Little Effect of Extrapair Paternity on the Opportunity for Sexual Selection in Savannah Sparrows (Passerculus Sandwichensis)

Abstract

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 (I s) 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 I s (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 sand-wichensis). 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 I s 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 I s in 2002 and for 23.6% of I s in 2003. Nonetheless, in absolute terms, the occurrence of EPP on Kent Island increased the opportunity for sexual selection less than twofold. Averaging over the two years, I s was only 78% higher than I s,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 (I s /I s,app) and only a marginally significant positive correlation between EPP and I s 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.

Behavioral Paternity Predicts Genetic Paternity in Satin Bowerbirds (Ptilonorhynchus Violaceus), a Species With a Non-Resource-Based Mating System

“…Excluding nonbreeding birds also allowed me to compare the opportunity for sexual selection in male and female breeding turkeys since I was not able to estimate the variance among females in the absence of this restriction. Due to the relatively low numbers of nests examined in any given year, annual data were pooled across years (Woolfenden et al 2002); interannual variation is clearly important (Freeman-Gallant et al 2005) but could not be considered in the present study except in the analysis of males that included nonbreeding individuals because of insufficient sample size in individual years. If two nests from the same female were sampled in the same year, the first nest was included if I collected the entire clutch prior to incubation; otherwise, the nest with the most samples was used in this analysis.…”

“…Given the widespread interest in sexual selection in birds, it is somewhat surprising that direct measures of I s are relatively uncommon compared to the growing number of species for which molecular parentage data are known (for reviews see Woolfenden et al 2002, Freeman-Gallant et al 2005, Hauber and Lacey 2005, Whittingham and Dunn 2005, Albrecht et al 2007). Existing studies have typically focused on pair-bonded passerines, frequently with the purpose of exploring the relative importance of extra-pair and within-pair offspring in determining the variance in male reproductive success (Webster et al 1995(Webster et al , 2007.…”

“…Including extra-pair young when calculating male fitness could reduce I s if males face trade-offs between gaining offspring in other nests and loss of paternity in their own broods. Alternatively, if males that produce more extrapair young also generate more within-pair offspring, variance in reproductive success will tend to increase (Webster et al 1995, Freeman-Gallant et al 2005. Empirical studies have found that, while the inclusion of information on molecular paternity always increases estimates of the opportunity for selection compared to estimates based only upon the fecundity of social mates, the importance of extra-pair offspring can vary dramatically, even across years in the same population (Freeman-Gallant et al 2005).…”

“…Alternatively, if males that produce more extrapair young also generate more within-pair offspring, variance in reproductive success will tend to increase (Webster et al 1995, Freeman-Gallant et al 2005. Empirical studies have found that, while the inclusion of information on molecular paternity always increases estimates of the opportunity for selection compared to estimates based only upon the fecundity of social mates, the importance of extra-pair offspring can vary dramatically, even across years in the same population (Freeman-Gallant et al 2005). Comparative studies are now beginning to examine across bird species how ecological and behavioral correlates can influence the opportunity for selection through changes in number of extra-pair young (Whittingham andDunn 2005, Albrecht et al 2007).…”

A distinct reed bunting dawn song and its relation to extrapair paternity

“…In socially monogamous passerine species, female choice is often not limited to the choice of a social partner because of extrapair mating (Griffith et al 2002). Extrapair paternity (EPP) can increase the variance in reproductive success of males (Webster et al 1995(Webster et al , 2007Møller 1998;Albrecht et al 2007; but see Freeman-Gallant et al 2005;Whittingham & Dunn 2005) and thus the selection pressure on sexually selected traits, such as song output or song diversity . Studies on various bird species suggest that song is important in extrapair mate choice (Hasselquist et al 1996;Kempenaers et al 1997;Forstmeier et al 2002;Byers 2007;Gil et al 2007).…”