Extra-pair paternity in the socially monogamous mountain bluebird Sialia currucoides and its effect on the potential for sexual selection

Sexual selection theory posits that ornamental traits can evolve if they provide individuals with an advantage in securing multiple mates. That male ornamentation occurs in many bird species in which males pair with a single female is therefore puzzling. It has been proposed that extra-pair mating can substantially increase the variance in reproductive success among males in monogamous species, thus increasing the potential for sexual selection. We documented the frequency of extra-pair paternity and examined its effect on variation in male reproductive success in the mountain bluebird Sialia currucoides , a socially monogamous songbird in which males possess brilliant plumage ornamentation. Extra-pair paternity was common in our Wyoming study population, with 72% of broods containing at least one extra-pair offspring. The standardized variance in actual male reproductive success (i.e., the total number of within-pair and extra-pair offspring sired) was more than seven times higher than the variation in apparent success (i.e., success assuming that no extra-pair mating occurred). Success at siring within-pair and extra-pair offspring both contributed to the variation in overall male reproductive success. Within-pair success, however, did not predict a male's level of extra-pair success, suggesting that males do not sacrifice within-pair paternity to gain extra-pair paternity. Calculation of the sexual selection (Bateman) gradient showed that males sire approximately two additional offspring for each extra-pair mate that we identified. Thus, in this sexually dichromatic species, extra-pair mating increases the variance in male reproductive success and provides the potential for sexual selection to act.     

Paternity analysis reveals opposing selection pressures on crown coloration in the blue tit (Parus caeruleus)

In socially monogamous species, extra-pair paternity can increase the variance in reproductive success and thereby the potential for sexual selection on male ornaments. We studied whether male secondary sexual ornaments are selected through within- and/or extra-pair reproductive success in the blue tit (Parus caeruleus). Male blue tits display a bright blue crown plumage, which reflects substantially in the ultraviolet (UV) and previously has been indicated to be an important sexual signal. We show that males with a more UV-shifted crown hue were less cuckolded, which probably resulted from female preference for more ornamented mates. By contrast, however, older males and males with a less UV-shifted hue sired more extra-pair young. This probably did not reflect direct female preference, since cuckolders were not less UV-ornamented than the males they cuckolded. Alternatively, a trade-off between UV ornamentation and other traits that enhance extra-pair success could explain this pattern. Our results might reflect two alternative male mating tactics, where more UV-ornamented males maximize within-pair success and less UV-ornamented males maximize extra-pair success. Since crown colour was selected in opposite directions by within-pair and extra-pair paternity, directional selection through extra-pair matings seemed weak, at least in this population and breeding season. Reduced intensity of sexual selection due to alternative mating tactics constitutes a potential mechanism maintaining additive genetic variance of male ornaments.     

Effects of extra-pair and within-pair reproductive success on the opportunity for selection in birds

The number of mates, their fecundity, and the number of extra-pairfertilizations can all affect male reproductive success in biparentalspecies. Extra-pair mating in birds has been of particular interest,because it could generate strong levels of sexual selectioneven when a species is socially monogamous. We examined howextra-pair fertilizations affect the opportunity for selectionin the sexually dimorphic common yellowthroat (Geothlypis trichas)and the sexually monomorphic house wren (Troglodytes aedon).We were able to identify sires for at least 95% of all nestlings,and, thus, we were able to make a nearly complete accountingof male reproductive success. Although extra-pair fertilizationswere common in yellowthroats (26% of young) it contributed little(21%) to the total variance in male reproductive success. Mostof the variance in reproductive success (58%) was attributableto the male's within-pair success, which was influenced primarilyby the number of young produced by each mate and the proportionof within-pair young sired. Despite a moderate level of extra-pairfertilizations (10% of young) in house wrens, almost all ofthe variance in male reproductive success (97%) was attributableto within-pair success, particularly the number of social mates.Although extra-pair fertilizations generally increase the variancein male reproductive success, within-pair reproductive successmay be the major source of variation in male reproductive success.Thus, sexual dimorphism in monogamous birds may be influencedmore by the number of mates and their fecundity than by extra-pairmatings.     

MEASURING THE EFFECTS OF PAIRING SUCCESS,EXTRA-PAIR COPULATIONS AND MATE QUALITY ON THE OPPORTUNITY FOR SEXUAL SELECTION

Sexual selection can act through variation in the number of social mates obtained, variation in mate quality, or variation in success at obtaining extra-pair fertilizations. Because within-pair fertilizations (WPF) and extra-pair fertilizations (EPF) are alternate routes of reproduction, they are additive, rather than multiplicative, components of fitness. We present a method for partitioning total variance in reproductive success (a measure of the opportunity for selection) when fitness components are both additive and multiplicative and use it to partition the variance into components that correspond to each mechanism of sexual selection. Computer simulations show that extra-pair fertilizations can either increase or decrease total variance, depending on the covariance between within-pair and extra-pair success. Simulations also suggest that for socially monogamous species, extra-pair fertilizations have a greater effect than variation in mate quality or pairing status on the opportunity for selection. Application of our model to data gathered for a population of red-winged blackbirds (Agelaius phoeniceus) indicates that most of the variance in male reproductive success was attributable to within-pair sources of variance. Nevertheless, extra-pair copulations increased the opportunity for selection because males varied both in the proportion of their social young that they sired and in the number of extra-pair mates that they obtained. Furthermore, large and positive covariances existed between the number of extra-pair mates a male obtained and both social pairing success and within-pair paternity, indicating that, in this population, males preferred as social mates also were preferred as extra-pair mates.     

Extrapair paternity and the opportunity for sexual selection in a socially monogamous passerine

Effects of alternate mating strategies on the opportunity forsexual selection are widely debated, and recent studies haveconcluded that the effects of extrapair (EP) paternity on theopportunity for sexual selection may have been overstated dueto 1) methodological limitations of empirical studies and 2)the potential for males to gain from additional within-pair(WP) reproductive opportunities. We therefore examined the impactof EP paternity on the opportunity for sexual selection in thesocially monogamous and single-brooded eastern kingbird (Tyrannustyrannus). EP paternity was common in all 3 years of our study(61% of 89 broods, 47% of nestlings) and realized reproductivesuccess (EP + WP young) ranged from 0 to 9 young/male/year.A total of 31% of males lost all WP paternity (24% sired neitherWP nor EP young, whereas 7% sired EP but not WP young), andvariance in male realized reproductive success was more than9 times greater than that of apparent reproductive success.Nearly half of EP mates were not nearest neighbors, and manywere separated by 3 or more territories (>1000 m). EP successwas independent of nest defense behavior, but early singingmales and males with high song rates were most successful atboth a population level and when cuckolders and cuckoldees werecompared. EP paternity contributed significantly to the opportunityfor sexual selection in kingbirds, and we suggest that thisis probably due to the low potential for WP variation in reproductivesuccess, apparent long-distance movements of one or both sexes,and consequent absence of reciprocal cuckoldry.     

Variation in social organization influences the opportunity for sexual selection in a social lizard

Social monogamy has traditionally been suggested to be maintained because of weak sexual selection on male partner acquisition. However, the ubiquitous incidence of extra-pair paternity suggests that sexual selection can be strong in monogamous systems, although studies partitioning variance in male reproductive success have come to mixed conclusions. Here, we use detailed field data to examine variance in male reproductive success and its implications for the maintenance of sociality in a population of the socially monogamous lizard Egernia whitii. We show that both within-pair and, to a lesser extent, extra-pair partner acquisition contribute to the variance in male reproductive success, resulting in considerable opportunity for sexual selection. Despite this, levels of multiple mating are lower in Egernia compared to other reptiles, suggesting that male partner acquisition is constrained. We suggest that this constraint may be a result of strong territoriality combined with sexual conflict over multiple mating generated by costs of extra-pair paternity to females as a result of facultative male care. This has the potential to limit sexual selection by reducing variance in male reproductive success and therefore contribute to the maintenance of complex social organization.     

Sexual selection and mating patterns in a mammal with female-biased sexual size dimorphism

In mammals, species with highly male-biased sexual size dimorphismtend to have high variance in male reproductive success. However,little information is available on patterns of sexual selection,variation in male and female reproductive success, and bodysize and mating success in species with female-biased size dimorphism.We used parentage data from microsatellite DNA loci to examinethese issues in the yellow-pine chipmunk (Tamias amoenus), asmall ground squirrel with female-biased sexual size dimorphism.Chipmunks were monitored over 3 years in the Kananaskis Valley,Alberta, Canada. We found evidence of high levels of multiplepaternity within litters. Variation in male and female reproductivesuccess was equal, and the opportunity for sexual selectionwas only marginally higher in males than females. Male and femalereproductive success both depended on mating success. We foundno evidence that the number of genetic mates a male had dependedon body size. Our results are consistent with a promiscuousmating system in which males and female mate with multiple partners.Low variation in male reproductive success may be a generalfeature of mammalian species in which females are larger thanmales.     

Extra-pair mating,male plumage coloration and sexual selection in yellow warblers (Dendroica petechia)

Extra-pair mating has been proposed as a source of sexual selection responsible for secondary sexual traits that are common among socially monogamous birds, although supporting evidence is scant. In the socially monogamous yellow warbler, males are larger than females, and unlike females, have extensive reddish streaking on their breasts. Using DNA fingerprinting we show that within-pair parentage was positively related to male size, and that extra-pair mating success was positively related to the amount of streaking on the breast. To our knowledge, this is the first intraspecific evidence of an association between a male plumage ornament and gains of extra-pair paternity that is apparently independent of age. This study confirms that extra-pair mating can be an important mechanism of sexual selection even when the most successful sires are commonly cuckolded, and refutes a previous hypothesis that the variation in plumage and behaviour among male yellow warblers is an example of alternative, equally successful, evolutionarily stable strategies (ESS). More generally, the demonstrated independence of within-pair and extra-pair success and their associated traits indicates that where animals have multiple secondary sexual traits, different traits may be selected by different mechanisms that contribute to total reproductive success.     

Postmating sexual selection: the effects of male body size and recovery period on paternity and egg production rate in a water strider

The role of male body size in postmating sexual selection wasexplored in a semiaquatic insect, the water strider Gerris lateralis.To separate effects of male size per se from those due to numericsperm competition, male recovery period (shown here to be proportionalto ejaculate size) was manipulated independently of body sizein a factorial experiment where virgin females were mated firstwith sterile males and then with focal males. Both relativemale fertilization success and female reproductive rate were measured.The number of sperm transferred increased with male recoveryperiod, an effect that was mediated by longer copulation duration,but there were no effects of body size on ejaculate size. Neithermale size nor recovery period had any significant direct effectson male fertilization success. However, copulation durationinfluenced relative fertilization success, suggesting that malesable to transfer more sperm also achieved higher fertilizationsuccess. Females exercised cryptic female choice by modulatingtheir reproductive rate in a manner favoring large males andmales that were successful in terms of achieving high relativefertilization success. Thus, successful males gained a twofoldadvantage in postmating sexual selection. This study has important implicationsfor previous estimates of sexual selection in this group of insectsbecause pre- and postmating sexual selection will be antagonisticdue to limitations in male sperm production: males mating frequently(high mating success) will on average transfer fewer sperm ineach mating and will hence tend to fertilize fewer eggs permating (low fertilization success).     

Little effect of extrapair paternity on the opportunity for sexual selection in Savannah sparrows (Passerculus sandwichensis)

Extrapair paternity (EPP) can dramatically increase the opportunity for sexual selection if relatively few males are able to monopolize the majority of fertilizations in a population. Although recent work with birds suggests that EPP can increase the standardized variance in male reproductive success (Is) as much as 13-fold, only a male's within-pair success is typically quantified with any accuracy. In most cases, nearly half of all extrapair young are of unknown parentage. A strong, negative correlation across studies between the proportion of extrapair young for which parentage is known and the apparent effect of EPP on Is (r(s) = -0.71, P = 0.013, N = 13 studies) suggests that the incomplete sampling of extrapair sires has greatly exaggerated the influence of EPP. To achieve a more thorough accounting of EPP and its importance to variation in male fitness, we used a suite of four to six microsatellite loci to identify extrapair young and their sires in a polygynous population of Savannah sparrows (Passerculus sandwichensis). Pooling over the 2002 and 2003 breeding seasons, 79 of 116 females (68.1%) produced young outside of the pairbond and 194 of 411 offspring (47.2%) were extrapair. We identified sires for 96.4% of all young (N = 396), including sires for 92.3% of the extrapair young (N = 179), allowing us to partition Is into within-pair and extrapair components. In both years, EPP-related fitness components generated more variation in male reproductive success than the number or quality of within-pair mates. Differences among males in the number of extrapair mates alone accounted for 56.6% of Is in 2002 and for 23.6% of Is in 2003. Nonetheless, in absolute terms, the occurrence of EPP on Kent Island increased the opportunity for sexual selection less than two-fold. Averaging over the two years, Is was only 78% higher than Is,app, the variance in male reproductive success that would have occurred had EPP been nonexistent and males sired all young on their territories. Likewise, across nine socially monogamous species, we found no correlation between the extent of EPP and its effect on the opportunity for sexual selection (Is/Is,app) and only a marginally significant positive correlation between EPP and Is itself. Taken together, our results suggest that the relationship between EPP and sexual selection in birds may be much less strong and much less straightforward than commonly thought.