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.     

Indiscriminate polyandry and male parental effort

Extrapair paternity 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 extrapair fertilizations (EPF) for reasons of genetic compatibility and/or in pursuit of genetic diversity for their offspring. In such cases, females cannot make a pre-copulatory selection of the optimal genetic partners, and therefore combine promiscuous copulation with the use of in copula and/or post-copulatory selection mechanisms to optimize the genetic endowment of their offspring—indiscriminate polyandry. Our results indicate that, when indiscriminate polygamy is constrained by the availability of extrapair male partners, there are three possible (parameter regime wise) evolutionary stable strategy solutions. (1) All females seek EPF, while all males restrict parental care. (2) All females seek EPF, while all males are unconditionally parental. (3) Females use a combination strategy where pursuit of EPF is mixed—on either a population, or an individual level—with genetic monogamy, while all males use a conditional paternal care strategy, which involves adjusting their parental efforts according to their certainty of paternity.     

Paternal genetic effects on offspring fitness are context dependent within the extrapair mating system of a socially monogamous passerine

Avian extrapair mating systems provide an interesting model to assess the role of genetic benefits in the evolution of female multiple mating behavior, as potentially confounding nongenetic benefits of extrapair mate choice are seen to be of minor importance. Genetic benefit models of extrapair mating behavior predict that females engage in extrapair copulations with males of higher genetic quality compared to their social mates, thereby improving offspring reproductive value. The most straightforward test of such good genes models of extrapair mating implies pairwise comparisons of maternal half-siblings raised in the same environment, which permits direct assessment of paternal genetic effects on offspring traits. But genetic benefits of mate choice may be difficult to detect. Furthermore, the extent of genetic benefits (in terms of increased offspring viability or fecundity) may depend on the environmental context such that the proposed differences between extrapair offspring (EPO) and within-pair offspring (WPO) only appear under comparatively poor environmental conditions. We tested the hypothesis that genetic benefits of female extrapair mate choice are context dependent by analyzing offspring fitness-related traits in the coal tit (Parus ater) in relation to seasonal variation in environmental conditions. Paternal genetic effects on offspring fitness were context dependent, as shown by a significant interaction effect of differential paternal genetic contribution and offspring hatching date. EPO showed a higher local recruitment probability than their maternal half-siblings if born comparatively late in the season (i.e., when overall performance had significantly declined), while WPO performed better early in the season. The same general pattern of context dependence was evident when using the number of grandchildren born to a cuckolding female via her female WPO or EPO progeny as the respective fitness measure. However, we were unable to demonstrate that cuckolding females obtained a general genetic fitness benefit from extrapair fertilizations in terms of offspring viability or fecundity. Thus, another type of benefit could be responsible for maintaining female extrapair mating preferences in the study population. Our results suggest that more than a single selective pressure may have shaped the evolution of female extrapair mating behavior in socially monogamous passerines.     

Sexual selection resulting from extrapair paternity in collared flycatchers

Extrapair paternity has been suggested to represent a potentially important source of sexual selection on male secondary sexual characters, particularly in birds with predominantly socially monogamous mating systems. However, relatively few studies have demonstrated sexual selection within single species by this mechanism, and there have been few attempts to assess the importance of extrapair paternity in relation to other mechanisms of sexual selection. We report estimates of sexual selection gradients on male secondary sexual plumage characters resulting from extrapair paternity in the collared flycatcher Ficedula albicollis, and compare the importance of this form of sexual selection with that resulting from variation in mate fecundity. Microsatellite genotyping revealed that 15% of nestlings, distributed nonrandomly among 33% of broods (N=79), were the result of extrapair copulations. Multivariate selection analyses revealed significant positive directional sexual selection on two uncorrelated secondary sexual characters in males (forehead and wing patch size) when fledgling number was used as the measure of fitness. When number of offspring recruiting to the breeding population was used as the measure of male fitness, selection on these traits appeared to be directional and stabilizing, respectively. Pairwise comparisons of cuckolded and cuckolding males revealed that males that sired young through extrapair copulations had wider forehead patches, and were paired to females that bred earlier, than the males that they cuckolded. Path analysis was used to partition selection on these traits into pathways via mate fecundity and sperm competition, and suggested that the sperm competition pathway accounted for between 64 and 90% of the total sexual selection via the two paths. The selection revealed in these analyses is relatively weak in comparison with many other measures of selection in natural populations. We offer some explanations for the relatively weak selection detected. Copyright 1999 The Association for the Study of Animal Behaviour.     

Extrapair paternity, inclusive fitness, and within-group benefits of helping in western bluebirds

In central coastal California, USA, 3–16% of western bluebird ( Sialia mexicana ) pairs have adult male helpers at the nest. Demographic data on a colour-ringed population over a 13-year period indicate that helpers gain a small indirect fitness benefit through increases in the number of young fledged from nests of close kin. A small proportion of adult helpers (16%) that were able to breed and help simultaneously had higher annual inclusive fitness than males that only bred. These males comprised such a minor proportion of helpers that the mean fitness of helpers was still lower than the mean fitness of independent breeders. We used DNA fingerprinting to determine whether extrapair fertilizations alter within-group benefits enough to tip the balance in favour of helping behaviour. Overall, 19% of 207 offspring were sired by males other than their social father and extrapair fertilizations occurred in 45% of 51 nests. Intraspecific brood parasitism was rare so that mean mother-nestling relatedness approximated the expected value of 0.5. Extrapair paternity reduced putative father-offspring relatedness to 0.38. Mean helper-nestling relatedness was 0.41 for helpers assisting one or both parents and 0.28 for helpers aiding their brothers. Helpers rarely sired offspring in the nests at which they helped. Helping was not conditional on paternity and helpers were not significantly more closely related to offspring in their parents' nests than to offspring in their own nests. Although helpers may derive extracurricular benefits if helping increases their own or their father's opportunities for extrapair fertilizations, within-nest inclusive fitness benefits of helping do not compensate males for failing to breed. Breeding failure and constraints on breeding are the most likely explanations for why most helpers help.     

Female extrapair mate choice in a cooperative breeder: trading sex for help and increasing offspring heterozygosity

Sexual conflict between males and females over mating is common. Females that copulate with extrapair mates outside the pair-bond may gain (i) direct benefits such as resources or increased paternal care, (ii) indirect genetic benefits for their offspring, or (iii) insurance against infertility in their own social mate. Few studies have been able to demonstrate the different contexts in which females receive varying types of benefits from extrapair mates. Here, I examined sexual conflict, female extrapair mate choice, and patterns of extrapair paternity in the cooperatively breeding superb starling Lamprotornis superbus using microsatellite markers. Although extrapair paternity was lower than many other avian cooperative breeders (14% of offspring and 25% of nests), females exhibited two distinct mating patterns: half of the extrapair fertilizations were with males from inside the group, whereas half were with males from outside the group. Females with few potential helpers copulated with extrapair mates from within their group and thereby gained direct benefits in the form of additional helpers at the nest, whereas females paired to mates that were relatively less heterozygous than themselves copulated with extrapair mates from outside the group and thereby gained indirect genetic benefits in the form of increased offspring heterozygosity. Females did not appear to gain fertility insurance from copulating with extrapair mates. This is the first study to show that individuals from the same population mate with extrapair males and gain both direct and indirect benefits, but that they do so in different contexts.     

AN EXPERIMENTAL STUDY OF PATERNITY AND TAIL ORNAMENTATION IN THE BARN SWALLOW (HIRUNDO RUSTICA)

Previous studies of the socially monogamous barn swallow (Hirundo rustica) have shown that males that most frequently engage in extrapair copulations and whose partners are least involved in copulations with extrapair males are those with long tail ornaments. In this study, through the use of three highly polymorphic microsatellite markers, we analyze the relationships between length of tail ornaments of male barn swallows and proportion of nestlings fathered in own broods, number of offspring fathered in broods of other pairs, and total number of offspring fathered, using both a correlational and an experimental approach. Consistent with our predictions, we show that males with either naturally long or experimentally elongated tails have higher paternity (proportion of biological offspring in own broods), and they produce more biological offspring during the whole breeding season than males with naturally short or experimentally shortened tails. Males with naturally long tails also had more offspring in extrapair broods than short-tailed males, but the effect of tail manipulation on the number of offspring fathered in extrapair broods, although being in the predicted direction, was not statistically significant. Cuckolded males that did not fertilize extrapair females had smaller postmanipulation tail length than cuckolders. We conclude that there is a causal, positive relationship between male tail length and paternity. Since female barn swallows have extensive control over copulation partners and heritability of tail length is high, this study shows that female choice is a component of selection for larger male ornaments. Benefits from extrapair fertilizations to females may arise because they acquire “good” genes for sexual attractiveness or high viability for their offspring.     

Male provisioning is negatively correlated with attempted extrapair copulation frequency in the stitchbird (or hihi)

In species with biparental care of offspring and high levels of extrapair parentage, paired males may suffer reduced fitness by investing in offspring that are not their own. In such instances, males are predicted to evolve some form of discrimination between kin and nonkin and vary investment accordingly. One simple form of discrimination is to follow behavioural cues to paternity. We investigated paternal contribution to chick rearing in a socially monogamous population of stitchbirds, Notiomystis cincta, with high levels of extrapair copulation (EPC) and extrapair paternity. The relative contribution of the male to chick provisioning was negatively correlated with attempted EPC frequency. Because the majority of EPC attempts are forced and conspicuous in stitchbirds they are an obvious cue for males to use. Copyright 2000 The Association for the Study of Animal Behaviour.     

Part-Time Mate Guarding Affects Paternity in Male Reed Buntings (Emberiza schoeniclus)

Male mate guarding by close following has been reported in many socially monogamous bird species and is generally believed to function as a paternity guard. Many aspects of the dynamics and effectiveness of this behavior are still however poorly understood. Here, we describe the temporal variation in mate guarding behavior in male reed buntings (Emberiza schoeniclus) with a particular focus on how males allocate their mating effort between mate guarding and extrapair mating in a context of intense sperm competition. In our highly synchronous study population most males have to balance the simultaneous and mutually exclusive demands of mate guarding and seeking extrapair copulations (EPCs). We found that males frequently switched between guarding their mates and performing intrusions to neighboring territories. Both activities seemed to have significant fitness payoffs, as male mate guarding effort had a positive effect on paternity, and a large fraction of extrapair fertilizations occurred during the days when the sire guarded its own female. The reed bunting is thus an example of how discontinuous or part‐time mate guarding can still be effective in securing paternity. Female reed buntings were not particularly active in initiating EPCs as they never were observed performing extraterritorial forays. We argue that the absence of female‐initiated EPCs is a prerequisite for males to trade mate guarding against seeking EPCs. Otherwise, if females circumvent male mate guarding by timing their EPCs to periods of male absence, males should guard their mates almost continuously or rely on alternative paternity guards.     

Extrapair paternity in the Hoopoe Upupa epops: an exploration of the influence of interactions between breeding pairs, non-pair males and strophe length

Previous studies of the Hoopoe Upupa epops have shown that the strophe length of male songs influences female mate choice, and is correlated with female reproductive rates and male production of fledglings in the male’s own brood. However, frequent interactions between breeding pairs and non‐pair males suggests that extrapair copulations could occur and could affect the real number of fledglings sired by males, and therefore the relationship between strophe length and breeding success. Here we analyse the incidence of interactions between breeding pairs and non‐pair males, and of extrapair paternity, the interrelation of these parameters, the influence of male strophe length on them, and whether extrapair fertilizations affect the correlation between strophe length and breeding success of males, in a colour‐ringed population of Hoopoes in south‐eastern Spain. Multilocus DNA‐fingerprinting revealed that 10% of the broods contained offspring sired by extrapair males, representing 7.7% of the chicks. However, the interactions between pairs and non‐pair males were more frequent, with more than 25% of broods being visited by non‐pair males, and about 10% being helped (fed or defended) by males other than the nest owner. Most of these relationships were apparently attempts by visitor males to obtain copulations with paired females, or to obtain access to such females or nests in future breeding attempts. However, there was no significant link between the detection of interactions with alien males in a nest and the occurrence of extrapair paternity in it, indeed extrapair paternity was found in only 30% of the nests with interactions, and therefore the detection of visits or helping by non‐pair males cannot be considered evidence of extrapair paternity in visited or helped broods. Males that sang with long strophes never suffered losses of paternity within their broods, while 25% of males that sang with short strophes did. However, these differences were not significant. Nevertheless, strophe length of males was significantly positively correlated with per brood and seasonal production of fledglings after accounting for losses of paternity within their own broods.     

Extrapair paternity in the blue tit (Parus caeruleus) : female choice, male charateristics, and offspring quality

Extrapair paternity is common in many birds, and it is now generallyaccepted that female choice plays an important role. However,die benefits that females obtain from extrapair paternity aremuch less dear. To test the hypothesis that females obtain indirectfitness benefits, we studied paternity in a blue tit populationover 4 years. Extrapair paternity occurred in 31-47% of allnests and accounted for 11-14% of all offspring. Most malesthat fathered extrapair young did not lose paternity themselves,males never "exchanged" paternity, and within nests the extrapairoffspring were usually fathered by a single male. Comparisonsbetween males that did and did not lose paternity and pairwisecomparisons between the extrapair male(s) and the within-pairmale showed that successful males had longer tarsi and sangon average longer strophes during the dawn chorus. Successfulmales weighed less (relative to their size) during the nettlingstage, but nevertheless they survived better. Male age did notinfluence their likelihood of losing paternity, but extrapairmales were usually older than the within-pair male they cuckolded.Within nests with mixed paternity, extrapair young were morelikely to survive than within-pair young in cases of partialbrood mortality. Our data also suggest that extrapair offspringwere more likely to be males. Because extrapair males were usuallyclose neighbors, male quality should be considered relativeto the quality of the neighbors. Despite this, we found consistencyin female choice over years. Our observations provide supportfor the hypothesis that female blue tits engage in extrapaircopulations to obtain good genes for their offspring.     

The evolution of paternity and paternal care in birds

Paternity has been hypothesized to be related to the evolutionof paternal care because (1) there should be selection formales not to invest in broods with an uncertain parentage,or (2) male extrapair activity is traded against paternal care.We used interspecific comparisons to discriminate between these alternatives. Male participation in three kinds of parentalcare (nest building, incubation, provisioning of offspring)increased with high paternity in their own nests. Male parentalactivities at some stages of the breeding cycle were significantlycorrelated. A multivariate analysis taking this intercorrelationbetween different components of care and potentially confoundingvariables such as precociality, polyandry, and sexual dichromatism into account revealed that paternity was significantly positivelyrelated to offspring provisioning, while male participationin the other components of parental care did not explain asignificant amount of interspecific variation in paternity.Analyses of evolutionary transitions between different dichotomizedstates of paternity and paternal care provided no clear conclusionsconcerning evolutionary scenarios. However, theoretical arguments and the results of the contrast analyses suggest that male provisioningof offspring evolved in response to paternity.     

Male parental care, female reproductive success, and extrapair paternity

Birds differ considerably in the degree of male parental care,and it has been suggested that interspecific variation in extrapairpaternity is determined by the relative importance of benefitsto females from male parental care and good genes from extrapairsires. I estimated the relationship between extrapair paternityand the importance of male parental care for female reproductivesuccess mainly based on male removal studies, using a comparativeapproach. The reduction in female reproductive success causedby the absence of a male mate was positively correlated withthe male contribution to feeding offspring. The frequency ofextrapair paternity was negatively related to the reductionin female reproductive success caused by the absence of a mate.This was also the case when potentially confounding variablessuch as developmental mode of offspring and sexual dichromatismwere considered. A high frequency of extrapair paternity occursparticularly in bird species in which males play a minor rolein offspring provisioning and in which attractive males providerelatively little parental care. Bird species with frequentextrapair paternity thus appear to be those in which direct fitness benefits from male care are small, females can readilycompensate for the absence of male care, and indirect fitnessbenefits from extrapair sires are important.     

Male reed buntings do not adjust parental effort in relation to extrapair paternity

Parental effort is considered to be costly; therefore, malesare expected to provide less care to unrelated offspring. Theoreticalmodels suggest that males should either reduce their care tothe entire brood or alternatively distinguish between relatedand unrelated nestlings and direct provisioning to kin whenpaternity is in doubt. Reed buntings (Emberiza schoeniclus)have been found to have high levels of extrapair paternity (EPP,i.e., offspring of a male other than the male attending thenest; 55% of offspring), and males are therefore under strongselection pressure to adjust their parental effort accordingto the proportion of EPP in their brood. In this study, we investigatedwhether male reed buntings exhibit a reduction in paternal care(incubation and provisioning nestlings) in relation to decreasedpaternity. We also assess whether males bias their provisioningtoward kin. We measured incubation time, provisioning rates,and food allocation to individual nestlings using video recordingsat the nests. Microsatellite DNA analysis was used to analyzethe paternity of offspring. In direct contrast to a previousstudy on the same species, our results provided no indicationthat males lowered their effort with decreased paternity. Furthermore,in nests of mixed paternity, males did not bias their provisioningbehavior to kin. It remains to be investigated whether the absenceof a relationship between paternity and paternal care can beascribed to absence of reliable paternity cues or whether thebenefits of reducing paternal care did not outweigh the costsin our study population. We found no evidence that the levelof paternal care affected male survival or offspring mass, suggestingthat both the benefits and costs of any reduction in paternalcare would have been low.     

Mate guarding, male attractiveness, and paternity under social monogamy

Socially monogamous species vary widely in the frequency ofextrapair offspring, but this is usually discussed assumingthat females are free to express mate choice. Using game-theorymodeling, we investigate the evolution of male mate guarding,and the relationship between paternity and mate-guarding intensity.We show that the relationship between evolutionarily stablemate-guarding behavior and the risk of cuckoldry can be complicatedand nonlinear. Because male fitness accumulates both throughpaternity at his own nest and through his paternity elsewhere,males evolve to guard little either if females are very faithfulor if they are very unfaithful. Attractive males are usuallyexpected to guard less than unattractive males, but within-pairpaternity may correlate either positively or negatively withthe number of extrapair offspring fertilized by a male. Negativecorrelations, whereby attractive males are cuckolded more, becomemore likely if the reason behind female extrapair behavior appliesto most females (e.g., fertility insurance) rather than thesubset mated to unattractive males (e.g., when females seek"good genes") and if mate guarding is efficient in controllingfemale behavior. We discuss the current state of empirical knowledgewith respect to these findings.     

Willow tit Parus montanus extrapair offspring are more heterozygous than their maternal half‐siblings

Through extrapair matings, males can sire additional offspring with low cost and females may look for direct benefits in form of food or additional paternal care or gain genetic benefits that increase offspring fitness. We studied the patterns of female mate choice and frequency of extrapair paternity in the socially monogamous willow tit Parus montanus using microsatellites. We also examined the effect of heterozygosity on the growth rate and survival of the chicks. We found 25 mixed‐paternity broods out of 117 broods of which both parents were sampled. Altogether, 6.7% of sampled chicks were classified as extrapair young. The pairwise relatedness of social pairs did not correlate with the percentage of extrapair young in the brood and there was no difference in heterozygosity between promiscuous and monogamous parents. However, the extrapair young were more heterozygous than the within‐pair young in the mixed‐paternity broods. The maternal half‐siblings in mixed paternity broods were similar in body size. Thus, there was no indication for different growth rate between the siblings, but there were indications that heterozygosity affects survival.     

Early arrival is not associated with more extra‐pair fertilizations in a long‐distance migratory bird

When assessing the benefits of early arrival date of migratory birds, a hidden and often ignored component of males’ fitness is the higher chance of early‐arriving birds to obtain extra‐pair fertilizations. Here we investigated how extra‐pair paternity might affect the relationship between male arrival date and number of fertilizations in a model study system, the European pied flycatcher Ficedula hypoleuca. For this purpose, we sampled and genotyped breeding pairs, unpaired males and offspring (including embryos from unhatched eggs when possible) of a Dutch pied flycatcher population. Detailed information on arrival date of males, egg laying date of their social mates and nest success was also recorded. Early‐arriving males had early‐laying females and males with early‐laying females had a higher probability of siring extra‐pair eggs and obtain more fertilizations. However, male arrival date alone did not correlate with the probability to gain extra‐pair paternity and neither to the amount of fertilized eggs. Both early‐ and late‐arriving males had a higher probability of losing paternity in their own nest compared to birds with an intermediate arrival date. Finally, late‐arriving males were more likely to remain unpaired but, interestingly, a few of these birds obtained paternity via extra‐pair copulations. Because earlier arrival date did not lead to more extra‐pair fertilizations and because such relationship seems to be driven mainly by the female's laying date, we conclude that the contribution of extra‐pair paternity to the overall fitness benefits of early male arrival date is relatively small.     

Breeding synchrony and extrapair fertilizations in two populations of red-winged blackbirds

We tested the relationship between synchrony of breeding andthe frequency of extrapair fertilizations (EPFs) in two populationsof red-winged blackbirds known to differ in female extrapairbehavior. We found no association between the number of simultaneouslyfertilizable females (temporal neighbors) and EPF rate in eitherpopulation, although a significant difference between populationsin the direction of this relationship (positive where femalesinitiated extrapair copulations and negative where males initiatedthem) suggested a modest difference in the influence of synchrony.Males losing offspring to EPFs tended to have more fertilizablefemales at that time than the actual sires in some analysesbut not in others. We also tested several assumptions underlyingtwo competing hypotheses for the effects of synchrony. We foundno evidence that females pursued extrapair copulations moreoften when other females were synchronous. Rather, females weremore likely to gain EFFs with exirapatr males whose social mateswere not yet building their nests. Synchrony also did not consistentlyaffect male pursuit of exirapair copulations or achievementof EPFs. These results suggest that timing of breeding has someeffects on extrapair activity, but that those effects are bothrelatively weak and influenced by other factors that vary betweenyears or populations.     

Polygyny and extrapair fertilizations in eastern red-winged blackbirds (Agelaius phoeniceus)

Parentage of nestling red-winged blackbirds (Agelaius phoeniceus)from an eastern population was determinedusing DNA fingerprintingtechniques. Of 235 nestlings surveyed, 58 had fingerprints excludingthemale, but none excluded the female tending the nest. Data onpairing status during the female's fertilizable period was availablefor 232 offspring; 55 (25%of 1988 nestlings, 23% of 1989 nestlings)of those were sired through extrapair copulations. Of these55 offspring, 33 could be assigned to nearby territory holders;16 of the remaining nestlings may have been sired by nearbymales that were not captured. During both years, 44% of territorialmales had more than one female nesting simultaneously on theirterritory. The number of extrapair fertilizations gained bymales increased significandy with harem size in 1 year. Paternity(die proportion of nesdings on the territory sired by die territoryholder) showed a positive but nonsignificant increase widi haremsize in bodi years. There was no apparent cost in paternityfor males guarding two or more fertilizable females at the sametime. The broods of females that were fertilizable at die sametime anodier female was setding on die same territory tendedto have a greater proportion of extrapair fertilizations (0.37)than did die broods of odier females within harems (0.15). Establishedfertilizable females were chased significantly more by die territoryowner and by extrapair males when a new female was setding.There were no associations between a male's paternity or successat gaining extrapair fertilizations and his age or color-bandcombination. Overall, extrapair fertilizations had litde effecton die relationship between fledgling success and harem sizeand appeared to have a minimal impact on die overall intensityof sexual selection on males.     

Cuckoldry and the stability of biparental care

In socially monogamous species, females may engage in extra-pair fertilizations to gain direct or indirect benefits not provided by the social mate, with the potential risk of a reduction in the social mate’s paternal effort. I present an ESS model of cuckoldry frequencies, which considers both facultative and nonfacultative male responses to losses in paternity. Two possible equilibria exist: stable social monogamy with varying degrees of extra-pair paternity, and polygamy with little or no male care. Monogamy with limited cuckoldry can be stable only if the initial cuckoldry frequency is low, intrinsic cuckoldry benefits are not high, males can reasonably accurately detect cuckoldry, and female compensation for losses in male care is incomplete. Deviations from these assumptions lead to stronger mate acquisition in males at the expense of paternal care, and eventually to runaway evolution towards polygamy. Average female fitness is reduced in the runaway, although it is initiated by females maximizing the survival of offspring – a potential “tragedy of the commons” in breeding systems.