QUANTITATIVE GENETICS OF SEXUAL SIZE DIMORPHISM IN THE COLLARED FLYCATCHER,FICEDULA ALBICOLLIS

Quantitative genetic theory predicts that evolution of sexual size dimorphism (SSD) will be a slow process if the genetic correlation in size between the sexes is close to unity, and the heritability of size is similar in both sexes. However, there are very few reliable estimates of genetic correlations and sex-specific heritabilities from natural populations, the reasons for this being that (1) offspring have often been sexed retrospectively, and hence, selection acting differently with respect to body size in the two sexes between measuring and sex identification can bias estimates of SSD; and (2) in many taxa, parents may be incorrectly assigned to offspring either because of assignment errors or because of extrapair paternity. We used molecular sex and paternity identification to overcome these problems and estimated sex-specific heritabilities and the genetic correlation in body size between the two sexes in the collared flycatcher, Ficedula albicollis. After exclusion of the illegitimate offspring, the genetic correlation in body size between the sexes was 1.00 (SE = 0.22), implying a severe constraint on the evolution of SSD in this species. Furthermore, sex-specific heritability estimates were very similar, indicating that neither sex will be able to evolve faster than the other. By using estimated genetic parameters, together with empirically derived estimates of sex-specific selection gradients, we further demonstrated that the predicted selection response in female tarsus length is displaced about 200% in the opposite direction from that to be expected if there were no genetic correlation between the sexes. The correspondence between the biochemically estimated rate of extrapair paternity (about 15 % of the young) and that estimated from the “heritability method” (11%) was good. However, the estimated rate of extrapair paternity with the heritability method after exclusion of the illegitimate young was 22%, adding to increasing evidence that factors other than extrapair paternity (e.g., maternal effects) may be resposible for the commonly observed higher mother-offspring than father-offspring resemblance.     

Extrapair paternity in hooded warblers

We examined the role of extrapair fertilizations (EPFs) in themating system of the hooded warbler (Wilsonia citrina), a monogamoussongbird. DNA fingerprinting revealed that 8 of 17 (47%) femaleshad extrapair young in their first or second brood, and 23 of78 (29%) nestlings were the result of EPFs. Extrapair youngwere signifkandy more likely to occur in first broods than insecond broods. The proportion of EPFs within a brood was stronglybirnodal among broods: nests had 50% or more extrapair youngor none. In seven of eight broods where EPFs occurred, an adjacentmale neighbor was identified as the actual father. Male-likecoloration in females did not reduce the likelihood of havingextrapair young. Females with extrapair young did not receiveless parental care from their mates. All males who obtainedEPFs were mated to fertile females or were feeding offspringat the time they most likely mated with the extrapair female.Our results are consistent with the female control hypothesis,which predicts that females benefit from extrapair copulations(EPCs) and have some control over which males, if any, obtainEPCs. However, we could not reject the alternative hypothesisthat some male neighbors are particularly dominant and aggressiveduring EPC attempts, so females accept these EPCs to minimizecosts.     

Effects of breeding density, synchrony, and experience on extrapair paternity in tree swallows

Breeding density, synchrony, and experience are expected toinfluence the frequency of extrapair paternity in birds. UsingDNA fingerprinting, we examined the effect of these factorson tree swallows nesting at relatively high (grids of nest-boxes)and low (solitary boxes at least 100 m from the nearest neighbor)densities and in relatively synchronous (Alberta) and asynchronous(Ontario) populations in Canada. The mean percentage of extrapairoffspring per nest did not differ significantly between birdsnesting in grids (43%, n = 22 families) and solitary boxes (57%,n = 12 families). Similarly, there was no significant differencein the mean percentage of extrapair offspring per nest betweenrelatively synchronous (60%, n = 12 Alberta families) and asynchronous(41%, n = 22 Ontario families) populations. We also found noconsistent pattern between extrapair paternity and breedingexperience among seven males and seven females examined overtwo to three breeding seasons. Female tree swallows can influencethe fertilization success of extrapair males by active selectionand rejection of copulation partners. We suggest that this abilitylimits the predicted effect of various ecological factors onthe frequency of extrapair paternity     

Mating systems, sperm competition, and the evolution of sexual dimorphism in birds

Comparative analyses suggest that a variety of factors influence the evolution of sexual dimorphism in birds. We analyzed the relative importance of social mating system and sperm competition to sexual differences in plumage and body size (mass and tail and wing length) of more than 1,000 species of birds from throughout the world. In these analyses we controlled for phylogeny and a variety of ecological and life-history variables. We used testis size (corrected for total body mass) as an index of sperm competition in each species, because testis size is correlated with levels of extrapair paternity and is available for a large number of species. In contrast to recent studies, we found strong and consistent effects of social mating system on most forms of dimorphism. Social mating system strongly influenced dimorphism in plumage, body mass, and wing length and had some effect on dimorphism in tail length. Sexual dimorphism was relatively greater in species with polygynous or lekking than monogamous mating systems. This was true when we used both species and phylogenetically independent contrasts for analysis. Relative testis size was also related positively to dimorphism in tail and wing length, but in most analyses it was a poorer predictor of plumage dimorphism than social mating system. There was no association between relative testis size and mass dimorphism. Geographic region and life history were also associated with the four types of dimorphism, although their influence varied between the different types of dimorphism. Although there is much interest in the effects of sperm competition on sexual dimorphism, we suggest that traditional explanations based on social mating systems are better predictors of dimorphism in birds.     

Genetic monogamy despite variable ecological conditions and social environment in the cooperatively breeding apostlebird

Mating strategies may be context‐dependent and may vary across ecological and social contexts, demonstrating the role of these factors in driving the variation in genetic polyandry within and among species. Here, we took a longitudinal approach across 5 years (2006–2010), to study the apostlebird (Struthidea cinerea), an Australian cooperatively breeding bird, whose reproduction is affected by ecological “boom and bust” cycles. Climatic variation drives variation in the social (i.e., group sizes, proportion of males and females) and ecological (i.e., plant and insect abundance) context in which mating occurs. By quantifying variation in both social and ecological factors and characterizing the genetic mating system across multiple years using a molecular parentage analysis, we found that the genetic mating strategy did not vary among years despite significant variation in rainfall, driving primary production, and insect abundance, and corresponding variation in social parameters such as breeding group size. Group sizes in 2010, an ecologically good year, were significantly smaller (mean = 5.8 ± 0.9, n = 16) than in the drought affected years, between 2006 and 2008, (mean = 9.1 ± 0.5, n = 63). Overall, apostlebirds were consistently monogamous with few cases of multiple maternity or paternity (8 of 78 nests) across all years.     

Extrapair paternity in chestnut-sided warblers is correlated with consistent vocal performance

The elaborateness of many bird songs is commonly presumed tohave evolved under the influence of sexual selection by femalemate choice. Thus, aspects of acoustic diversity, such as songrepertoire size, are seen as likely targets of female choice.In many songbird species with song repertoires, however, therepertoires are small. In such species, female choice mightbe based on song features other than, or in addition to, songdiversity. To investigate this conjecture, I assessed singingand paternity in a population of chestnut-sided warblers (Dendroicapensylvanica), a species in which song repertoires are of modestsize. Twenty-two song traits were evaluated to determine whichones best predicted male extrapair reproductive success. Thecandidate traits encompassed measures of song diversity (e.g.,song repertoire size), gross-scale song performance (e.g., singingrate), and fine-scale song performance (e.g., variability amongsongs in a bout). Regression analysis revealed that the bestpredictor of extrapair success was singing with little variability.In particular, the most successful males sang with consistentpitch and timing, as well as high pitch. The greater extrapairsuccess of males with more consistent vocal performance maybe due to female preference for such performance, which couldbe an indicator of male quality.