High Rates Of Extra-pair Paternity Research Articles

Infrequent Extra-Pair Paternity in the Banded Wren, a Synchronously Breeding Tropical Passerine

Abstract. Many species of socially monogamous birds engage in extra-pair copulations, but the extent of extra-pair paternity varies considerably by species. The ecological conditions contributing to interspecific variation in rates of extra-pair paternity remain unclear. We used a combination of single-nucleotide polymorphisms and microsatellites to document extra-pair paternity in the Banded Wren (Thryothorus pleurostictus), a socially monogamous neotropical passerine that lives in a highly seasonal environment. The rate of extra-pair paternity was relatively low, with extra-pair males siring 4% of offspring in 10% of nests. This low rate is consistent with current hypotheses predicting low rates of extra-pair paternity in nonmigratory species in which survivorship of adults is high, and it is contrary to the pattern predicted by hypotheses linking synchronous breeding with high rates of extra-pair paternity. Extra-pair males sang better and were more genetically similar to females than the males they cu...

Genetic Parentage and Local Population Structure in the Socially Monogamous Upland Sandpiper

Abstract. For a single lineage of birds, the diversity of mating systems and parental care among shorebirds (Charadrii) is high, which has made them an important group for investigations of the evolution of social mating systems. From 2003 to 2007, we studied a population of the Upland Sandpiper (Bartramia longicauda) at Konza Prairie Biological Station in northeastern Kansas. Our objectives were to determine the genetic mating system of this socially monogamous shorebird and to determine whether spatial patterns of philopatry and nest placement might affect opportunities for extra-pair mating. We used six microsatellite markers to estimate rates of extra-pair paternity in 58 family groups (107 parents, 184 offspring). We found that 30% of broods contained extra-pair offspring, representing 15% of chicks, the highest rate of extra-pair paternity ever reported in a socially monogamous shorebird. High rates of extra-pair paternity were not due to the degree of relatedness between partners in mated pairs, an...

Molecular evidence for extra-pair paternity and intraspecific brood parasitism in the Black-headed Gull

Social monogamy is common among birds, while genetic monogamy is supposed to be rare. I have investigated the genetic mating system of the Black-headed Gull Larus ridibundus in which, as in seabirds and most other long-lived and socially monogamous birds, extra-pair paternity (EPP) is typically rather infrequent. The parentage of 79 chicks from 30 broods was determined based on the analysis of six microsatellite markers. In this study population, evidence of allelic inconsistencies between putative parents and chicks as a result of both EPP and intraspecific brood parasitism (ISBP) was found in 43% of nests . Extra-pair paternity was detected in 33% (10/30) of broods, and 20% (16/79) of all nestlings were sired by extra-pair males. In addition, 9% (7/79) of chicks out of five nests (17%) were not the offspring of either member of the pair, indicating ISBP. These findings reveal a moderate rate of ISBP and a high rate of EPP in the Black-headed Gull compared with other related species and shows that the Black-headed Gulls successfully participate in extra-pair copulations.

Multiple paternity in the cooperatively breeding fish Neolamprologus pulcher

In cooperative breeders, mature males may compete for fertilizations. In this study, we measured the degree of multiple paternity in a natural population of a cooperatively breeding fish. Neolamprologus pulcher (Perciformes: Cichlidae) is a highly social cichlid endemic to Lake Tanganyika. We used highly variable microsatellite loci to survey 12 groups with an average number of 10.6 brood care helpers per group and a total of 43 offspring (mean 3.6 per brood). In 11 of 12 groups, all young were assigned to the dominant female. The dominant male sired all offspring in three groups, part of the offspring in four groups, and in five groups, he had no paternity at all. In total, 44.2% of young were not fathered by the current male territory owner. Multiple paternity was found in 5 of 12 broods (41.7 %), with 8 of 35 young (22.9 %) being sired by males other than the respective territory owners. This is an exceptionally high rate of extra-pair paternity among cooperatively breeding vertebrates. Neither helpers present in these territories during collection nor neighbouring males were unequivocally assigned to have sired these extra-pair young. However, behavioural observations suggest that male helpers may have produced these young before being expelled from the territory in response to this reproductive parasitism. We discuss these results in the light of reproductive skew theory, cooperative breeding in vertebrates and alternative reproductive tactics in fish.

Male age predicts extrapair and total fertilization success in the socially monogamous coal tit

An important step to a better understanding of the adaptive significance of extrapair mating behavior in socially monogamous species is to uncover the identity of extrapair sires (EPS). Here, we used a combination of multilocus DNA fingerprinting and microsatellite analysis to identify EPS in the socially monogamous coal tit (Periparus ater), a passerine bird with high rates of extrapair paternity. We then analyzed how fertilization success was related to male age in 2 consecutive first brood periods based on knowledge of the exact age of the majority of territorial males. EPS were significantly older compared with the males they cuckolded. Furthermore, extrapair and, as a consequence, also total fertilization success were positively related to male age, while within-pair success was not. Interestingly, fertilization success did not increase linearly with male age but leveled off for older age classes and was most parsimoniously described by the inverse term of male age. Results were consistent over the 2 years, while the demography of the study population differed with respect to the age distribution of territorial males. Furthermore, we also show that individual males increased their extrapair fertilization (EPF) success with age indicating that cross-sectional analyses were not confounded by cohort effects. Together with the results from other species, these findings suggest that male age (or a strong correlate thereof) is a major determinant of EPF success in several socially monogamous bird species.

Extra-pair paternity and the evolution of testis size in a behaviorally monogamous tropical mammal, the large treeshrew (Tupaia tana)

Monogamy is rare in mammals (<5% spp.) but occurs in greater frequency among primates (15%) and their close relatives, the treeshrews (100%; Order: Scandentia). Two genetic studies of parentage in monogamous primates revealed high rates of extra-pair paternity (EPP), but to date parentage has not been studied in a treeshrew species. I analyzed the genetic parentage of 22 offspring from two populations of large treeshrews in Sabah, Malaysia (NE Borneo), using seven autosomal microsatellite loci and one mitochondrial DNA marker. Half of these offspring were sired by males that were not the presumed partner of the mother (50% EPP), and three litters exhibited evidence of multiple paternity. However, comparative analysis indicated that the high rate of EPP in Tupaia tana is not associated with intense sperm competition. Relative testis size of treeshrews was similar to testis size in 22 primate species with uni-male mating systems but smaller than 44 primates with multi-male mating systems. After factoring out the effects of body size and phylogeny, I also found that the evolution of multi-male mating systems was significantly associated with the evolution of larger testis size. Male–female pairs of T. tana occupy joint territories but forage and sleep alone (“dispersed pair-living”), and I argue that this form of behavioral monogamy renders mate guarding ineffective. The adaptive advantages of behavioral monogamy likely differ from the advantages driving EPP in large treeshrews. However, small testis size suggests that behavioral monogamy is not masking a dispersed multi-male mating system in this species.

MHC-associated mating strategies and the importance of overall genetic diversity in an obligate pair-living primate

Mate choice is one of the most important evolutionary mechanisms. Females can improve their fitness by selectively mating with certain males. We studied possible genetic benefits in the obligate pair-living fat-tailed dwarf lemur (Cheirogaleus medius) which maintains life-long pair bonds but has an extremely high rate of extra-pair paternity. Possible mechanisms of female mate choice were investigated by analyzing overall genetic variability (neutral microsatellite marker) as well as a marker of adaptive significance (major histocompatibility complex, MHC-DRB exon 2). As in human medical studies, MHC-alleles were grouped to MHC-supertypes based on similarities in their functional important antigen binding sites. The study indicated that females preferred males both as social and as genetic fathers for their offspring having a higher number of MHC-alleles and MHC-supertypes, a lower overlap with female’s MHC-supertypes as well as a higher genome wide heterozygosity than randomly assigned males. Mutual relatedness had no influence on mate choice. Females engaged in extra-pair mating shared a significant higher number of MHC-supertypes with their social partner than faithful females. As no genetic differences between extra-pair young (EPY) and intra-pair young (IPY) were found, females might engage in extra-pair mating to ‘correct’ for genetic incompatibility. Thus, we found evidence that mate choice is predicted in the first place by the ‘good-genes-as-heterozygosity hypothesis’ whereas the occurrence of extra-pair matings supports the ‘dissassortative mating hypothesis’. To the best of our knowledge this study represents the first investigation of the potential roles of MHC-genes and overall genetic diversity in mate choice and extra-pair partner selection in a natural, free-living population of non-human primates.

Paternity and performance of golden-winged warblers (Vermivora chrysoptera) and golden-winged X blue-winged warbler (V. pinus) hybrids at the leading edge of a hybrid zone

We examined two components of reproduction in a population of golden-winged warblers in the initial stages of hybridization with blue-winged warblers. First, we used genetic analyses of mate choice to determine whether copulations outside the social pairbond (extra-pair copula- tions; EPCs) occur in these species and their hybrids. Second, we compared several aspects of reproduction (pairing success, clutch size, hatching success, fledging success, and loss of paternity) in nests raised by phenotypic golden-winged warblers to those raised by hybrid individ- uals. Together, these data provide us with the first quantitative analysis of reproduction within this hybrid system. Our data suggest several reasons why the level of hybridization between these species is likely being under- estimated. First, many birds in our population showed only subtle phenotypic signs of introgression indicating that hybrid status can only be determined by close examination. Second, high rates of extra-pair paternity indicate that we cannot base our estimates of hybridization on pairbond data alone. More than 30% of nestlings were the result of EPCs, occurring in 55% of all nests. Third, there was no difference in the number of hybrid or "pure" golden-winged warbler males chosen as social mates, relative to their abundance. Indeed, based on several components of nesting success, it appears as though hybrids are having as great realized reproductive success as are phenotypic golden-winged warblers in our population. Accordingly, we argue that hybrid reproductive success and extra-pair fertilizations are likely playing a major role in the ongoing hybridization between blue-winged and golden-winged warblers. Studies such as this one are important to our understanding of the process and implications of hybridization, and findings may be applicable to other hybrid zones.

Extra-pair paternity in alpine marmots, Marmota marmota: genetic quality and genetic diversity effects

Assuming that a male’s genetic characteristics affect those of his offspring, extra-pair copulation has been hypothesized to increase heterozygosity of the progeny—the “genetic compatibility” hypothesis—and the genetic diversity within litters—the “genetic diversity” hypothesis. We tested these two hypotheses in the alpine marmot (Marmota marmota), a socially monogamous mammal showing a high rate of extra-pair paternity (EPP). In a first step, we tested the assumption that a male’s genetic characteristics (heterozygosity and genetic similarity to the female) affect those of his offspring. Genetic similarity between parents influenced offspring heterozygosity, offspring genetic similarity to their mother, and litter genetic diversity. The father’s heterozygosity also influenced litter genetic diversity but did not affect offspring heterozygosity. Hence, heterozygosity seems not to be heritable in the alpine marmot. In a second step, we compared genetic characteristics of extra-pair young (EPY) and within-pair young (WPY). EPY were less genetically similar to their mother but not more heterozygous than WPY. EPY siblings were also less genetically similar than their WPY half siblings. Finally, the presence of EPY promoted genetic diversity within the litter. Thus, our data support both the “genetic compatibility” and the “genetic diversity” hypotheses. We discuss further investigations needed to determine the primary causes of EPP in this species.

Extra‐pair paternity in the strongly monogamous Wandering AlbatrossDiomedea exulanshas no apparent benefits for females

Although 92% of avian species are socially monogamous, extra‐pair copulation (EPC), resulting in extra‐pair paternity (EPP), is a common reproductive strategy in birds. Among seabirds, in which the rate of social monogamy reaches 100%, Procellariiformes (albatrosses and petrels) show low EPP rates, with the noticeable exception of the only albatross investigated in this regard, the Waved AlbatrossPhoebastria irrorata. This species, in which forced copulations are known to occur, showed a surprisingly high rate of EPP (25% of chicks). We investigate here EPP rates in another albatross species, the Wandering AlbatrossDiomedea exulans, subject to a demographic survey conducted for 38 years. We combined data on pair bonds with analysis of ten microsatellite loci and found that 10.7% of 75 chicks had an extra‐pair sire. Although there was some evidence for inbreeding avoidance, within‐pair and extra‐pair chicks showed similar levels of heterozygosity, and the incidence of EPP was independent of age, experience or past reproductive success. Hence, we found no evidence that females benefit from EPCs. Owing to the male‐biased sex ratio in adults, widowed and divorced males required more time to find a new mate (+28 and +72%, respectively) than did females. Combined with high sexual size dimorphism, this phenomenon might promote the forced copulations observed in this species. Our data therefore suggest that EPC is beneficial to unpaired males but occurs at random in females, consistent with the hypothesis that EPP results solely from forced EPCs. However, the importance of the latter for EPP and the part played by solitary males require further investigation.

Extrapair paternity and mate choice in a chickadee hybrid zone

The dynamics of hybrid zones are likely to be influenced greatly by patterns of mate choice, including “cryptic” choice mediated through extrapair copulations. To understand changes in hybrid zones over time and space, a detailed examination of mating patterns and correlates is needed. We studied the role of extrapair fertilizations (EPFs) in the breeding biology of hybridizing black-capped and Carolina chickadees in southeastern Pennsylvania over 4 years, using microsatellite DNA markers. We detected extrapair offspring (EPO) in 56% of 90 broods examined; these accounted for at least 26% of 477 offspring. Chickadees do not appear to use EPFs to reduce costs of heterospecific pairing: EPFs were no more likely to occur in genetically dissimilar (heterospecific) social pairs than in pairs where social mates were genetically similar. However, females paired with black-capped–like males were more likely to have EPO. Females that acquired EPFs did not obtain these from males genetically similar to themselves; instead, all females, regardless of their genotype or that of their social mate, tended to prefer Carolina-like males as extrapair partners. There was no relationship between the presence of EPO and hatching or fledging success. High rates of extrapair paternity and apparent female preference for Carolina-like males suggest that mate choice is an important influence in ongoing northward movement of this hybrid zone.

Genetic similarity between pair mates is not related to extrapair paternity in the socially monogamous coal tit

The benefits of extrapair mating behaviour for females of socially monogamous bird species are unclear despite substantial research effort. The genetic compatibility hypothesis proposes genetic benefits such that females paired to social mates of low genetic compatibility avoid or diminish negative fitness consequences by mating with a more compatible extrapair mate, resulting in offspring of higher genetic quality. Furthermore, within the context of inbreeding depression observed in natural populations, a high degree of overall genetic similarity between social pair mates may be regarded as a special case of genetic incompatibility. We tested the hypothesis that female extrapair matings represent an adaptive behavioural response to avoid negative consequences of being paired to a genetically similar social pair male in the coal tit, Parus ater, a socially monogamous passerine with high rates of extrapair paternity. In contrast to what was predicted, we found no evidence for a positive association between the genetic similarity of social pair mates (measured as band-sharing coefficients from multilocus DNA fingerprints) and the occurrence of extrapair paternity. Furthermore, the genetic similarity of the cuckolding female with its social mate was not higher when compared pairwise to that of its extrapair mate in 63 uniquely composed triplets. Finally, three parameters of reproductive success were not related to the genetic similarity of social pair mates. We conclude that avoiding the potentially negative fitness consequences of being paired to a genetically similar social pair mate did not select for and thus cannot maintain female extrapair mating behaviour in our study population. (c) 2004 The Association for tire Study of Animal Behaviour Published by Elsevier Ltd. All rights reserved.

Cuckoldry and recapture probability of adult males are not related in the socially monogamous coal tit (Parus ater)

Despite substantial research effort, the benefits of female extra-pair matings in socially monogamous bird species remain elusive. The “good genes” hypothesis assumes that females engage in extra-pair copulations with males of superior genetic quality compared to their respective social mate. Therefore, a negative association between the degree of cuckoldry and male survival is predicted, if genetic quality is phenotypically reflected by high viability. Furthermore, genetic sires of extra-pair offspring (EPO) should survive better than the social fathers they cuckolded. We tested these predictions in a nestbox population of the coal tit (Parus ater), a socially monogamous passerine with low breeding dispersal and high rates of extra-pair paternity (EPP). Based on 257 genotyped first broods of two consecutive years, we found no relationship between the incidence of EPP or the proportion of EPO within a given brood and male or female recapture probabilities. Furthermore, recapture rates did not differ between social and genetic fathers of EPO or males that did or did not appear as extra-pair sires in other broods. Our results were not affected by differential (short-range) breeding dispersal with respect to EPP or by other potentially confounding variables. Hence, they are not in accordance with the “good genes as viability genes” hypothesis.

Extrapair paternity and the evolution of bird song

Bird song is usually considered to have evolved in the context of sexual selection. Because extrapair paternity is a major component of sexual selection, mating advantages at the social level for males that produce songs of high quality may be transformed into higher success in extrapair paternity. Therefore, males with longer and more complex songs should suffer less from extrapair paternity intraspecifically, whereas species with high rates of extrapair paternity, reflecting intense sperm competition, should produce more elaborate songs. Although some intraspecific studies demonstrated a negative link between features of songs and extrapair paternity in own nest, others failed to detect such a relationship. Contrary to expectation, a metaanalysis of all studies revealed no significant intraspecific evidence for songs being associated with extrapair paternity. In addition, in comparative analyses based on generalized least squares (GLS) models, we found that no measures of song complexity and temporal output were significantly related to extrapair paternity interspecifically, even when potentially confounding factors such as social mating system, life history, migration, habitat, or sexual dichromatism were held constant. Only plumage dichromatism was significantly related to extrapair paternity. The absence of both intra- and interspecific relationships between measures of song variability and extrapair paternity suggests that factors other than postmating sexual selection have been the important evolutionary forces shaping differences in song. Key words: bird song, evolution, extrapair paternity, generalized least squares, meta-analysis, repertoire size, sexual selection. [Behav Ecol 15:508–519 (2004)]

Extra-pair Paternity in the Dusky Warbler, Phylloscopus fuscatus: a Test of the 'Constrained Female Hypothesis'

The 'constrained-female hypothesis' (CFH) states that females risk losing their social partner's assistance in brood care when they engage in extra-pair copulations. Hence, it is predicted that individual differences in female extra-pair behaviour can be explained in terms of (1) the amount of male care that females risk to lose (2) the intrinsic ability of females to raise offspring without male help, and (3) food availability, facilitating uni-parental care. The dusky warbler, a species with a polygynous mating system and a high rate of extra-pair paternity (45% of the offspring) offers an excellent opportunity to test all these predictions. In accordance with prediction (1) I find that females in pairing situations in which they cannot expect to receive male assistance in brood care are more likely to have extra-pair young than females in situations where bi-parental care is the rule. Using age and bill morphology as predictors for a female's ability to raise nestlings alone, I find no direct support for prediction (2). Finally, variation in food availability (3) had no influence on extra-pair paternity. Overall, the CFH provides some explanation for why female promiscuity differs between situations of bi-parental and uni-parental care, but the large amount of remaining variation can neither be explained by food abundance nor by individual female characteristics.

Environmental conditions affect the magnitude of inbreeding depression in survival of Darwin's finches.

Understanding the fitness consequences of inbreeding (inbreeding depression) is of importance to evolutionary and conservation biology. There is ample evidence for inbreeding depression in captivity, and data from wild populations are accumulating. However, we still lack a good quantitative understanding of inbreeding depression and what influences its magnitude in natural populations. Specifically, the relationship between the magnitude of inbreeding depression and environmental severity is unclear. We quantified inbreeding depression in survival and reproduction in populations of cactus finches (Geospiza scandens) and medium ground finches (Geospiza fortis) living on Isla Daphne Major in the Galápagos Archipelago. Our analyses showed that inbreeding strongly reduced the recruitment probability (probability of breeding given that an adult is alive) in both species. Additionally, in G. scandens, first-year survival of an offspring with f = 0.25 was reduced by 21% and adults with f = 0.25 experienced a 45% reduction in their annual probability of survival. The magnitude of inbreeding depression in both adult and juvenile survival of this species was strongly modified by two environmental conditions, food availability and number of competitors. In juveniles, inbreeding depression was only present in years with low food availability, and in adults inbreeding depression was five times more severe in years with low food availability and large population sizes. The combination of relatively severe inbreeding depression in survival and the reduced recruitment probability led to the fact that very few inbred G. scandens ever succeeded in breeding. Other than recruitment probability, no other trait showed evidence of inbreeding depression in G. fortis, probably for two reasons: a relatively high rate of extrapair paternity (20%), which may lead to an underestimate of the apparent inbreeding depression, and low sample sizes of highly inbred G. fortis, which leads to low statistical power. Using data from juvenile survival, we estimated the number of lethal equivalents carried by G. scandens, G. fortis, and another congener, G. magnirostris. These results suggest that substantial inbreeding depression can exist in insular populations of birds, and that the magnitude of the inbreeding depression is a function of environmental conditions.

ENVIRONMENTAL CONDITIONS AFFECT THE MAGNITUDE OF INBREEDING DEPRESSION IN SURVIVAL OF DARWIN'S FINCHES

Understanding the fitness consequences of inbreeding (inbreeding depression) is of importance to evolutionary and conservation biology. There is ample evidence for inbreeding depression in captivity, and data from wild populations are accumulating. However, we still lack a good quantitative understanding of inbreeding depression and what influences its magnitude in natural populations. Specifically, the relationship between the magnitude of inbreeding depression and environmental severity is unclear. We quantified inbreeding depression in survival and reproduction in populations of cactus finches (Geospiza scandens) and medium ground finches (Geospiza fortis) living on Isla Daphne Major in the Galápagos Archipelago. Our analyses showed that inbreeding strongly reduced the recruitment probability (probability of breeding given that an adult is alive) in both species. Additionally, in G. scandens, first-year survival of an offspring with f = 0.25 was reduced by 21% and adults with f = 0.25 experienced a 45% reduction in their annual probability of survival. The magnitude of inbreeding depression in both adult and juvenile survival of this species was strongly modified by two environmental conditions, food availability and number of competitors. In juveniles, inbreeding depression was only present in years with low food availability, and in adults inbreeding depression was five times more severe in years with low food availability and large population sizes. The combination of relatively severe inbreeding depression in survival and the reduced recruitment probability led to the fact that very few inbred G. scandens ever succeeded in breeding. Other than recruitment probability, no other trait showed evidence of inbreeding depression in G. fortis, probably for two reasons: a relatively high rate of extrapair paternity (20%), which may lead to an underestimate of the apparent inbreeding depression, and low sample sizes of highly inbred G. fortis, which leads to low statistical power. Using data from juvenile survival, we estimated the number of lethal equivalents carried by G. scandens, G. fortis, and another congener, G. magnirostris. These results suggest that substantial inbreeding depression can exist in insular populations of birds, and that the magnitude of the inbreeding depression is a function of environmental conditions.

High rates of extra-pair young in the pair-living fat-tailed dwarf lemur, Cheirogaleus medius

Cheirogaleids are one of the most primitive extant primate taxa in the world. Their lifestyle and mating system, therefore, have been considered to be representative for social systems in primate ancestors. Accepted models of social evolution in primates state that pair-bonding has evolved secondarily from diurnal group-living taxa and should therefore be constrained primarily to diurnal species. In contrast to these assumptions, the nocturnal fat-tailed dwarf lemur (Cheirogaleus medius) lives in permanent pairs, with obligate paternal care probably representing the evolutionary basis of pair-living. In this sociobiological field study, we analyzed the reproduction strategy of C. medius in the tropical forest of western Madagascar. In the rainy seasons from 1995 to 1999, 173 individuals of C. medius were captured and individually marked and 131 were genetically characterized through seven microsatellite loci. Additionally, 36 of these individuals were radio-tracked and observed. For 53 genotyped individuals, including 16 offspring, information about pair-bonding and family structure was known from field observations. Genetic analyses revealed that yearlings and infants living with an adult pair were in all cases sibs of the social mother. However, C. medius does not restrain from extra-pair copulations (EPCs) and a high rate of extra-pair paternity (44%) was detected. Males sired offspring with their female partners as well as with extra-pair females within the same year, indicating that males may increase their reproductive success by EPCs without necessarily running the risk of cuckoldry. Females on the other hand do not seem to run the risk of reduced paternal care, either because males cannot detect relatedness of young, or because they might even increase their inclusive fitness by raising offspring of closely related males. Since females reproduce preferentially with territory holders and no paternity could be assigned to floating males, superior genetic quality of the males might be crucial for female choice.

Comparative evidence for a positive association between divorce and extra-pair paternity in birds

Although most bird species are socially monogamous, they show a large variation in both divorce rate and the proportion of extra-pair paternity (EPP). Recently, adaptive explanations of avian monogamy have considered divorce and EPP as related behavioural strategies by which individuals paired with low quality mates can improve their breeding status within ecological and time constraints. It has been suggested that, at both the intra- and inter-specific levels, divorce rate should be associated with the frequency of EPP. Divorce and EPP could coexist as alternative strategies whose relative frequencies would depend on ecological conditions, resulting in a negative association between the two phenomena. Or, they can be seen as synergistic tactics co-varying with variation in mate quality between populations or species and are thus positively associated. Here, applying two different comparative methods: the comparison of independent linear contrast and the pairwise comparison of closely related taxa, we show that high rates of divorce are positively associated with high rates of extra-pair paternity in socially monogamous species of birds, even when controlling for survival rate as a potentially confounding variable. This constitutes the first comparative evidence for a relation between divorce and adultery in animals. We discuss this result in relation to recent hypotheses on the adaptiveness of divorce and extra-pair copulations in birds and natural variation in mate quality among populations or species. The pattern found is most consistent with the hypothesis that birds may divorce to gain a better breeding position, but alternatives cannot be fully ruled out.