Dalliances and doubtful dads: what determines extra-pair paternity in socially monogamous wandering albatrosses?

Genetic techniques have revealed surprisingly high rates of extra-pair paternity (EPP) in socially monogamous albatrosses. We sought to establish social and genetic influences on EPP in wandering albatrosses (Diomedea exulans) at Marion Island, where EPP rates were 14–24?% in three successive seasons. EPP probably resulted from both female solicited extra-pair behaviours and male forced copulations. EPP was not linked to breeding experience nor with poor reproductive performance, despite a tendency for pairs to consistently produce either EPP or within-pair paternity (WPP) chicks. Mate guarding may inhibit extra-pair behaviour; however, parental arrival date and presence in the colony prior to laying did not correlate with EPP. There was little support for genetic advantages to producing EPP chicks, but the population is characterised by low genetic variability, which may result in mate incompatibility. Mates of pairs that failed and pairs producing EPP young tended to be more similar genetically to their partners than mates producing WPP young, suggesting that EPP may counter mate incompatibility. EPP and WPP chicks grow equally well, so cuckolded males did not reduce investment in EPP chicks. The lack of discriminatory behaviour by cuckolded males together with low genetic diversity in the population may allow continued high levels of EPP. In albatrosses, pair bonds are typically long lasting and the costs of forming new pairings may discourage mate swapping. Females may undertake extra-pair copulations as an adaptive alternative to mate swapping because the costs of extra-pair behaviour are small.     

Multiple sexual pigments,assortative social pairing,and genetic paternity in the yellow warbler (Setophaga petechia)

In socially monogamous species, extra-pair paternity may increase the reproductive success of highly ornamented males, mediating the evolution of sexual ornaments. However, ornaments may also attract social mates, and a tradeoff between extra-pair paternity (EPP) and within-pair paternity (WPP) may complicate mating strategies. Further, in many socially monogamous species, females are also ornamented, and the relationship between female ornamentation and patterns of EPP has been neglected. We investigated the patterns of genetic paternity with respect to carotenoid- and melanin-based pigmentation in yellow warblers (Setophaga petechia) of both sexes. We asked whether males face tradeoffs between EPP and WPP, how paternity patterns relate to carotenoid- versus melanin-based pigmentation, and whether less EPP occurs in broods when males and females are assortatively paired. Males faced a tradeoff between EPP and WPP. Moreover, non-additive relationships existed between paternity patterns and the two pigment types in both sexes. Males with high melanin coverage but dull carotenoid pigmentation achieved EPP but lost WPP, whereas males with high levels of both pigment types had high WPP but gained little EPP. A parallel pattern occurred in females. Warblers paired assortatively by pigmentation and EPP was less common in broods when the sexes were assortatively paired by carotenoid pigmentation. Results suggest that the most colorful birds obtain high quality social mates and advance reproductive success through WPP, show that correlations can arise between female ornamentation and patterns of EPP, and also uniquely suggest that social pairing patterns may influence extra-pair mating strategies.     

Sex allocation and paternity in a cooperatively breeding passerine: evidence for the male attractiveness hypothesis?

Sex allocation theory predicts that female birds with high-quality mates will benefit from producing more sons, since sons will inherit their father’s superior traits and enjoy a great reproductive success, whereas females with low-quality mates will benefit from producing more daughters, since the variance in reproductive success among daughters is typically lower. The male attractiveness hypothesis may apply to extra-pair paternity (EPP) because socially monogamous females routinely mate with higher quality males outside the pair bond. We test these predictions using the Tibetan ground tit (Pseudopodoces humilis), a sexually monomorphic, socially monogamous, facultatively cooperative breeder. There was greater variation in actual reproductive success among males than females due to EPP. An excess of sons was detected for bi-parental (i.e., non-cooperative) broods wherein EPP was mainly sired by bi-parental males. The pattern was attributed to male-biased sex ratios produced for both EPP and within-pair offspring within the same broods. The reason for the latter case might be a random allocation of more offspring to sons by the potentially EPP-exposed females that have an inability to control fertilization by specific males. In cooperative broods where EPP mostly resulted from within-group helpers of presumed low-quality, as indicated by their failure in acquiring a social mate, there was a non-significant tendency for EPP offspring to be daughters and for within-pair offspring in the same broods to be unbiased. These results support the EPP-related male attractiveness hypothesis especially in terms of the overproduction of sons. Offspring produced through quasi-parasitism was unbiased towards either sex, suggesting a weak female choiceness with respect to the quality of host males.     

Do male plumage and song characteristics influence female off-territory forays and paternity in the hooded warbler?

Extra-pair fertilizations are common in many socially monogamous species, and paternity studies have indicated that females may use male vocal performance and plumage ornaments as cues to assess male quality. Female off-territory forays may represent a key component of female choice and male extra-pair mating success, and female foray behaviour is expected to be strongly influenced by indictors of male quality. In this study, we examined how male song and ornamentation affect how often females left their territories, which males they visited and extra-pair paternity in a socially monogamous passerine, the hooded warbler (Wilsonia citrina). We radiotracked 17 females during the fertile period and quantified male vocal performance (song output and rate) and plumage characteristics (size of the black melanin hood and colour of the black hood, yellow cheeks and breast areas). We obtained blood samples and determined paternity at 35 nests including those of 14 females that we radiotracked. Eleven (65%) of the 17 females forayed off-territory, whilst fertile and female foray rate was positively correlated with the number of extra-pair young in the nest. Females that left their territories more frequently were paired with males that sang at a low rate. In addition, extra-pair mates had higher song rates than the social mates they cuckolded (5.3 songs/min vs. 4.4 songs/min). Female off-territory forays or extra-pair paternity were not significantly related to male plumage characteristics. Our results indicate that a high song rate influences both the foray behaviour of a male’s social mate and the likelihood that he will sire extra-pair offspring with neighbouring females.     

Non-aggressive mate guarding by the blue-footed booby: a balance of female and male control

Thirteen pairs of blue-footed boobies (Sula nebouxii) were observed on their colony. Pairs courted frequently and, on average, copulated 24 times during the 30 days before laying, with 38% of those copulations occurring in the last 5 days (presumed fertile period). Males and females increased attendance at the nest site as laying approached. Seven females performed an average seven extra-pair copulations, with 1–2 paired male neighbors, but these were less concentrated in the presumed fertile period than within-pair copulations, and the last two copulations of all 7 females were with their social mates. Rates of female extra-pair copulations were six times lower when their social mate was present, and during the presumed fertile period, no female performed an extra-pair copulation in the presence of her mate. Males did not respond to infidelity of social mates with aggression, prompt copulation, retaliatory copulation, or increase in copulation. Seven of 13 males performed an average of five extra-pair copulations, with 1–3 paired female neighbors, before their own mates began egg-laying. The males' extra-pair copulations represented only 4% of their total copulations during their own mates' presumed fertile periods. Females, the larger sex, apparently control sexual access and copulate with extra males to achieve extra-pair fertilization. Males pursue a mixed strategy: they copulate with extra females, mostly outside their own mate's presumed fertile period, and they copulate increasingly with their social mate as laying approaches, probably assuring some paternity by mate guarding, involving attendance and courtship. Behavior of males and females is also consistent with other hypotheses for extensive joint nest site attendance: pairbonding, copulation access, and territory acquisition. Received: 14 November 1997 / Accepted after revision: 16 May 1998     

Interactive effects of male and female age on extra-pair paternity in a socially monogamous seabird

Females sometimes obtain older sires for their offspring through extra-pair interactions, but how female age influences paternity is largely unexplored and interactive effects across the age span of both sexes have not been analyzed. To test whether female choice of sire age varies with female age in the blue-footed booby (Sula nebouxii), we examined associations between ages of both partners and the probability of extra-pair paternity (EPP) in 350 broods of parents up to 22 years old in a single breeding season. Extra-pair paternity enables a female to select an alternative sire for her offspring and could function to avoid or achieve particular combinations of parental ages. A male age?×?female age interaction revealed that in young females (≤4 years), EPP decreased with increasing age of the social partner, whereas in old females (≥8 years), it increased. Moreover, sires of extra-pair (EP) chicks of young females paired to young males were on average 6.33 years older than the females’ social partners. Since female boobies control copulatory access, this pattern could imply that young females choose old sires for their proven genetic quality and that old females avoid very old males because matings with them may risk infertility or genetic defects in offspring. Taking female age into account and observing across the whole age span may be necessary for understanding female age-based mate choice.     

Search costs influence the spatial distribution,but not the level,of extra-pair mating in tree swallows

Costs of searching for a mate are an important component of models of sexual selection, yet they have rarely been examined in wild populations of vertebrates. In this paper, we report an experiment in which we handicapped female tree swallows by clipping some flight feathers. This manipulation increased the costs of flight and searching for extra-pair mates. Despite these costs, handicapped females had the same level of extra-pair mating (percentage of extra-pair young, percentage of broods with extra-pair young, and the number of extra-pair sires per brood) as control females. However, handicapped females were more likely to have young sired by extra-pair males that lived closer to her nest than control females. This change in the distribution of extra-pair mating was most likely due to female choice rather than male coercion, and it suggests that extra-pair mating has significant benefits to females. One important implication of our study is that ecological and social factors that influence search costs could affect the spatial distribution of extra-pair sires and, consequently, the intensity of sexual selection. These effects may have been overlooked in previous studies that did not identify extra-pair sires.     

Mate guarding in the swallowHirundo rustica

Summary The importance of mate guarding by males in the monogamous swallowHirundo rustica was studied by temporarily detaining the males. Mate guarding reduced the frequency of extra-pair copulations and of sexual chases involving female mates. Males participated in sexual chases more frequently if they had a non-fertile female. Neighbouring males of ‘widowed’ females increased their own mate guarding presumably in response to the experimentally increased rate of sexual chases. Neighbouring males with a fertile female increased their mate guarding more than did males with a non-fertile female. Addition of eggs to swallow nests in the post-fledging period of the first brood induced mate guarding by male nest owners. These males also copulated more frequently with their mates than did control males. Neighbouring male swallows responded to the increased mate guarding by showing sexual interest in the guarded females. removal of eggs from swallow nests during the laying period, leaving only one egg in the nest, resulted in reduced nest attendance by females. Male mates responded by increasing their mate guarding intensity as compared to controls, and neighbouring males showed an increased sexual interest in these females.     

Extra-pair paternity and tail ornamentation in the barn swallow Hirundo rustica

Females of socially monogamous species may copulate with attractive non-mates to obtain access to the genes of such males, and a preference for attractive copulation partners may result in sexual selection. Extra-pair copulations are common in the socially monogamous barn swallow Hirundorustica, and a 2-year study of paternity using multi-locus DNA fingerprinting demonstrated that 33% of 63 broods and 28% of 261 offspring were sired by extra-pair males. The frequency of extra-pair offspring within broods was highly skewed with the majority of all broods having either no extra-pair offspring or only extra-pair offspring. Individual pairs were consistent in their frequency of extra-pair paternity among broods, and the repeatability of extra-pair paternity of multiple broods of the same female was statistically significant. The proportion of extra-pair offspring was negatively related to the tail length of the male attending the nest. Behavioural observations showed that extra-pair fertilizations were more likely in broods raised by females that had been observed to engage in extra-pair copulations. The frequency of extra-pair offspring was unrelated to the intensity of two male paternity guards, mate guarding and the rate of intra-pair copulations. In an analysis of extra-pair paternity and male parental care in different broods of the same male, male barn swallows fed their offspring relatively less frequently if the brood contained more extra-pair offspring. Therefore, female barn swallows pursue extra-pair copulations with attractive males, which may result in sexual selection, even though extra-pair paternity is costly for females due to the reduction of paternal care by their social mates. Received: 24 January 1997 / Accepted after revision: 2 August 1997     

Positive association between social and extra-pair mating in a polygynous songbird, the dickcissel (Spiza americana)

In polygynous species, males appear to gain additional offspring by pairing with multiple females simultaneously. However, this may not be true if some females copulate outside of the social pair bond. Polygynous males could experience lower paternity because of trade-offs among gaining multiple social mates, guarding fertility with these mates, and pursuing extra-pair matings. Alternatively, polygynous males could simultaneously gain extra social mates and have high paternity, either because of female preferences or because of male competitive attributes. We tested four predictions stemming from these hypotheses in a facultatively polygynous songbird, the dickcissel (Spiza americana). Unlike most previous studies, we found that males with higher social mating success (harem size) also tended to have higher within-pair paternity and that the number of extra-pair young a male sired increased significantly with his social mating success. Females that paired with mated males were not more likely to produce extra-pair young. In contrast, extra-pair paternity was significantly lower in the nests of females whose nesting activity overlapped that of another female on the same territory. This pattern of mating was robust to differences in breeding density. Indeed, breeding density had no effect on either extra-pair mating or on the association between polygyny and paternity. Finally, nest survival increased with harem size. This result, combined with the positive association between polygyny and paternity, contributed to significantly higher realized reproductive success by polygynous male dickcissels.     

Extra-pair paternity and the opportunity for sexual selection in a socially monogamous bird (Dendroica petechia)

We used multi-locus DNA fingerprinting to characterise the genetic mating system of the socially monogamous yellow warbler (Dendroica petechia). Over 2 years there were no instances of brood parasitism, but 59% of families (n = 90) contained extrapair sired young and 37% of offspring (n = 355) were of extra-pair paternity. Most hypotheses for extra-pair mating in monogamous species assume a paternity benefit to extra-pair sires, and focus on the benefit(s) to females. However, the assumption of male benefit has been little tested. Among yellow warblers, known extra-pair sires were just as likely to be cuckolded as any male in the population, and there was at least one reciprocal exchange of extra-pair paternity. Nevertheless, among known extra-pair sires, the paternity gains from extra-pair paternity were, on average, greater than the losses in their own families. These results show there is a paternity benefit to certain males. However, the benefit is not absolute but relative and therefore more difficult to measure. The results also suggest that patterns of extra-pair fertilisation are not determined by female choice alone. Most confirmed extra-pair mates were territorial neighbours, but some resided as far as three territories apart, and greater spatial separation was implied in other cases. Thus, the opportunity for extra-pair mating is great. We estimate that as a result of extra-pair fertilisations, variance in male mating success is increased somewhere between 3-fold and 15-fold over that which would result from within-pair reproduction alone. These findings affirm the potential importance of extra-pair reproduction for sexual selection in monogamous species and they support earlier suggestions that extra-territorial forays by male yellow warblers are for the purpose of extra-pair mating.     

Polygyny and extra-pair paternity enhance the opportunity for sexual selection in blue tits

Polygyny and extra-pair paternity are generally thought to enhance sexual selection. However, the extent to which these phenomena increase variance in male reproductive success will depend on the covariance between success at these two strategies. We analysed these patterns over four breeding seasons in facultatively polygynous blue tits Cyanistes caeruleus. We found that both polygyny and extra-pair paternity increased variance in male reproductive success and that standardised variance in annual number of genetic fledglings was 2.6 times higher than standardised variance in apparent success when assuming strict monogamy. Nevertheless, male success at securing within-pair paternity was unrelated to success at gaining extra-pair paternity and, when considering the positive effect of age on extra-pair success and attracting a second female, polygynous males were no more likely to sire extra-pair fledglings. Overall, polygynous males fledged more genetic offspring than monogamous males, but first-year polygynous males lost a greater share of within-pair paternity. A literature review suggests that this adverse effect of polygyny on within-pair paternity is frequent among birds, inconsistent with the prediction that females engage in extra-pair copulation with successful males to obtain good genes. Furthermore, a male's share of paternity was repeatable between years, and among females of polygynous males within years, such that a compatibility function of extra-pair copulations was likewise unsupported. Instead, we suggest that the observed patterns are most consistent with a fertility insurance role for extra-pair copulations, which does not exclude the greater opportunity for sexual selection through differential ability of males to gain paternity.     

Benefits of extra-pair mating may depend on environmental conditions—an experimental study in the blue tit (Cyanistes caeruleus)

Extra-pair mating constitutes a relatively common reproductive strategy in many socially monogamous bird species. This strategy may considerably improve reproductive success of males, but female benefits from extra-pair matings still remain unclear and empirical evidence is scarce. This may be because genetic benefits of extra-pair mating are not always revealed. It is possible that they are shown only in unfavourable environmental conditions and hence problems arise with detecting differences between within- and extra-pair offspring whose performance is measured under favourable conditions. In order to test this prediction, we manipulated environmental conditions by altering brood sizes of blue tits and compared phenotypic characteristics of within- and extra-pair offspring in mixed-paternity broods. We found that extra-pair young exhibited a higher response to phytohemagglutinin in comparison to within-pair young, but this was only observed among nestlings from experimentally enlarged broods. These results indicate that genetic benefits may interact with the environment, and thus benefits of extra-pair mating are likely to become visible only when conditions are relatively unfavourable.     

Selection for male choice based primarily on mate compatibility in the oldfield mouse, Peromyscus polionotus rhoadsi

Despite the consensus that mate choice acts as a mechanism for selection of secondary sexual traits, the evolutionary forces affecting mate preferences themselves remain controversial. In this study, we first demonstrated selection acting directly on the mate preferences of monogamous male oldfield mice, Peromyscus polionotus rhoadsi. One group of male oldfield mice were allowed to express a social preference between two potential mates, and were subsequently paired with either their preferred or rejected female. Among these pairs, those containing preferred females produced more offspring than did those containing rejected females. We next demonstrated that this fitness advantage depended primarily on compatibility between the members of a mated pair. A second group of male oldfield mice were not allowed the opportunity to express a social preference between potential mates. Rather, these males were paired with females that had been either preferred or rejected by males in the first group. Among these pairs, those containing preferred females did not produce more offspring than those containing rejected females. In other words, individual mate preferences had fitness consequences only for those males that expressed them, demonstrating that these preferences were based primarily on compatibility between mates.     

Correlates of extra-pair fertilization success in hooded warblers

We examined correlates of extra-pair fertilization (EPF) success in the hooded warbler (Wilsonia citrina), a species where females pursue extra-pair matings. The good genes hypothesis predicts that females choose extra-pair mates with morphological or behavioral traits that reflect differences in male genetic quality. EPFs were common, as 35.3% (42/119) of broods contained extra-pair young (EPY) and 26.7% (95/356) of nestlings were the result of EPFs. There was a strong skew in male EPF success which increased the variance in annual male mating success 2–3 fold. However, male morphology did not predict EPF success, as extra-pair males were not older or larger than the males they cuckolded. Likewise, there were no significant correlations between the proportion of extra-pair young in a brood and male size or age. The good genes hypothesis predicts that high-quality males will be consistently preferred as genetic mates, but the number of young sired by a male with his social mate was not consistent from one year to the next. There was a significant negative correlation between female age and proportion of EPY produced, which could result if older females obtain higher-quality social mates. We found no strong evidence that females choose extra-pair mates for good genes, but females may use behavioral rather than morphological cues to assess relative male quality. Received: 31 May 1996 / Accepted after revision: 17 October 1996     

Multiple paternity and offspring quality in tree swallows

There is mounting evidence in a variety of taxa that females increase offspring quality by mating with multiple males, often resulting in multiple paternity. In birds, however, few studies have explicitly examined the benefits of mating with several different males; instead, the focus has been on whether or not extra-pair mating occurs, and its adaptive significance remains controversial. We examined the hypothesis that offspring quality, particularly immune response (phytohaemagglutinin assay) and growth, increases with the number of sires in broods of socially monogamous tree swallows (Tachycineta bicolor). We found one of the highest known levels of multiple paternity in birds (84% of nests with two or more extra-pair young had at least two extra-pair sires). Among nests with extra-pair young, the number and diversity of sires continued to increase linearly with the number of extra-pair young, so there was no evidence that some males monopolized paternity at high levels of extra-pair fertilization. Indeed, the number of sires was actually greater than expected in large broods, suggesting that some females might be seeking more mates. We found no effect of the number of sires on nestling immune response or growth. In mixed paternity broods, the immune response of extra-pair young did not differ from that of their within-pair half-siblings. However, among all broods, nestlings had a stronger immune response in nests with at least one extra-pair nestling than in nests with all within-pair nestlings. These results are not consistent with a good genes benefit of extra-pair mating, but they do suggest that there are environmental effects associated with extra-pair mating that increase nestling immune response. These environmental effects could produce indirect genetic effects on sexual selection if they are heritable. The extraordinarily high number of sires in this species highlights a relatively unexplored source of sexual selection in birds.     

Major histocompatibility complex and mate choice in a monogamous rodent

A growing number of studies indicate that females can increase the viability of their offspring by gaining direct benefits such as parental care or genetic advantages through selective mating with certain males. Among the best candidates for the genetic basis of mate choice in vertebrates are the genes of the major histocompatibility complex (MHC) because these highly polymorphic genes may increase offspring viability and provide direct cues for mate choice. A free-ranging, pair-living rodent was used as an example to investigate MHC-dependent mate choice in an obligate monogamous species, the Malagasy giant jumping rat Hypogeomys antimena. Two possible mechanisms of mate choice were tested. First, mate choice may occur to increase the heterozygosity of MHC genes in the progeny and, second, mates might choose each other according to the degree of dissimilarity of their functional MHC DRB (exon 2) proteins in order to maximise the allelic divergence in their offspring. Analyses of 65 Hypogeomys couples failed to confirm associations of mating patterns with the MHC genotype to increase heterozygosity or MHC allelic divergence in the progeny. Also, no evidence for mechanisms to increase the allelic divergence was found in sex-specific analyses where a male or female, respectively, migrated to and was accepted by a territory and burrow holder of the opposite sex. However, the frequency distribution of 0, 1 or 2 new alleles potentially available for the progeny differed significantly when a new male was chosen by a territory-holding female. In contrast to current models, genetically similar instead of dissimilar mates seem to be the preferred choice. This is the first study investigating the role of the MHC in mate selection in an obligate monogamous rodent.Communicated by G. Wilkinson     

Mate guarding,copulation strategies and paternity in the sex-role reversed,socially polyandrous red-necked phalarope<Emphasis Type="Italic"> Phalaropus lobatus</Emphasis>

In a recent review, Westneat and Stewart (2003) compiled evidence that extra-pair paternity results from a three-player interaction in which sexual conflict is a potent force. Sequentially polyandrous species of birds appear to fit this idea well. Earlier breeding males may attempt to use sperm storage by females to obtain paternity in their mates subsequent clutches. Later-breeding males may consequently attempt to avoid sperm competition by preferring to pair with previously unmated females. Females may bias events one way or the other. We examined the applicability of these hypotheses by studying mating behavior and paternity in red-necked phalaropes (Phalaropus lobatus), a sex-role reversed, socially polyandrous shorebird. Male red-necked phalaropes guarded mates more strongly than other shorebirds. Males increased within-pair copulation attempts during their mates fertile period, and maintained or further increased attempts towards the end of laying, suggesting an attempt to fertilize the females next clutch; these attempts were usually thwarted by the female. Paired males sought extra-pair copulations with females about to re-enter the breeding pool. Multilocus DNA fingerprinting showed that 6% of clutches (4/63) each contained one chick sired by a male other than the incubator, producing a population rate of these events of 1.7% (n=226 chicks). Male mates had full paternity in all first clutches (n=25) and 15 of 16 monogamous replacement clutches. In contrast, 3 of 6 clutches of second males contained extra-pair young likely fathered by the females previous mate. Previously mated female phalaropes may employ counter-strategies that prevent later mating males from discriminating against them. The stability of this polyandrous system, in which males provide all parental care, ultimately may depend on females providing males with eggs containing primarily genes of the incubating male, and not a previous mate.Communicated by M. Webster     

Breeding synchrony and paternity in the barn swallow (Hirundo rustica)

The barn swallow (Hirundo rustica) is a socially monogamous passerine which usually breeds in colonies where extra-pair copulations are frequent. Males intensively guard their mates during the female fertile period. Since males are more likely to be available for extra-pair copulations when their mate is not fertile, synchrony in timing of breeding may affect paternity of individual males. In this study, we analysed the change in mate-guarding rate by males in relation to the fertility condition of the female, and the relationships between breeding synchrony and density with paternity in first broods of 52 male barn swallows. Paternity (proportion of nestlings fathered in own brood) was assessed by typing of three highly polymorphic microsatellite loci. Mate guarding by males peaked during the fertile period of their mates. Paternity increased as breeding synchrony in the colony increased. Paternity of barn swallows is positively associated with the degree of exaggeration of male tail ornaments. The relationship between male ornamentation and paternity was partly mediated by an effect of ornament size on breeding synchrony. We suggest that females might delay breeding with low-quality males to enhance their opportunities for being fertilised by high-quality extra-pair males. Received: 16 January 1998 / Accepted after revision: 25 October 1998     

Extra-pair paternity in willow ptarmigan broods: measuring costs of polygyny to males

Willow ptarmigan are one of only three monogamous grouse species in North America. However, in some populations between 5 and 20% of individuals pair polygynously. It has been suggested that monogamy may be maintained by the high cost of polygyny to males. We have used DNA fingerprinting to assess the actual reproductive success of both monogamous and polygynous adults. We determined whether or not the putative parents were the biological parents of the chicks from 38 broods. Of these clutches 30 were from monogamous matings, and 8 were from bigamous matings. Of the 207 chicks from monogamous matings 96% were within-pair offspring, compared to 67% of the 49 chicks from bigamous matings. All extra-pair offspring chicks resulted from extra-pair fertilizations (EPFs), and there were no instances of intraspecific nest parasitism. Mate guarding by monogamous males seems to be a highly effective method for maintaining genetic monogamy, as the only cases in which EPFs occurred were when the resident female left the territory for a few days or when a second female visited the territory. Our results support the notion that certainty of parentage may be one factor constraining willow ptarmigan males to be monogamous.