect of Social Environment on Male–Male Competition in Guppies (Poecilia reticulata)

AbstractWe examined male–male competition in guppies (Poecilia reticulata) to test for evidence of hierarchy formation and any subsequent effects on male mating success by comparing the interactions of pairs of males that were siblings and life‐long tank mates with those of unrelated pairs that had never met. These pairs of males were first observed in the absence of a female; then a female was added to gauge the effects of the initial male–male interactions on male sexual behaviour. The unfamiliar/unrelated pairs engaged in significantly more aggressive interactions such as physical contacts, nipping and chasing than the familiar/related pairs. Based on several previous studies, we suggest that familiarity played a greater role than relatedness in the differences in behaviour that we observed. Our results suggest that, in some circumstances, more aggressive males may have more mating opportunities than less aggressive males. Our results also indicate that males adjust their aggressive and courtship behaviours to the perceived intensity of competition for mates, based on the number of mature males in their rearing tanks. We suggest that male–male competition for mating opportunities may play a more important role in the guppy mating system than previously thought.

Size Dimorphism, Intrasexual Competition, and Sexual Selection in Wattled Jacana (Jacana Jacana), A Sex-Role-Reversed Shorebird in Panama

AbstractWe studied sexual size dimorphism, intrasexual competition, and sexual selection in an individually marked population of Wattled Jacanas (Jacana jacana) in the Republic of Panama. Males are the sole incubators of eggs (28-day incubation) and primary providers of chick care (50–60 days). Females were 48% heavier than, and behaviorally dominant over, males. Females also showed greater development of secondary sexual characters (fleshy facial ornamentation and wing spurs) than males. Both sexes defended territories throughout the year against same-sex conspecifics. Competition for territorial space was intense, and many individuals of both sexes did not become breeders. Resident females further competed with one another to accumulate multiple mates, resulting in a mating system of simultaneous polyandry. Female and male residents (territory holders) were larger, heavier, and more ornamented than adult floaters of the same sex. Larger and heavier females also had more mates than smaller females. Body size was thus a critical predictor of success in intrasexual competition for territories (both sexes) and for mates (females). Three measures of sexual selection—(1) sex difference in the opportunity for sexual selection, (2) female-to-male ratio of potential reproductive rates, and (3) operational sex ratio—each indicated that sexual selection is currently operating more strongly on females than on males (female-to-male ratios ranged from 1.43:1 to 2.22:1). Values of 1.61:1 and 1.43:1 represent the first published quantitative estimates of the opportunity for sexual selection for any sex-role-reversed bird. Our study supports the theory that when increased parental care entails reduced opportunities for future reproduction, asymmetries in parental care behaviors of the sexes can influence the intensity of competition for mates and the direction and strength of sexual selection.

Temperature affects male and female potential reproductive rates differently in the sex-role reversed pipefish, Syngnathus typhle

The differences in potential reproductive rate between the sexes can be used to predict the operational sex ratio and the patterns and intensity of mating competition and hence sexual selection in a population. This article describes how one environmental component, temperature, affects potential reproductive rates of the two sexes in the paternally brooding, sex-role reversed pipefish (Syngnathus typhle). Males brooded embryos much longer (on average 58 days) in cold water (about 10°C) than in warmer water (35 days at about 15°C). As a consequence, the potential reproductive rate (number of eggs brooded per day) of males was significantly higher in warm water. In females, however, potential reproductive rate, i.e., number of eggs produced per day given an unlimited access to mates, was not significantly different between temperatures. In both sexes, potential reproductive rate was positively related to body size. At both temperatures, females had the potential to reproduce faster than males. As a result, the operational sex ratio will become female biased and sex-roles reversed, as is the case in this species. Since temperature differently influenced the potential reproductive rates of males and females, with the sexual difference larger at lower temperatures, more intense female-female competition is predicted at low temperatures.

Geographic Variation of Sexual Dimorphism in the Raccoon Procyon lotor

-Geographic variation of sexual dimorphism for 22 cranial characters was investigated in raccoons (Procyon lotor) from 87 localities. Geographic variation in the degree of sexual dimorphism occurred only for canine width and the degree of sexual dimorphism in canine width was significantly less for raccoons from W of the Continental Divide than those from E of the Continental Divide. The lack of geographic variation in sexual dimorphism of greatest length of skull indicates that sexual dimorphism in body size of raccoons also does not vary geographically (skull length is highly correlated with body size). Thus, it appears that selection pressures responsible for sexual dimorphism in raccoons are acting uniformly throughout much of the range of the species. Geographic variation in the degree of sexual dimorphism for canine width may be the result of geographic variation in the intensity of intrasexual selection (i.e., variation in the intensity of male-male competition for mates during the breeding season). If so, the apparent lack of geographic variation in sexual size dimorphism of raccoons may indicate that male body size is not under the same kinds of selection pressures (e.g., intrasexual selection) as canine size.

Interaction of Wild Males and Laboratory-Adapted Females of the Mexican Fruit Fly (Diptera: Tephritidae) in Natural Habitats

Despite >40 yr of ecological and behavioral research on reproductive interactions involved in sterile insect release programs, experimental measurement of the intensity of intermale competition for mates and the role of sterile females in these programs has rarely been addressed. Experiments described in this study were designed to evaluate these factors. Virgin females from a mass-rearing strain (mass reared for 36 yr) of the Mexican fruit fly, Anastrepha ludens (Loew), were released and subsequently recaptured along with native Mexican fruit flies in natural habitats in Nuevo Leon, Mexico. Tests were carried out at six locations and replicated four times. Recaptured females were dissected to determine mating status. Data were analysed to determine whether the released females mated with native males, and to evaluate the influence of test site, test date, and wild male and female density on rates of recapture of the released females and their mating status. Results showed that 5–50% of the recaptured females mated with native males. This indicates that released females (even from a strain adapted to mass rearing) may occupy a significant amount of native male reproductive effort and that males in native populations have excess reproductive capacity. Number of native males captured per test was significantly correlated with the percentage of mated females. Plots of percent mating versus number of native males captured showed that when number of captured males was high, percent females mated was always high, but when numbers of males captured was low, percent mating could still be high. A severe freeze during the fourth test greatly reduced the native population. Released females were recaptured up to 7 wk after release and 4 wk after the freeze, indicating that the mass-reared strain had sufficient physiological-behavioral tolerance to survive extreme environmental stress.

Plasma steroid hormone levels in free-living Wilson's phalaropes, Phalaropus tricolor

Blood samples collected from free-living Wilson's phalaropes during the reproductive season were analyzed for testosterone, dihydrotestosterone, estradiol-17β, and progesterone. Levels of testosterone were seven times greater in nonincubating males than in females. During incubation males underwent a reduction in testosterone and dihydrotestosterone to levels similar to those of females. Estradiol-17β values were higher in females than in incubating males and comparable to values reported for other avian species. Progesterone was significantly greater in females than in incubating or nonincubating males. Maximum levels of progesterone were detected in laying females. Progesterone levels increased in males during incubation while testosterone and dihydrotestosterone levels decreased. These results indicate that the greater intensity of competition for mates among females and exclusive male parental care characteristic of this species is not based upon a reversal of the typical avian levels of androgens and estrogens in males and females.