Fertility Assurance Research Articles

Alternative reproductive strategies provide a flexible mechanism for assuring mating success in the European badgers (Meles meles): An investigation from hormonal measures

Selection-pressures differ with population density, but few studies investigate how this can affect reproductive physiology. European badger (Meles meles) density varies from solitary to group-living across their range, with reported mating periods throughout the entire year to specific seasonal periods. Badger reproduction is evolutionarily distinct, interrupting the direct progression from conception to gestation with delayed implantation (DI), allowing for superfecundation (SF). To establish the tactical mating flexibility afforded by DI*SF, we used cross-sectional population-level seasonal variation of circulating sex-steroids for 97 females from a high-density population. Oestradiol was highest in spring among non-parous females, then lower in summer, and remained low during following seasons, suggesting that the mating period was restricted to just spring. Oestrone was consistently higher than oestradiol; it was elevated in spring, lowest during summer, peaked in autumn, and remained elevated for pregnant females in winter. This suggests that oestrone sustains pre-implanted blastocysts throughout DI. Progesterone was low throughout, except during winter pregnancy, associated with implantation and luteal development. In contrast to multiple mating periods reported by lower-density studies, our oestradiol data suggest that, at high-density, females exhibit only one mating period (congruent with testosterone patterns in males studied previously in this same population). While additional mating periods during DI enhance fertility assurance at low-density, at high-density, we propose that when coitus is frequent, fertilisation is assured, precluding the need for further cycles and associated mating risks. This endocrinologically flexible DI*SF mating strategy likely represents a form of balancing selection, allowing badgers to succeed at a range of regional densities.

Flexible polyandry in female flies is an adaptive response to infertile males

AbstractInfertility is common in nature despite its obvious cost to individual fitness. Rising global temperatures are predicted to decrease fertility, and male sterility is frequently used in attempts to regulate pest or disease vector populations. When males are infertile, females may mate with multiple males to ensure fertilization, and changes in female mating behavior in turn could intensify selection on male fertility. Fertility assurance is a potentially wide-spread explanation for polyandry, but whether and how it actually contributes to the evolution of polyandry is not clear. Moreover, whether a drop in male fertility would lead to a genetic increase in polyandry depends on whether females respond genetically or through behavioral plasticity to male infertility. Here, we experimentally manipulate male fertility through heat-exposure in Drosophila pseudoobscura, and test female discrimination against infertile males before and after mating. Using isogenic lines, we compare the roles of behaviorally plastic versus genetically fixed polyandry. We find that heat-exposed males are less active and attractive, and that females are more likely to remate after mating with these males. Remating rate increases with reduced reproductive output, indicating that females use current sperm storage threshold to make dynamic remating decisions. After remating with fertile males, females restore normal fecundity levels. Our results suggest that male infertility could explain the evolution of adaptively flexible polyandry, but is less likely to cause an increase in genetic polyandry.

Heteropaternity of twins in ring-tailed lemurs (Lemur catta).

This project investigated paternity among 14 sets of twins born into a colony of ring-tailed lemurs (Lemur catta) on St. Catherines Island, GA. Female L. catta commonly mate with >1 male, and twins of different sexes confirm that dizygotic twinning can occur in this species. However, no study has previously evaluated twins using genetic data to measure the proportion of mono versus dizygotic twinning in L. catta, and no study has tested for heteropaternity (sirehood by two different males) in this species. We predicted that L. catta would show a predominance of dizygosity and evidence for heteropaternity. Paternity was determined for 28 infants (14 sets of twins) via paternity exclusion analyses using 10 polymorphic microsatellite markers, and data were collected on sexual behavior across four mating seasons during the first cycle of mating in each year, which overlapped with the conceptive periods of these infants. All twins were found to be dizygotic, as evidenced by dissimilar multilocus genotypes between the two infants of each twin pair. Heteropaternity was found in 3 of 14 (21% of) twin pairs, whereas the remaining 11 pairs of twins (79%) shared the same sire. In one case of heteropaternity, one sire was a resident male, and one sire was an extra-group male. An extra-group male also sired one of the same-sire twin pairs. All other sires of twins were group residents. Female multiple mating, which is common to many primates including L. catta, can be viewed as an adaptive reproductive strategy whose function-in addition to fertility assurance or stimulating sperm competition-may be to increase the genetic diversity of a female's offspring, even among infants born in the same birth event. Therefore, during the course of primate evolution, the adaptive benefits of heteropaternity would be expected to have contributed to positive selection for female promiscuity.

Multiple paternity in wild house mice (Mus musculus musculus): effects on offspring genetic diversity and body mass

Multiple mating is common in many species, but it is unclear whether multiple paternity enhances offspring genetic diversity or fitness. We conducted a survey on wild house mice (Mus musculus musculus), and we found that in 73 pregnant females, 29% of litters had multiple sires, which is remarkably similar to the 23–26% found in feral populations of Mus musculus domesticus in the USA and Australia, respectively. The question is: How has selection maintained multiple mating in these subspecies since the evolutionary divergence, ca. 2800–6000 years ago? We found no evidence that multiple paternity enhanced females’ litter size, contrary to the fertility assurance or genetic benefits hypotheses. Multiple paternity was associated with reduced mean and variance in offspring body mass, which suggests that females allocate fewer resources or that there is increased intrauterine conflict in multiple-versus single-sired litters. We found increased allelic diversity (though not heterozygosity) in multiple-sired litters, as predicted by the genetic diversity hypothesis. Finally, we found that the dams’ heterozygosity was correlated with the mean heterozygosity of their offspring in single-and multiple-sired litters, suggesting that outbred, heterozygous females were more likely to avoid inbreeding than inbred, homozygous females. Future studies are needed to examine how increased genetic diversity of litters and smaller mean (and variance) offspring body mass associated with multiple paternity affect offspring fitness.

Causes of extra-pair paternity and its inter-specific variation in socially monogamous birds

Based on molecular technology, researchers find that extra-pair paternity (EPP) prevails among socially monogamous bird species. This phenomenon challenges traditional views of sexual selection and mating system, and has become one of the hot-spots in the avian behavior ecology. This review focuses on the evolutionary causes leading to EPP and the potential factors affecting it. Early studies of EPP in birds used a wide variety of tools, including plumage color polymorphism, polymorphic enzymes, and sex-differences in estimation of the heritability of morphological traits. Although each of these methods can be used to estimate the likelihood that EPP are present or absent in a population, none of them provide enough accurate estimation to allow meaningful cross-species comparison. Currently, studies of EPP mainly use “DNA-methods”, namely multi-locus mini-satellite fingerprints, single-locus mini-satellite fingerprints, and micro-satellite genotyping, because they can provide accurate outcome of paternity identification and their results are reproducible. From the point of the female, the driving forces for EPP are that females may gain direct benefits or indirect genetic benefits from EPP. Hypotheses explaining the benefits include fertility assurance, good genes, genetic compatibility, and genetic diversity. Despite large numbers of theoretically plausible explanations for EPP, there have been few direct empirical tests that can provide unambiguous support for only one type of explanation. The most straightforward test of the genetic benefit hypothesis of extra-pair copulation is a comparison of the performance of maternal half-siblings from multiply sired broods. Some highly-cited landmark studies spectacularly support the genetic benefit hypothesis, while other studies failed to reveal any systematic differences in maternal half-sibling performance, even in the same species or in taxonomically closely-related species. Briefly, the point that purpose of extra-pair copulation is to gain genetic benefits is still facing great challenges. Therefore, some researchers suggest that more attention should be focused on the interactions between parties involved in the extra-pair paternity phenomenon. Others, however, believe that such interactions can also be of a cooperative nature, and involve exchange of direct benefits. In brief, an approach focusing on interactions (involving either conflict or cooperation) seems to be the most promising direction to improve our understanding of the phenomenon of extra-pair paternity. As for the factors leading to intra- and inter-specific variation in the level of EPP, current researches mainly focus on the breeding density, breeding synchrony, the complexity of the habitat, paternal care, adult mortality, food availability, and genetic diversity. Explaining intra- and inter-specific variation in the extent of EPP has been difficult, but an appreciation of the problems of small sample sizes, and an ever-increasing comparative database have led to several recent advances. It now seems probable that differences between species in the rate of EPP are due to a combination of differences in life history, pattern of parental care, and local opportunities for promiscuity. In a word, although there have been a lot of theoretical and empirical researches about EPP of birds, no consensus on the basic questions has been received in this area. Thus, more scientific statistical analysis method and more empirical experiments are still badly needed to improve the theoretical system.

Do male secondary sexual characters signal ejaculate quality? A meta‐analysis

There are two reasons why researchers are interested in the phenotypic relationship between the expression of male secondary sexual characters (SSCs) and 'ejaculate quality' (defined as sperm/ejaculate traits that are widely assumed to increase female fertility and/or sperm competitiveness). First, if the relationship is positive then females could gain a direct benefit by choosing more attractive males for fertility assurance reasons ('the phenotype-linked fertility' hypothesis). Second, there is much interest in the direction of the correlation between traits favoured by pre-copulatory sexual selection (i.e. affecting mating success) and those favoured by post-copulatory sexual selection (i.e. increasing sperm competitiveness). If the relationship is negative this could lead to the two forms of selection counteracting each other. Theory predicts that the direction of the relationship could be either positive or negative depending on the underlying genetic variance and covariance in each trait, the extent of variation among males in condition (resources available to allocate to reproductive traits), and variation among males in the cost or rate of mating. We conducted a meta-analysis to determine the average relationship between the expression of behavioural and morphological male secondary sexual characters and four assays of ejaculate quality (sperm number, viability, swimming speed and size). Regardless of how the data were partitioned the mean relationship was consistently positive, but always statistically non-significant. The only exception was that secondary sexual character expression was weakly but significantly positively correlated with sperm viability (r = 0.07, P < 0.05). There was no significant difference in the strength or direction of the relationship between behavioural and morphological SSCs, nor among relationships using the four ejaculate quality assays. The implications of our findings are discussed.

No Detectable Fertility Benefit from a Single Additional Mating in Wild Stalk-Eyed Flies

BackgroundMultiple mating by female insects is widespread, and the explanation(s) for repeated mating by females has been the subject of much discussion. Females may profit from mating multiply through direct material benefits that increase their own reproductive output, or indirect genetic benefits that increase offspring fitness. One particular direct benefit that has attracted significant attention is that of fertility assurance, as females often need to mate multiply to achieve high fertility. This hypothesis has never been tested in a wild insect population.Methodology/Principal FindingsFemale Malaysian stalk-eyed flies (Teleopsis dalmanni) mate repeatedly during their lifetime, and have been shown to be sperm limited under both laboratory and field conditions. Here we ask whether receiving an additional mating alleviates sperm limitation in wild females. In our experiment one group of females received a single additional mating, while a control group received an interrupted, and therefore unsuccessful, mating. Females that received an additional mating did not lay more fertilised eggs in total, nor did they lay proportionately more fertilised eggs. Female fertility declined significantly through time, demonstrating that females were sperm limited. However, receipt of an additional mating did not significantly alter the rate of this decline.Conclusions/SignificanceOur data suggest that the fertility consequences of a single additional mating were small. We discuss this effect (or lack thereof), and suggest that it is likely to be attributed to small ejaculate size, a high proportion of failed copulations, and the presence of X-linked meiotic drive in this species.

Recovery, Preparation, Storage and Utilization of Spermatozoa for Fertility Preservation in Cancer Patients and Sub-Fertile Men

Sperm cryopreservation is an important part of an infertility program for patients undergoing infertility treatments, fertility assurance for vasectomy cases, and for fertility preservation due to cancer or other medical conditions. With recent developments in reproductive technology, even men with severely impaired sperm parameters can benefit from cryopreservation as procedures such as intra-cytoplasmic sperm injection (ICSI) require only a few sperm to achieve fertilization and pregnancy. The increasing success of cancer treatment and concerted efforts to ensure quality of life after successful treatment have placed great emphasis on the need to preserve the reproductive capability of young men. It is a highly effective method of protecting male fertility potential, and involves collection, freezing, and long-term storage of sperm. Based on the etiological condition of the patients, sperm can be collected by ejaculation or by surgical retrieval from epididymis or testes. The option to bank sperm should be offered systematically to all patients who may benefit. However, this is not a standard of practice yet; it may be overlooked due to lack of physician awareness regarding the need for fertility preservation and the effectiveness of this option, and/or overestimating the limitations of poor baseline sperm quality leading physician to view cryopreservation as futile. Failure to offer cryopreservation ignores the only possible reproductive option available to certain patients.

Female multiple mating and paternity in free-ranging North American red squirrels

Multimale mating (MMM) is common in female mammals, but our understanding of its evolutionary significance in this taxon lags behind that in others (e.g. invertebrates and birds). To date, the majority of research on mammalian MMM has been conducted under laboratory conditions, and the extent to which these findings have relevance for natural populations is little known. We quantified MMM in a free-ranging population of North American red squirrels, Tamiasciurus hudsonicus , and investigated its hypothesized influences and consequences. We observed 85 mating chases, representing 62 individual females, over three mating seasons and calculated standardized indexes of MMM and number of copulations. Extrinsic elements of mating chases (i.e. an index of the number of attending males and date) explained variation in MMM, while traits of the females (age and pre-oestrous body mass) did not correlate with MMM. Hypotheses of cryptic direct benefit (fertility assurance and infanticide avoidance) were not supported because MMM did not influence pregnancy rate, litter size or nest fate. There was also no correlation between MMM and offspring quality, multiple paternity or litter allelic diversity, and therefore, no support for hypotheses of genetic benefit. In addition to not accruing observable benefits to MMM, females did not incur a detectable cost, thus rendering MMM selectively neutral. Rather, in line with traditional explanations, female mating behaviour in red squirrels appears to be a passive response to selection on multifemale mating in males.

The Evolution of Infidelity in Socially Monogamous Passerines: Neglected Components of Direct and Indirect Selection

A recent study by Goran Arnqvist and Mark Kirkpatrick in the American Naturalist (165:S26-S37) suggested that female polyandry in birds is not driven by females because quantitative genetic approximations of selection demonstrated that indirect selection for female infidelity is weaker than natural selection against it. Instead, it was argued that extrapair copulations are the result of antagonistic selection on male behavior driving female coercion. While the approach and framework of the study were very good, the conclusions of the study were premature because a number of potential adaptive components of polyandry were unaccounted for, and several critical assumptions are unsupported by the current empirical data. Our understanding of extrapair paternity in birds, and perhaps polyandry in general, will be improved by a better empirical understanding of the direct benefits of fertility assurance and postcopulatory cryptic female choice and the relationship between polyandry and male investment. In addition, we need to develop a greater awareness of the limitations of trying to study behavior by proxy in the molecular laboratory. Together, these challenges and the framework recently presented should improve our understanding of the true function of extrapair paternity in birds.

The Evolution of Infidelity in Socially Monogamous Passerines: Neglected Components of Direct and Indirect Selection

A recent study by Göran Arnqvist and Mark Kirkpatrick in the American Naturalist (165:S26–S37) suggested that female polyandry in birds is not driven by females because quantitative genetic approximations of selection demonstrated that indirect selection for female infidelity is weaker than natural selection against it. Instead, it was argued that extrapair copulations are the result of antagonistic selection on male behavior driving female coercion. While the approach and framework of the study were very good, the conclusions of the study were premature because a number of potential adaptive components of polyandry were unaccounted for, and several critical assumptions are unsupported by the current empirical data. Our understanding of extrapair paternity in birds, and perhaps polyandry in general, will be improved by a better empirical understanding of the direct benefits of fertility assurance and postcopulatory cryptic female choice and the relationship between polyandry and male investment. In addition, we need to develop a greater awareness of the limitations of trying to study behavior by proxy in the molecular laboratory. Together, these challenges and the framework recently presented should improve our understanding of the true function of extrapair paternity in birds.

Female multiple mating for fertility assurance in red flour beetles (Tribolium castaneum)

The costs of mating with multiple partners include expenditure of energy and time and a reduction in lifespan, but females of many taxa mate with several different partners shortly after their first copulation. Often it is not clear what females gain from this behaviour. In this study, we used the red flour beetle (Tribolium castaneum Herbst, 1797) to test the hypothesis that females mate with multiple males for fertility assurance because the first copulation often does not lead to offspring production. We found that the probability of producing offspring, as measured by the proportion of females that produced offspring, was not affected by multiple mating when females were mated to virgin partners. However, when females were mated to nonvirgin partners, multiple mating led to an increased probability of producing offspring. To establish the mechanism through which multiple mating enhanced the probability of producing offspring, we further investigated whether this result was observed because multiple mating provided genetically compatible sperm or because it provided sufficient sperm. Viability of larvae from multiply mated females was higher than that of larvae from singly mated females, but the total number of adult offspring produced was not significantly different between the two groups. The capacity of males to inseminate females decreased in successive matings after the initial copulation, suggesting that ineffective copulations between virgin females and nonvirgin males are likely due to male sperm depletion. Therefore, mating with multiple males increased the probability that females would produce offspring and served as fertility assurance.

Fertility assurance and breeding synchrony

Extrapair fertilizations are an important factor in reproduction of many bird species. It has been suggested that pursuit of extrapair fertilizations provides a selective pressure capable of affecting the “ecological” attributes of some bird species. It is known that in some cases the pursuit of extrapair copulations is motivated by fertility assurance. That is, there are populations in which (i) some males and females cannot produce viable offspring, and (ii) there is no pre-copulatory way to choose a compatible mate. In such situations individuals mate for “economic” reasons and attempt to secure reproduction by copulating with multiple partners. Here we formulate and analyze a simple game theoretical model addressing the interplay between environmental constraints and the pursuit of fertility assurance via extrapair copulations in determining breeding synchrony. Our results indicate that breeding synchrony in such cases is determined by the magnitude of the costs (due to environmental constraints) of breeding asynchronously versus the worth of the attendant increase in extrapair opportunities.

Multiple copulations in natural populations of lizards: evidence for the fertility assurance hypothesis

Females of many species mate multiply to obtain material or genetic benefits. However, because of problems associated with estimating number of copulations, our understanding of female mating patterns in natural populations is very limited. We studied correlates of multiple mating over three years in populations of the viviparous common lizard, Lacerta vivipara, a species in which the number of copulations can be reliably inferred from the number of mating scars. Our results show that females with a high number of copulations had lower clutch size, but heavier offspring (controlled for a potential relationship between the two), than females with fewer copulations. This pattern could be driven by sexual selection, ecological, and environmental factors. More promiscuous females had lower proportion of infertile eggs, controlling for clutch size. Thus, one of the primary reasons for multiple copulations in natural populations of L. vivipara could be sperm limitation, or to avoid infertile males.

Rapid female multiple mating in red flour beetles (Tribolium castaneum)

Many female insects mate with multiple males within a single fertile period despite costs such as expenditure of energy and time and contraction of sexually transmitted diseases. In the red flour beetle, Tribolium castaneum, females remate with different males within minutes of the first copulation. If rapid multiple mating is adaptive then multiply mated females should have higher fitness than singly mated females. In this study, we determined the remating frequency of female beetles, characterized female mating behavior, and examined the fitness consequences of female multiple mating. We found that female T. castaneum mated, on average, with 4–6 nonvirgin males within a 1-h observation period. The number of males present in a mating arena did not significantly affect copulation frequency or the intermating interval. However, number of males present significantly affected the length of a single copula as a result of disturbance by rival males when more males were present. Female mating with multiple males in 24 h did not significantly improve egg production, F1-adult production, egg-to-adult viability, fertility retention, and female survivorship. Thus, multiple mating did not enhance long-term female fitness. Polyandrous mating behavior may have evolved through other mechanisms such as fertility assurance and increased offspring genetic diversity or fitness.

Relatedness, body size and paternity in the alpine newt, Triturus alpestris.

Sexual selection has traditionally been investigated assuming that male quality is as skewed as patterns of male reproductive success can sometimes be. Recently, female choice has been investigated under the model of genetic compatibility, which assumes that each individual female has her own 'best' mate and there is no overall optimal choice for all females. We investigated female mate choice in the newt species Triturus alpestris, a member of a genus where female choice has been investigated only within the context of the optimal male (female choice for condition-dependent traits). We provided females with two males that differed in one condition-dependent trait (body size) and overall genetic composition. Both male body size and female body size did not influence paternity, but the degree of genetic relatedness between females and potential mates did. Two components of fitness (fecundity and hatching success) did not differ between singly and multiply sired clutches, indicating that females do not employ polyandry as a means of increasing offspring fitness through genetic bet-hedging. Instead, we hypothesize that females may mate initially for fertility assurance, but prefer less-related males as the most genetically compatible mates.

Fertility Assurance through Extrapair Fertilizations and Male Paternity Defense

Extrapair paternity has been observed in many formally monogamous species. Male pursuit of extrapair fertilizations is explained by the advantages of having offspring that receive essential paternal care from other males. Since females are capable of exercising a degree of control over the post-copulatory sperm competition, extrapair paternity cannot persist unless it confers fitness benefits on cuckolding females. Thus, extrapair paternity involves cooperation between mated females and extrapair males. On the other hand, paired males frequently exhibit strategies that minimize their loss of paternity and/or conserve paternal investment if paternity is lost. Hence, extrapair attributes of diverse species and populations reported in the literature are particular solutions of evolutionary games involving gender-specific cuckolding/anti-cuckolding strategies. Here we use methods of evolutionary game theory to study the role of male paternity guarding strategies in situations where females seek extrapair fertilizations for reasons of genetic compatibility and/or in pursuit of genetic diversity for their offspring. Our results indicate that in these circumstances pursuit of extrapair fertilizations is the only evolutionary stable female strategy. Males, on the other hand, have two, mutually exclusive, evolutionary stable strategies: full time pursuit of extrapair fertilizations and a compromise strategy wherein they protect in-pair paternity during their mate's fertile periods and pursue extrapair paternity the rest of the time. The relative merits of these two strategies are determined by the efficiency of male in-pair paternity defense, breeding synchrony, fitness advantages of extrapair over in-pair offspring, and the intensity of competition for extrapair fertilizations from floater males.

Fertility Assurance Through Extrapair Fertilization, and Male Parental Effort

Extrapair paternity (EPP) has been observed in many formally monogamous species. Male pursuit of extrapair fertilizations (EPF) is explained by the advantages of having offspring that receive essential paternal care from other males. Because females are capable of exercising a degree of control over the post-copulatory sperm competition, EPP’s persistence indicates that females benefit from EPF. Thus, EPP involves cooperation between mated females and extrapair males. On the other hand, mated males exhibit a spectrum of anti-cuckolding strategies. Hence, extrapair attributes of diverse species and populations reported in the literature are particular solutions of evolutionary games involving gender-specific cuckolding/anti-cuckolding strategies. Here we use game theoretical methods to study the effect of male paternal effort conserving strategies in situations where females seek EPF for reasons of genetic compatibility and/or in pursuit of genetic diversity for their offspring. Our results indicate that in these circumstances pursuit of EPF is the only evolutionary stable female strategy. Males, on the other hand, have two, mutually exclusive, evolutionary stable strategies: males that restrict parental care regardless of their mate’s fidelity, and males that never restrict parental care. That is, when females seek EPF for reasons of fertility assurance and/or genetic diversity, the conditional male strategy—therein the male’s parental efforts are based on his certainty of paternity—loses in competition with the unconditional strategies.

Postcopulatory vocalizations of fallow bucks: who is listening?

Fallow bucks (Dama dama) produce a postcopulatory vocalization (PCV), consisting of an increase in the short-term groaning rate during the first min after mating. In this article, we consider two main hypotheses to assess the possible function of the postcopulatory vocalization. First, the PCV could be directed at females, and used to advertise the current fertility status of the male. Second, the PCV could be directed at males, and used to transmit an intrasexual threat signal. We found that during days when a male gained many matings, his PCVs did not decline, and males with larger intervals between matings did not produce higher PCVs. Lower PCVs were not associated with infertile matings, and for females that mated twice within the same estrus, the PCVs of their first matings were not lower than other PCVs. In addition, higher PCVs were not associated with shorter intervals to a male’s next mating. Thus, there was no evidence to suggest that the PCV was involved in transmitting a signal of fertility assurance, either to females that had mated, or to those that were about to mate. We found that PCVs declined as males reached the end of their mating success, therefore suggesting that PCVs are more likely to be involved in transmitting an intrasexual threat signal related to current condition and/or motivation. We suggest that this signal is probably involved in mate guarding. Key words: postcopulatory vocalization, fallow deer, Dama dama, signal, fertility, body condition, motivation, mate guarding. [Behav Ecol 12:41–46 (2001)]

Immune defence, extra–pair paternity, and sexual selection in birds

Secondary sexual characters have been suggested to reliably reflect the ability of individuals to resist debilitating parasites, and females may gain direct or indirect fitness benefits from preferring the most extravagantly ornamented males. Extra–pair paternity provides an estimate of an important component of sexual selection in birds. Species with a high frequency of extra–pair paternity have a variance in realized reproductive success that is greater than the variance in apparent reproductive success, and extra–pair copulations and hence extra–pair paternity by females are often directly associated with the expression of male secondary sexual characters. If sexually dichromatic species have experienced a long period of antagonistic coevolution with their parasites, such species should have evolved larger immune defence organs than sexually monochromatic species. Bird species with sexual dichromatism had larger spleens for their body size than monochromatic species in a comparative analysis. Furthermore, species with a high frequency of extra–pair paternity were sexually dichromatic and had large spleens for their body size. These results are consistent with the hypothesis that females of dichromatic bird species seek extra–pair copulations to obtain indirect fitness benefits in terms of superior resistance of their offspring to virulent parasites.