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Extra-pair copulation
Extra-pair copulation
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Extra-pair copulation (EPC) is a mating behaviour in monogamous species. Monogamy is the practice of having only one sexual partner at any one time, forming a long-term bond and combining efforts to raise offspring together; mating outside this pairing is extra-pair copulation.[1] Across the animal kingdom, extra-pair copulation is common in monogamous species, and only a very few pair-bonded species are thought to be exclusively sexually monogamous. EPC in the animal kingdom has mostly been studied in birds and mammals.[2][3][4] Possible benefits of EPC can be investigated within non-human species, such as birds.[5]

For males, a number of theories are proposed to explain extra-pair copulations. One such hypothesis is that males maximise their reproductive success by copulating with as many females as possible outside of a pair bond relationship because their parental investment is lower, meaning they can copulate and leave the female with minimum risk to themselves. Females, on the other hand, have to invest a lot more in their offspring; extra-pair copulations produce a greater cost because they put the resources that their mate can offer at risk by copulating outside the relationship.[1] Despite this, females do seek out extra pair copulations,[6] and, because of the risk, there is more debate about the evolutionary benefits for females.

In human males

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Extra-pair copulation in men has been explained as being partly due to parental investment.[7] Research has suggested that copulation poses more of a risk to future investment for women, as they have the potential of becoming pregnant, and consequently require a large parental investment of the gestation period, and then further rearing of the offspring.[7] Contrastingly, men are able to copulate and then abandon their mate as there is no risk of pregnancy for themselves, meaning there is a smaller risk of parental investment in any possible offspring.[8] It has been suggested that,[8] due to having such low parental investment, it is evolutionarily adaptive for men to copulate with as many women as possible. This will allow males to spread their genes with little risk of future investment but it does come with the increased risk of sexually transmitted infections (STIs).[9]

Various factors can increase the probability of EPC in males. Firstly, males with low levels of fluctuating asymmetry are more likely to have EPCs.[10] This may be due to the fact that signals of low fluctuating asymmetry suggest that the males have "good genes", making females more likely to copulate with them as it will enhance the genes of their offspring, even if they do not expect long-term commitment from the male.[10] Psychosocial stress early on in life, including behaviours such as physical violence and substance abuse, can predict EPC in later life.[11] This has been explained as being due to Life History Theory, which argues that individuals who are reared in environments where resources are scarce and life expectancy is low, are more likely to engage in reproductive behaviours earlier in life in order to ensure the proliferation of their genes.[12] Individuals reared in these environments are said to have short life histories. With respect to Life History Theory, these findings have been explained by suggesting that males who experienced psychosocial stress early in life have short life histories, making them more likely to try and reproduce as much as possible by engaging in EPC to avoid gene extinction.[11]

Despite the possibility of increased reproductive success resulting from EPC, there are risks associated with the behavior. For example, costs may include discovery of sexual behavior outside the pair bond, with the risk of breakdown of the relationship with a long-term partner; in some cases, a discovered EPC leads to violence from partners.[11][a] Men may also refrain from EPC to minimize risk of STI transmission which can be common in EPCs. The extra-pair partners in the EPC may have a higher number of sexual partners and therefore be at greater risk of STIs; this would counter the lower incidence of STI transmission among sexually exclusive, monogamous couples. Mitigating against improved reproductive success from EPC is the loss of enhanced child-rearing environments for any resulting offspring. In an evolutionary context, long-term pair-bonded couples would have greater success rearing children, increasing survival rates; offspring from EPC may have a reduced chance of survival in the absence of cooperative child-rearing from both parents, reducing the probability of the male's genes surviving to the next generation.[13]

In human females

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From an evolutionary perspective, females have to invest a lot more in their offspring than males due to prolonged pregnancy and child rearing; a child has a better chance of survival and development with two parents involved in child-rearing.[14] Therefore, extra-pair copulations have a greater cost for women because they put the support and resources that their mate can offer at risk by copulating outside the relationship.[1] There is also the increased risk of sexually transmitted infections,[1] which is suggested as a possible evolutionary reason for the transition from polygamous to monogamous relationships in humans.[15] Despite this, females do seek out extra-pair copulation, with some research finding that women's levels of infidelity are equal to that of men's, although this evidence is mixed.[16] Due to the increased risk, there is more confusion about the evolutionary benefits of extra-pair copulation for females.

The most common theory is that women mate outside of the monogamous relationship to acquire better genetic material for their offspring. A female in a relationship with a male with 'poorer genetic quality' may try to enhance the fitness of her children and therefore the continuation of her own genes by engaging in extra-pair copulation with better quality males.[14] A second theory is that a woman will engage in extra-pair copulation to seek additional resources for herself or her offspring.[1] This is based on observations from the animal world in which females may copulate outside of their pair-bond relationship with neighbours to gain extra protection, food or nesting materials. Finally, evolutionary psychologists have theorized that extra-pair copulation is an indirect result of selection on males. The alleles in males that promote extra-pair copulation as an evolutionary strategy to increase reproductive success is shared between sexes leading to this behaviour being expressed in females.[6]

There are also social factors affecting the likelihood of extra-pair copulation. Both males and females have been found to engage in more sexual behaviour outside of the monogamous relationship when experiencing sexual dissatisfaction in the relationship,[16] although how this links to evolutionary theory is unclear. Surveys have found cultural differences in attitudes towards infidelity, though it is relatively consistent that female attitudes are less favorable toward infidelity than male attitudes. For women, the social costs of EPC may be greater, including higher levels of social disapprobation. This has a range of consequences, depending on cultural and personal circumstances. Known or suspected EPC by women may result in lowered life chances for any offspring, or the woman being at higher risk of violence and homicide, for example.[a][17][18]

Other animals

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As well as humans, EPC has been found in many other socially monogamous species.[2][3][4][19] When EPC occurs in animals which show sustained female-male social bonding, this can lead to extra-pair paternity (EPP), in which the female reproduces with an extra-pair male, and hence produces EPO (extra-pair offspring).[20]

Due to the obvious reproductive success benefits for males,[20] it used to be thought that males exclusively controlled EPCs.[5] However, it is now known that females also seek EPC in some situations.[5]

In birds

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Pair of zebra finches: Bird Kingdom, Niagara Falls, Ontario, Canada

Extra-pair copulation is common in birds.[21] For example, zebra finches, although socially monogamous, are not sexually monogamous and hence do engage in extra-pair courtship and attempts at copulation.[22] In a laboratory study, female zebra finches copulated over several days, many times with one male and only once with another male. Results found that significantly more eggs were fertilised by the extra-pair male than expected proportionally from just one copulation versus many copulations with the other male.[23] EPC proportion varies between different species of birds.[1] For example, in eastern bluebirds, studies have shown that around 35% of offspring is due to EPC.[3] Some of the highest levels of EPP are found in the New Zealand hihi or stitchbird (Notiomystis cincta), in which up to 79% of offspring are sired by EPC.[24] EPC can have significant consequences for parental care, as shown in azure-winged magpie (Cyanopica cyanus).[25]

In socially polygynous birds, EPC is only half as common as in socially monogamous birds. Some ethologists consider this finding to be support for the 'female choice' hypothesis of mating systems in birds.[26]

In mammals

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Pair of white-handed gibbons

EPC has been shown in monogamous mammals, such as the white-handed gibbon.[27] A study of one group found 88% in-pair copulation and 12% extra-pair copulation.[28] However, there is much variability in rates of EPC in mammals.[29] One study found that this disparity in EPC is better predicted by the differing social structures of different mammals, rather than differing types of pair bonding. For example, EPC was lower in species who live in pairs compared to those who live in solitary or family structures.[29]

Reasons for evolution

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Some argue that EPC is one way in which sexual selection is operating for genetic benefits which is why the extra-pair males involved in EPC seem to be a non-random subset.[2] There is some evidence for this in birds.[30] For example, in swallows, males with longer tails are involved in EPC more than those with shorter tails.[31] Also female swallows with a shorter-tailed within-pair mates are more likely to conduct EPC than those whose mates have longer tails.[31] A similar pattern has been found for black-capped chickadees, in which all extra-pair males had higher rank than the within-pair males.[32] But some argue that genetic benefits for offspring is not the reason females participate in EPC.[5] A meta-analysis of genetic benefits of EPC in 55 bird species found that extra-pair offspring were not more likely to survive than within-pair offspring. Also, extra-pair males did not show significantly better 'good-genes' traits than within-pair males, except for being slightly larger overall.[5]

Another potential explanation for the occurrence of EPC in organisms where females solicit EPC is that the alleles controlling such behaviour are intersexually pleiotropic. Under the hypothesis of intersexual antagonistic pleiotropy, the benefit males get from EPC cancels out the negative effects of EPC for females. Thus, the allele that controls EPC in both organisms would persist, even if it would be detrimental to the fitness of females. Similarly, according to the hypothesis of intrasexual antagonistic pleiotropy, the allele that controls EPC in females also controls a behaviour that is under positive selection, such as receptiveness towards within-pair copulation.[33]

Notes

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References

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Extra-pair copulation

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Extra-pair copulation (EPC) refers to sexual between individuals outside of an established social , distinct from copulations occurring prior to pair formation. This behavior is prevalent in socially monogamous , where individuals form long-term partnerships for breeding and but nonetheless pursue additional matings. EPC often results in extra-pair paternity (EPP) or maternity, where are sired by or gestated from non-social partners, with molecular genetic studies revealing EPP rates of approximately 10-20% of young in many bird and varying levels in mammals. From an evolutionary perspective, males typically gain direct fitness benefits through EPC by increasing their fertilizations without additional , while females may accrue indirect genetic benefits by selecting superior sires for "good genes" or direct benefits such as resources in some contexts, though for consistent female advantages remains debated amid sexual conflicts over control. These dynamics challenge assumptions of strict , highlighting mixed reproductive strategies that enhance overall and male reproductive skew. In humans, historical EPP rates in Western European populations have been estimated at 1-2% per generation using genetic methods, though higher rates—up to 17% in certain traditional societies—correlate with factors like or concurrency, underscoring contextual variability rather than universal prevalence. Empirical quantification via paternity testing has resolved prior overestimations from non-genetic surveys, emphasizing causal roles of opportunity, mate guarding, and genetic compatibility in EPC occurrence.

Definition and Terminology

Core Definition

Extra-pair copulation (EPC), also termed extra-pair mating, denotes the act of between an individual and a partner outside its established , predominantly observed in socially species. Social monogamy involves long-term primarily for mutual benefits such as biparental care, maintenance, and provisioning, yet EPC allows deviation from genetic monogamy. This behavior results in extra-pair fertilizations (EPF), where gametes from an extra-pair partner contribute to , often manifesting as extra-pair paternity (EPP) or maternity in genetic parentage analyses. EPC encompasses both voluntary solicitations and coercive encounters, with the former typically involving female choice of alternative males and the latter often male-initiated to bypass mate guarding. In molecular studies using markers or single-nucleotide polymorphisms, EPP rates vary widely: for instance, averaging 11-15% across socially monogamous birds, though exceeding 50% in some families like the . These discrepancies highlight EPC's prevalence despite potential costs like retaliation from pair mates or reduced paternal investment. The evolutionary persistence of EPC underscores its role in , enabling males to amplify via multiple fertilizations without commensurate parental effort, as evidenced by higher lifetime fitness gains for males achieving EPP in species like the . For females, EPC may confer indirect genetic benefits, such as superior viability from high-quality extra-pair sires, though empirical support remains mixed and context-dependent.

Distinction from Other Mating Behaviors

Extra-pair copulation (EPC) is characterized by sexual matings between individuals who maintain a primary social pair bond, distinguishing it from mating systems lacking such bonds or involving multiple socially recognized partners. In contrast to promiscuity, where animals engage in indiscriminate matings with numerous partners without forming exclusive pairs, EPC occurs covertly within socially paired contexts, often in species classified as monogamous based on observable pair bonds. This behavior leads to discrepancies between social and genetic mating systems, as evidenced by extra-pair paternity rates exceeding 70% in species like the superb fairy-wren, where s solicit copulations from non-social s while relying on the for biparental care. Unlike , in which a forms multiple social bonds with s and often invests in resources for several, EPC involves no additional social commitments, preserving the primary 's structure despite genetic . , conversely, features a with multiple social partners, whereas EPC typically manifests in socially monogamous setups without altering social pairings. EPC further contrasts with within-pair copulations, which reinforce mutual investment and defense in the social pair, whereas extra-pair events prioritize genetic gains—such as superior alleles or diversity—without reciprocal parental duties from the extra-pair individual. It differs from serial or divorce, where pair bonds dissolve before new ones form, as EPC maintains the existing bond intact amid opportunistic matings. These distinctions highlight EPC as a strategy embedded in, rather than replacing, social monogamy, often undetected through behavioral observation alone.

Evolutionary Foundations

Adaptive Benefits for Males

Extra-pair copulations enable males to increase their by siring offspring with multiple females beyond their social , without incurring additional costs. This strategy aligns with Bateman's principle, where male fitness gains from mating multiply with the number of partners, as production is relatively inexpensive for males compared to females. In socially monogamous birds, such as passerines, males that combine within-pair and extra-pair paternity achieve higher total reproductive output than those restricted to social alone. For instance, empirical studies demonstrate that extra-pair paternity significantly elevates male lifetime by diversifying genetic contributions across broods. Extra-pair paternity also amplifies variance in male , allowing superior males—often those with advantageous traits like age or condition—to propagate genes more extensively. In a meta-analysis of avian studies, extra-pair fertilizations nearly doubled the relative variance in male , underscoring the selective advantage for males engaging in such behavior. This increased variance promotes intensity, favoring traits that enhance extra-pair mating opportunities.

Adaptive Benefits for Females

Females in socially monogamous species may derive indirect genetic benefits from extra-pair copulations (EPC) by mating with genetically superior extra-pair males, thereby enhancing quality while relying on the social mate for . Under the good genes hypothesis, extra-pair sires provide alleles that improve viability, survival, or attractiveness, as females preferentially solicit copulations from high-quality males exhibiting traits like larger song repertoires or better condition. This strategy allows females to separate genetic and direct benefits of mating, with empirical evidence from birds showing extra-pair (EPO) often outperforming within-pair (WPO) in fitness metrics such as fledging success or recruitment rates. The compatible genes hypothesis posits that EPC increases offspring heterozygosity or avoids deleterious recessive combinations, particularly beneficial in populations with moderate inbreeding risk. In reed buntings, for instance, females mating with unrelated extra-pair males produced offspring with higher tarsus length and mass, correlating with elevated lifetime compared to WPO. Studies in species like collared flycatchers and blue tits further indicate that EPO exhibit superior immune responses or telomere lengths under stressful conditions, supporting context-dependent genetic advantages when social mates are suboptimal. Direct benefits, such as material resources from extra-pair males, are rarer but documented in breeders where EPC secures additional helpers for brood defense or provisioning. In facultatively birds, female-initiated EPC has been linked to increased male for antipredator vigilance, indirectly boosting nest rates by up to 20% in high-predation environments. However, these benefits are modulated by environmental factors; in stable habitats, genetic gains predominate, while in variable or harsh conditions, fertility insurance against social mate becomes prominent, with EPO rates rising when pair males show low sperm quality. Overall, female EPC evolves when net fitness returns from enhanced quality exceed risks like retaliation or reduced paternal .

Genetic and Population-Level Effects

Extra-pair copulation (EPC) enhances by promoting mating beyond social pairs, thereby increasing heterozygosity and reducing the fixation of deleterious alleles within populations. In blue tit (Cyanistes caeruleus) populations, higher rates of extra-pair paternity (EPP), ranging from 0% to over 50% of offspring in some cases, correlate positively with measures of , including polymorphic loci ratios (0.30–0.82) and gene diversity (0.015–0.228). This pattern suggests EPC acts as a mechanism to maintain or amplify , particularly in species facing potential risks from social . Females often derive genetic benefits from EPC through selection for compatible or high-quality genes in extra-pair sires, as evidenced by context-dependent advantages in offspring viability and performance. Meta-analyses across bird species indicate that EPP rates rise when social mates share high genetic similarity (measured via microsatellites), enabling females to avoid inbreeding depression and produce more heterozygous offspring with potentially superior fitness. Such genetic gains are not universal, however, and depend on environmental factors like resource availability or sire quality, with some studies showing no direct link to provisioning or survival in cooperative breeders. At the population level, EPC influences (N_e) by skewing variance in male , though impacts are generally modest and variable. In a socially monogamous , EPP increased variance in by up to 150% in one breeding , reducing N_e by approximately 4%, while in another it slightly elevated N_e by 8% through broader sire representation. Higher further amplifies EPP rates—e.g., 56.4% of broods in dense reed bunting (Emberiza schoeniclus) subpopulations contained extra-pair young—potentially boosting overall but also intensifying and mating opportunities. EPC can promote population-level cooperation by incentivizing extra-pair sires to invest in communal defense, thereby enhancing offspring survival across social units. In pied flycatchers (Ficedula hypoleuca), males siring extra-pair offspring in neighboring nests approached predators closer (mean distances of 6–14 m vs. 98 m for non-sires) and mobbed more intensely, suggesting EPC extends indirect fitness benefits and stabilizes group-level antipredator behaviors. These dynamics underscore EPC's role in balancing individual reproductive strategies with emergent population resilience, without evidence of destabilizing effects on overall genetic health in most studied systems.

Occurrence in Non-Human Animals

Prevalence in Birds

Social monogamy predominates among birds, occurring in approximately 81-90% of and typically involving long-term pair bonds with biparental care. Despite this, extra-pair copulations (EPCs) are widespread, resulting in extra-pair paternity (EPP) that undermines genetic in most examined populations. Genetic analyses across over 500 studies and more than 300 have revealed EPP in 77% of 271 for which data are available, with species-level EPP rates ranging from 0% to 65.2%. Comprehensive reviews indicate that true genetic —defined as 0% EPP—is rare, occurring in fewer than 25% of socially monogamous studied. Population-level EPP rates vary substantially but average around 10% of and 20% of broods containing at least one extra-pair young, based on meta-analyses of molecular paternity assignments. In passerines, the most intensively studied avian order, rates are often higher, with extra-pair comprising 15-20% on average in many populations. Factors such as breeding synchrony, , and the degree of male correlate with elevated EPP frequencies, as higher synchrony and density facilitate EPC opportunities while reduced may reflect lower paternity assurance. EPP is less prevalent in certain taxa, such as raptors and , where rates often fall below 5%, compared to oscines where they exceed 20% in some cases. These patterns underscore that while EPCs are a near-universal feature of avian mating systems, their reflects ecological and behavioral trade-offs rather than uniform .

Prevalence in Mammals

Social is rare among mammals, occurring in only 3–5% of species, with examples including certain , like prairie voles, and carnivores such as some foxes and wolves. Despite these pair bonds, genetic —exclusive reproduction within the social pair—is even less common, as extra-pair copulations (EPC) frequently result in extra-pair paternity (EPP) or extra-group paternity (EGP). Empirical studies using genetic markers reveal high prevalence of EPC in socially monogamous mammals. A comparative analysis of 29 species found extensive EGP, with 46% exhibiting rates exceeding 20% of offspring sired outside the , underscoring EPC's role in mammalian systems. Multiple paternity within litters, a direct indicator of female EPC, averages 46.1% across mammalian populations—more than double the 19.5% rate in birds—based on a Bayesian of genetic data from diverse species. Rates vary widely, from 0% to 92% of offspring per species, influenced by factors like rather than pair-bond intensity alone. Social organization strongly predicts EPP levels: pair-living show higher genetic fidelity than those in multi-male groups, where opportunities for EPC increase due to proximity and reduced mate guarding. For instance, in group-dwelling socially monogamous mammals, EPP can approach or exceed 50% in some populations, reflecting females' access to alternative sires. These patterns highlight that while social monogamy facilitates biparental care, it rarely precludes EPC, which contributes to variance in male .

Patterns in Other Taxa

In , multiple paternity arises frequently through and storage, enabling females to mate with several males over extended periods. A review of 40 found multiple paternity in 62% of clutches, often facilitated by long-term viability in oviducts, which allows post-copulatory via selective fertilization. In such as the water skink (Eulamprus tympanum), females engaging in multiple matings produce larger clutches and more compared to single-mated females, suggesting direct benefits from extra-pair copulations despite no evidence of genetic benefits like increased viability. Extra-pair paternity also occurs in territorial like the green anole (), where females in female-defense systems solicit copulations from non-territorial males, potentially gaining superior genes or hedging against social mate infertility. Amphibians exhibit as a dominant pattern, with females often multiply during explosive breeding events, but true extra-pair copulation in socially paired contexts is rare due to limited . A of 28 and showed that multiple increases female reproductive output by 15-20% on average, primarily through higher egg production rather than offspring quality, though benefits vary by and are not universally tied to . In Neotropical frogs like Dendropsophus ebraccatus, which exhibit resource-defense , genetic analysis revealed unexpected high paternity fidelity within breeding bouts, with overlapping clutches sired predominantly by a single male despite opportunities for multiple ; this suggests mate guarding constrains extra-pair events, though persists across bouts. Extra-pair in amphibians may evolve indirectly via on male traits, but empirical support for adaptive benefits remains equivocal compared to reptiles. Among fishes, extra-pair paternity varies widely in biparental species, often eroding social . In the black-chinned (Sarotherodon melanotheron), brood-tending pair-bonded males sire only 63% of , with females engaging in extra-pair copulations to secure diverse sires, potentially reducing or enhancing performance in variable environments. Conversely, genetic prevails in some colonial nest-builders like the daisy triplefin (Forsterygion varium), where no extra-pair young were detected across 350+ genotyped individuals, attributed to synchronous spawning and male nest guarding that limits female solicitation of outsiders. In other species, such as reed buntings' analogs in , extra-pair events occur in 27% of broods without clear inbreeding avoidance drivers, indicating opportunistic mating over adaptive strategies. In , extra-pair reproduction manifests in socially monogamous or subsocial taxa, often yielding high paternity loss. The Nicrophorus vespilloides shows 54.8% extra-pair and 70% multi-sired broods, where females pursue additional matings post-pairing to boost via sperm replenishment, despite risks of over biparental care. Similarly, in the wood-feeding Cryptocercus punctulatus, field families exhibit common extra-pair paternity, though some apparent cases stem from or sampling artifacts rather than copulation alone. Multiple mating in fruit flies enhances female nonuniformly, with post-mating gains in egg viability tied to proteins, underscoring physiological rather than genetic benefits in many systems. Across these taxa, extra-pair patterns generally promote female reproductive assurance but impose costs like reduced paternal investment, with prevalence linked to mobility, , and guarding efficacy.

Forced Extra-Pair Copulations

Forced extra-pair copulations (FEPCs) involve males coercing mating with females bonded to other males, typically through or , bypassing female choice to potentially increase the coercing male's . This behavior manifests as , where males exploit opportunities for fertilization despite female resistance, which may include evasion, vocalization, or physical defense. In species with social or pair bonds, FEPCs target paired females to sire reared by the social mate, though success varies due to female countermeasures and low fertilization rates in some cases. In birds, FEPCs are well-documented, particularly in waterfowl and seabirds, where males employ forced mounting even in cloacal-kissing species lacking a penis. For instance, in Ross's and lesser snow geese, extra-pair copulations comprise 46–56% of all attempts and are invariably forced, yet genetic analyses reveal few resulting extra-pair fertilizations, suggesting effective female resistance or timing mismatches. Common murres exhibit frequent FEPCs alongside intense mate guarding, with males remaining vigilant at colonies to deter rivals, indicating that coercion serves both reproductive and competitive functions. In white-bearded manakins, FEPCs correlate with mate guarding behaviors, as territorial males risk paternity loss while pursuing coerced matings elsewhere. Recent studies on species like the Eurasian tree sparrow confirm FEPCs as aggression tools against intruders rather than genuine sexual interest, with neighboring males targeting paired females to disrupt competitors. Among mammals, FEPCs occur less frequently in socially monogamous contexts but are observed in and cetaceans, often involving group . In female-bonded such as chimpanzees, socially paired females experience heightened sexual from non-pair males, potentially elevating extra-pair paternity risks despite pair bonds. Bottlenose dolphins demonstrate tactics where alliances of males isolate and force copulations with females, including those in consortships, leading to documented extra-pair offspring. In other taxa like , scorpionflies exemplify extreme FEPCs via specialized claspers for restraint, though fertilization success depends on nuptial gifts or female morphology. Evolutionary drivers of FEPCs hinge on male fitness gains from siring without investment, balanced against costs like or retaliation. Females counter with behavioral avoidance, morphological defenses (e.g., elongated genitalia in waterfowl to displace unwanted ), and post-copulatory mechanisms, often rendering FEPCs low-yield. Empirical data underscore variability: while some FEPCs yield paternity, many fail, supporting the view that coercion persists as a high-risk in with asynchronous breeding or weak pair enforcement.

Patterns in Humans

Estimated Rates of Extra-Pair Paternity

Estimates of extra-pair paternity (EPP), defined as the proportion of offspring not genetically related to the social father in pair-bonded couples, have been derived primarily from genetic paternity testing using markers such as Y-chromosome STRs or autosomal SNPs in historical and contemporary pedigrees. In Western European populations, genetic analyses of over 500 married couples spanning 400 years indicate EPP rates ranging from 0.4% to 5.9% per generation, with an overall average below 1% and no consistent upward trend over time. Similarly, a study of 1,273 conceptions across 335 years in , , found non-paternity rates below 1%, often attributable to rare cases like or misattributed paternity rather than . These low figures contrast with self-reported rates, which exceed 20% in surveys, suggesting that extra-pair copulations (EPCs) frequently do not result in conception or detectable EPP due to timing or factors. Methodological advancements, such as double-blind genetic sampling to minimize , have refined these estimates, revealing that earlier anecdotal claims of 10-30% EPP—often cited without genetic verification—overstate in stable monogamous societies. Peer-reviewed genetic studies consistently EPP at 1-2% per generation in such contexts, challenging evolutionary models assuming high routine cuckoldry as a driver of male or paternity assurance behaviors. Variability arises from sampling biases; for instance, clinical data from disputed paternities yield inflated rates (up to 10-17%), while population-wide genetic screens in unselected families yield lower figures. Cross-cultural comparisons highlight greater variability: in the polygynous Himba pastoralists of , a 2020 double-blind genetic study of 99 couples reported an exceptionally high EPP rate of 48%, with 70% of couples having at least one extra-pair child, tolerated within their flexible mating norms where women often have multiple partners. An earlier self-reported analysis in the same group found 17% of marital births attributed to EPP, linked to female choice for heritable traits. However, such rates are outliers; in most studied human populations emphasizing social monogamy, EPP remains under 5%, influenced by factors like relationship stability, contraceptive use, and cultural enforcement of pair bonds rather than universal . These findings underscore that while EPC occurs across societies, realized EPP is constrained by biological and social mechanisms, with genetic evidence privileging low baseline rates in historical monogamous lineages.

Sex Differences in Motivation and Frequency

Men exhibit higher reported rates of extra-pair copulation (EPC) compared to women in large-scale surveys of marital infidelity. Analysis of data indicates that 20% of ever-married men and 13% of ever-married women report having had sex with someone other than their while married. These disparities emerge more prominently with age; rates are comparable among younger adults aged 18–29 (10% for men, 11% for women), but men consistently outpace women thereafter, with gaps reaching 18 percentage points among those aged 80 and older. Sex differences in EPC frequency align with broader patterns in sociosexual orientation, where men score higher on unrestricted attitudes toward , correlating with increased likelihood of extradyadic involvement. Genetic modeling of self-reported extrapair behavior estimates within-sex at 62% for men and 40% for women, suggesting a stronger heritable component in male tendencies. Motivationally, men more often pursue EPC for sexual gratification, variety, or opportunities with casual partners, with studies showing they are 1.75 times more likely than women to involve non-committed partners. In contrast, women report EPC driven primarily by emotional or relational factors, such as dissatisfaction in the primary , sexual , or contemplation of relationship dissolution, and are more prone to emotional or cyber-. These patterns hold across large samples, including one exceeding 94,000 respondents, where men engaged in repeat sexual infidelity more frequently while women's often involved known individuals. Empirical reviews attribute men's sexual motivations to lower costs of additional matings, while women's relational drivers reflect higher selectivity tied to greater reproductive investment.

Detection and Paternity Uncertainty

In humans, paternity uncertainty originates from the biological asymmetry of reproduction, where males lack direct sensory confirmation of conception during internal , creating a of cuckoldry even in socially monogamous pairs. This adaptive challenge has persisted across evolutionary history, evidenced by widespread extramarital affairs in diverse cultures and physiological indicators of , such as human testicular size intermediate between promiscuous chimpanzees and pair-bonding gorillas. Male sexual jealousy, disproportionately triggered by cues of partner sexual rather than emotional bonds, functions as a psychological safeguard, with such suspicions linked to heightened vigilance and conflict, including spousal violence in extreme cases. Detection of extra-pair copulations relies on indirect behavioral, perceptual, and , as males historically monitored partners for inconsistencies like unexplained absences, altered routines, or third-party reports. Mate guarding tactics, such as proximity maintenance and resource display, preempt detection needs by reducing opportunities for , while post-hoc assessments draw on phenotypic resemblance between putative fathers and to gauge genetic relatedness. Perceived father-child similarity positively predicts paternal investment, implying an evolved mechanism that adjusts effort based on inferred paternity probability. Empirical studies reveal moderate accuracy in infidelity detection through nonverbal signals. Listeners can identify individuals with past cheating histories from brief voice recordings, rating them as more prone to based on acoustic cues like pitch variation or tension, independent of explicit content. In the Himba pastoralists of , both sexes accurately pinpoint extra-pair paternity in assigned cases at rates exceeding chance, likely via community knowledge of liaisons rather than physiological markers. However, women outperform men in spotting emotional infidelity cues, while male sensitivity skews toward sexual threats, reflecting sex-specific evolutionary priorities. Prior to , introduced widely in the , uncertainty remained high and unresolvable without invasive methods like historical enforcement or practices in some societies. Genetic assays now confirm biological paternity with over 99.99% accuracy using markers like short tandem repeats, drastically reducing ambiguity but highlighting how ancestral detection limitations amplified perceived risks, fostering robust anti-cuckoldry adaptations despite empirically low extra-pair paternity incidence in most sampled populations.

Controversies and Empirical Debates

Variability in Reported Rates Across Populations

Reported rates of extra-pair paternity (EPP), a genetic measure of extra-pair copulation outcomes, exhibit substantial variation across human populations, spanning from under 1% in many historical and contemporary samples to over 40% in specific groups, challenging uniform assumptions about human mating fidelity. Genetic studies, which provide objective evidence via DNA analysis of parent-offspring mismatches, consistently reveal lower average rates—typically 1-2% per generation—in large-scale European historical datasets spanning centuries, with peaks around 6% during periods of socioeconomic upheaval like the early modern era. These low figures align with self-reported infidelity surveys in Western populations, where admitted extra-pair copulations remain rare among married individuals, though underreporting due to social stigma likely inflates perceived monogamy. In contrast, a 2020 double-blind genetic study of the Himba pastoralists in northern documented an exceptionally high EPP rate of 48%, with 70% of couples experiencing at least one extra-pair child, attributed to cultural norms tolerating flexible paternity assignment and low emphasis on exclusive . This outlier contrasts sharply with rates below 5% in most monogamous societies and correlates with elevated in the population, suggesting that reduced pressure may facilitate higher extra-pair mating without barriers. Similarly, self-reported data from women in a traditional African society indicated 17% of marital births resulting from extra-pair liaisons, linked to enhanced female through partner choice. Such elevated rates in non-Western, kin-based groups highlight how cultural practices—like partible paternity beliefs or polygynous tendencies—can amplify EPP by diminishing male investment in paternity assurance. This variability underscores methodological and interpretive debates: genetic assays in small, non-representative samples (e.g., the Himba study, n=99 couples) may overestimate population-wide norms due to , while historical European data, drawn from broader Y-chromosome and surname analyses, better reflect long-term trends under stricter monogamous institutions. Peer-reviewed genetic prioritizes over survey-based estimates, as the latter often suffer from desirability bias, with Western respondents understating by factors of 2-5 compared to anonymous physiological or genetic validations. Socioeconomic factors, including and legal enforcement of , appear causally linked to suppression of EPP, as evidenced by declining rates post- in . Overall, while EPP averages low globally, inter-population differences reflect adaptive responses to local , kinship structures, and mating enforcement mechanisms rather than fixed species-typical fidelity.

Implications for Social Monogamy and Paternity Assurance

Extra-pair copulations (EPCs) in humans erode the alignment between social and genetic monogamy, creating persistent paternity uncertainty that challenges the stability and evolutionary rationale of pair bonds. Social facilitates biparental care and resource provisioning, which enhance survival, but EPCs introduce the risk of cuckoldry, where males invest in non-biological . This discrepancy has driven the evolution of male-specific adaptations to assure paternity, including heightened triggered by cues of female , such as emotional closeness to rivals or changes in appearance. Paternity assurance tactics in humans encompass behavioral vigilance, such as mate guarding through proximity monitoring and restricting female autonomy, which correlates with perceived risk and is more pronounced during fertile phases despite . Empirical studies indicate these behaviors reduce EPC opportunities, with men exhibiting greater mate retention efforts in long-term relationships characterized by higher perceived cuckoldry threats. Historical data from Western European populations reveal extra-pair paternity rates averaging 0.76% across 16 studies spanning the 17th to 20th centuries, suggesting that while cuckoldry occurs infrequently, its reproductive costs—equivalent to losing 23 years of paternal investment per incident—exert strong selective pressure for assurance mechanisms. These dynamics imply that human social monogamy persists not as strict genetic fidelity but as a conditional balancing paternal investment benefits against cuckoldry costs, with cultural norms like contracts and laws functioning as extended safeguards to enforce paternity certainty. In contexts of high EPC prevalence, such as among certain groups, elevated paternity uncertainty correlates with reduced male provisioning and increased conflict, underscoring how unresolved assurance challenges can destabilize pair bonds. Evolutionary models posit that prolonged mate guarding, necessitated by uncertain , may have contributed to patriarchal structures, where male control over sexuality minimizes extra-pair fertilization risks at the expense of autonomy. However, debates persist on the efficacy of these tactics, as strategies for concealed EPCs—such as deception during —can evade guarding, perpetuating low-level paternity uncertainty even in ostensibly monogamous societies.

Critiques of Evolutionary Interpretations

Critiques of evolutionary interpretations of extra-pair copulation (EPC) often center on the lack of empirical support for proposed adaptive benefits, particularly the "good genes" hypothesis, which posits that females engage in EPC to obtain genetic advantages for while securing biparental care from social partners. Multiple avian studies have tested predictions that extra-pair (EPO) exhibit superior viability, , or compared to within-pair , yet findings frequently show no such advantages or even fitness costs. For instance, in collared flycatchers, EPO experienced lower lifetime , contradicting indirect genetic benefit models. Similarly, in blue-footed boobies, EPO displayed no elevated or reproductive output over a 10-year period, undermining claims of genetic quality gains from extra-pair sires. In great tits, patterns of extra-pair paternity failed to align with good genes predictions, with inconsistent yearly variation and no evidence of heritable viability benefits. These results suggest that EPC may not confer the hypothesized indirect fitness gains, potentially reflecting null or negative selection rather than . Alternative explanations frame EPC as a of other evolved traits, such as generalized choosiness or , rather than a dedicated strategy for genetic bet-hedging. In tree swallows, while EPC correlated with increased genetic diversity or compatibility, it did not support good genes effects, implying incidental rather than selected outcomes. Reviews of costs associated with EPC highlight risks like reduced paternal investment, retaliation, and transmission, which may outweigh rare benefits in many contexts, especially where EPO rates are low. In humans, estimated extra-pair paternity rates of 1-3% across populations indicate weak selective pressure for adaptive EPC, as high costs (e.g., social sanctions, paternity loss) would erode advantages if benefits were substantial. Philosophical and methodological critiques further challenge evolutionary accounts, arguing that they rely on unfalsifiable reverse-engineering of Pleistocene conditions without or consideration of developmental plasticity. Philosopher David Buller has contended that evolutionary psychology's mating strategy claims, including those involving , overinterpret data as domain-specific adaptations while neglecting learning and environmental influences. Empirical debates persist due to challenges like small sample sizes in genetic paternity analyses, reliance on correlational fitness proxies, and variability in detecting subtle genetic effects, leading some researchers to question adaptationist primacy over simpler proximate or byproduct mechanisms. These critiques emphasize that while EPC occurs widely, its persistence may stem from constraint or rather than targeted selection for genetic benefits.

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