Corey R. Freeman-Gallant

Social pairing and female mating fidelity predicted by restriction fragment length polymorphism similarity at the major histocompatibility complex in a songbird

Female birds often copulate outside the pair‐bond to produce broods of mixed paternity, but despite much recent attention the adaptive significance of this behaviour remains elusive. Although several studies support the idea that extra‐pair copulations (EPCs) allow females to obtain ‘good genes’ for their offspring, many others have found no relationship between female mating fidelity and traits likely to reflect male quality. A corollary to the good genes hypothesis proposes that females do use EPCs to increase the quality of young, but it is the interaction between maternal and paternal genomes — and not male quality per se — that is the target of female choice. We tested this ‘genetic compatibility’ hypothesis in a free‐living population of Savannah sparrows (Passerculus sandwichensis) by determining whether females mated nonrandomly with respect to the major histocompatibility complex (Mhc). During both the 1994 and 1995 breeding seasons, female yearlings (but not older birds) avoided pairing with Mhc‐similar males (P < 0.005). The Mhc similarity between mates also predicted the occurrence of extra‐pair young in first broods (P < 0.007) and covaried with estimates of genome‐wide levels of similarity derived from multilocus DNA fingerprinting profiles (P = 0.007). The overall genetic similarity between adults tended to predict female mating fidelity, but with less precision than their Mhc similarity (P = 0.09). In contrast, females appeared insensitive to the size, weight or age of males, none of which explained variation in female mating fidelity. Taken together, these results are consistent with the hypothesis that females sought complementary genes for their offspring and suggest either that the benefits of heterozygosity (at the Mhc) drive female mating patterns or that the avoidance of inbreeding is an ultimate cause of social and genetic mate choice in Savannah sparrows.

DNA Fingerprinting Reveals Female Preference for Male Parental Care in Savannah Sparrows

According to sexual selection theory, females choose mates to ensure access to high quality resources, male parental care, or good genes. This last hypothesis has been hotly debated on both theoretical and empirical grounds. In contrast, female preference for male parental care has received less attention, primarily because the potential benefits of paternal effort seem obvious. The fitness relations are less clear in double-brood species, however, because females can base mating decisions on their prior experience with male parental care. Here, the extent of male parental care delivered to first-brood offspring may indicate male genetic quality and/or be the target of female manipulation via her subsequent mating fidelity. In Savannah Sparrows (Passerculus sandwichensis), multi-locus DNA fingerprinting of 203 adults and young revealed substantial female infidelity in first and second broods: overall, 24 of 80 first-brood young (30%) and 13 of 80 second-brood young (16.3%) were the product of extra-pair fertilizations. Among 12 females altering fidelity between broods, absolute male feeding rate to first-brood offspring was a strong, positive predictor of change in female fidelity. Because the extent of male parental care reflects a male’s viability, the data support a ‘good-genes’ interpretation.

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 (Is) as much as 13‐fold, only a males 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 (rs=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.

SEXUAL SELECTION, MULTIPLE MALE ORNAMENTS, AND AGE- AND CONDITION-DEPENDENT SIGNALING IN THE COMMON YELLOWTHROAT

In many animals, sexual selection has resulted in complex signaling systems in which males advertise aspects of their phenotypic or genetic quality through elaborate ornamentation and display behaviors. Different ornaments might convey different information or be directed at different receivers, but they might also be redundant signals of quality that function reliably at different times (ages) or in different contexts. We explored sexual selection and age‐ and condition‐dependent signaling in the common yellowthroat (Geothlypis trichas), a sexually dichromatic warbler with two prominent plumage ornaments—a melanin‐based, black facial “mask” and carotenoid‐based, UV‐yellow “bib.” In a three‐year study, variance among males in the number of social (Mw) and extra‐pair (Me) mates generated strong sexual selection on mask and bib attributes. Some traits (mask size, bib yellow brightness) were correlated with male age and did not experience selection beyond age‐related increases in Mw and Me. Other traits showed age‐specific (bib size) or age‐reversed (ultraviolet brightness) patterns of selection that paralleled changes in the information‐content of each ornament. The components of male fitness generating selection in young versus old males were distinct, reflecting different sources of variation in male fertilization success. Age‐ and context‐dependent changes in the strength, direction, and target of selection may help explain the maintenance of multiple ornaments in this and other species.

Apparent heritability of parental care in Savannah sparrows

Parental effort measures the total amount of time and energy allocated toward care of offspring, and it is often an obvious and variable component of an animals life history (Winkler and Wilkinson 1988, Clutton-Brock 1991). Among birds, intraspecific variation in parental effort is more quantitative than qualitative. Individual adults differ in feeding rate (Yasukawa et al. 1990, Wright and Cuthill 1992), intensity of offspring defense (Winkler 1992), and tendency to engage in prolonged associations with young (Weatherhead and McRae 1990). Like all quantitative traits, intraspecific variation in parental effort can be attributed to genetic and environmental causes. However, recent work has focused mostly on environmental sources of variation (Ketterson and Nolan 1994, Gowaty 1996a), and individual birds often are viewed as decision-making machines that respond adaptively to changes in their physical or social environments (Winkler and Wilkinson 1988). Indeed, birds modify parental effort in response to numerous proximate factors, including genetic parentage (Lifjeld et al. 1998), brood size (Ruusila and P6ysa 1998), and offspring age and condition (Whittingham and Robertson 1993). In no case have environmental factors accounted completely for differences among individuals in the intensity of parental care, however, and some role for genetic or cultural determination seems likely. For quantitative traits, genetic and environmental effects can be distinguished by examining the phenotypic resemblance of relatives. In cases of high narrow-sense heritability (h2), additive genetic effects contribute substantially to overall phenotypic variation, and relatives strongly covary (Falconer and Mackay 1996). Among vertebrates, estimates of heritability are lacking for measures of parental effort and for most quantitative behaviors in the field (Hailman 1986, Boag and van Noordwijk 1987, Mousseau and Roff 1987; but see Waser and Jones 1989). In general, heritabilities for behavioral traits are low (Lemon 1993, Berthold and Pulido 1994), suggesting that quantitative differences among individuals are largely driven by the environment. Here, we examine the degree to which male and female feeding rates are heritable in an insular population of Savannah Sparrows (Passerculus sandwichensis). We show that quantitative differences in feeding rates among males can be attributed to the be-

MHC VARIATION IS RELATED TO A SEXUALLY SELECTED ORNAMENT, SURVIVAL, AND PARASITE RESISTANCE IN COMMON YELLOWTHROATS

Hamilton and Zuk proposed that females choose mates based on ornaments whose expression is dependent on their genetically based resistance to parasites. The major histocompatibility complex (MHC) plays an important role in pathogen recognition and is a good candidate for testing the relationships between immune genes and both ornament expression and parasite resistance. We tested the hypothesis that female common yellowthroats prefer to mate with more ornamented males, because it is a signal of their MHC‐based resistance to parasites and likelihood of survival. In this species, females prefer males that have larger black facial masks as extrapair mates. Using pyrosequencing, we found that mask size was positively related to the number of different MHC class II alleles, as predicted if greater variation at the MHC allows for the recognition of a greater variety of pathogens. Furthermore, males with more MHC class II alleles had greater apparent survival, and resistance to malaria infection was associated with the presence of a particular MHC class II allele. Thus, extrapair mating may provide female warblers with immunity genes that are related to parasite resistance, survival, and the expression of a male ornament, consistent with good genes models of sexual selection.

Variation at the major histocompatibility complex in Savannah sparrows

The class I and class II genes of the major histocompatibility complex (Mhc) encode dimeric glycoproteins responsible for eliciting the adaptive immune response of vertebrates. Recent work with birds suggests that the number, size, and arrangement of these genes can differ markedly across species, although the extent of this variation, and its causes and consequences, are poorly understood. We have used a 157‐base‐pair (bp) portion of the second exon of a class II B gene to probe the Mhc in a free‐living population of Savannah sparrows (Passerculus sandwichensis). Segregation analysis of Mhc bands suggests that class II B genes can be found in two independently assorting clusters, as previously described for domestic chickens (Gallus gallus) and ring‐necked pheasants (Phasianus colchicus) but unlike gene organization in mammals. The Mhc in Savannah sparrows appears large (with many class II B genes) and variable; we found 42 unique genotypes among 48 adults breeding on Kent Island, New Brunswick, Canada in 1995. Savannah sparrows are long‐distance migrants, and these results support recent predictions that migratory birds should show higher levels of Mhc polymorphism and/or a greater number of genes than sedentary species. Savannah sparrows are also socially polygynous with high levels of extra‐pair paternity, suggesting that a history of sexual selection might also influence the size and/or structure of the avian Mhc.

Sexual selection and the geography of Plasmodium infection in Savannah sparrows ( Passerculus sandwichensis )

According to Hamilton and Zuks hypothesis of parasite-mediated sexual selection, host-parasite coevolution maintains variation in male genetic quality and allows for strong intersexual selection in species with high rates of infection. In birds, most interspecific tests of this hypothesis relate the prevalence of blood parasites to some measure of the intensity of sexual selection. Such tests often rely on limited sampling of single populations to estimate species-wide infection rates, and many tests are thus vulnerable to intraspecific (geographic) variation in the evolutionary ecology of disease. Here, we used molecular techniques to examine variation in the prevalence of Plasmodium spp. across 14 populations of Savannah sparrows, Passerculus sandwichensis, in eastern North America. Plasmodium could not be detected in any of 68 island birds, but 34 of 119 (29%) mainland males, and 7 of 43 (16%) mainland females were infected. Among mainland birds, infection was common in southern populations but rare in New Brunswick, Canada. Overall, the prevalence of Plasmodium ranged from 0 to 60% across populations, although only 17.8% of birds were infected in the pooled (species-wide) sample. The extent of this geographic variation suggests that limited sampling of single populations is unlikely to yield accurate estimates of species-wide infection rates. However, among mainland Savannah sparrows, the prevalence of malaria correlated strongly with average male size and the degree of sexual size dimorphism. We speculate that either sexual selection leads to male-biased infection or, conversely, that high rates of infection promote the evolution of strong intersexual selection.

Extra-pair paternity in monogamous and polygynous Savannah sparrows,Passerculus sandwichensis☆☆☆

Abstract Extra-pair paternity can influence mating systems by affecting the fitness costs associated with polygyny. Polygyny is disadvantageous to males when the time and energetic demands of multiple pairings decrease either a males success at gaining extra-pair fertilizations or his ability to ensure paternity among harem members. In Savannah sparrows, Passerculus sandwichensis on Kent Island, New Brunswick, Canada, multilocus DNA fingerprinting of 136 adults and young revealed substantial female infidelity: overall, 31 of 92 young (33.7%) in 15 of 24 nests (62.5%) were the product of extra-pair fertilizations. Male mating status was a strong predictor of paternity. Each of seven monogamous females produced at least one extra-pair offspring, but only six of 11 primary females (54.5%) and two of six secondary females (33.3%) were unfaithful. As a result, nearly 80% of the young in nests of polygynous males resulted from within-pair fertilizations, compared with only 40% of the young in nests of monogamous males. Kent Island Savannah sparrows are simultaneously polygynous, and the absence of paternity costs associated with polygyny is surprising. The observed pattern of paternity suggests the operation of female choice, although male control of parentage cannot be excluded.

DNA Fingerprinting Data and the Problem of Non-Independence Among Pairwise Comparisons

Multilocus DNA fingerprinting is commonly used to assess genetic similarity within and between geographically disjunct populations. Typically, the proportion of DNA fingerprinting bands shared between two individuals (SXY) is calculated for all possible pair‐wise comparisons and the resulting data analysed parametrically to test differences in mean band‐sharing among groups. The degree to which covariation among interdependent SXY values (Sab ‐ Sbc) biases the analyses is often unknown. Here, we assess the extent of covariation in four DNA fingerprinting studies and evaluate the effectiveness of two corrective procedures, a permutation test and a subsampling routine using only independent pairwise comparisons drawn without replacement from the overall data. Covariation among interdependent SXY values was significantly greater than zero in every data set examined, including those from a bee, a rodent, and two passerine birds. Permutation tests did not correct for interdependence and yielded significance values nearly identical to those derived from uncorrected parametric procedures. In contrast, the subsampling procedure yielded corrected estimates of the standard error that were two to four times larger than those derived parametrically. As a result, comparisons that were significant using parametric tests were either non‐significant or only marginally significant with the subsampling routine. We conclude that interdependence among SXY values poses a substantial obstacle to hypothesis testing that must be addressed in future studies.