Evolutionary changes in color patches of blackbirds are associated with marsh nesting

Fully unraveling the mechanisms of sexual selection requiresan understanding of the variation in secondary sexual traitsacross species in a monophyletic assemblage and an understandingof the evolutionary relationships between those species. Therole of red and yellow male plumage coloration in territorydefense and sexual selection has been well studied in the red-wingedblackbird (Agelaius phoeniceus), and males of many other closerelatives of this species also have what appear to be carotenoid-pigmentedpatches in their plumage. We explored variation in male plumagecoloration across species of New World blackbirds (family Icteridae):traits known to be involved in sexual selection in this group.We document that blackbird lineages in which extant speciesbreed in marshes tend to have evolved from an all-black ancestralplumage to one exhibiting carotenoid plumage patches. The twomost likely hypotheses to explain this pattern are (1) increasedsexual selection intensity in marshes because of increasedvariance in territory quality and (2) increased frequency ofmale-male territorial interactions because of an increaseddensity of territories in marshes, but other hypotheses cannotbe ruled out. This pattern is consistent with either intersexualor intrasexual selection and warrants further investigation.     

NATURAL SELECTION AND SEXUAL SIZE DIMORPHISM IN RED-WINGED BLACKBIRDS

Patterns of overwinter mortality in the sexually dimorphic red-winged blackbird (Agelaius phoeniceus) were examined to test the predictions of the sexual-selection hypothesis that male size is limited by directional selection favoring small males and that female size is maintained by stabilizing selection wherein extreme phenotypes experience higher mortality. Museum specimens collected from Ontario over a 95-yr period were used to compare the sizes of males and females collected in fall and spring. In a separate field study, body sizes of returning and nonreturning male and female red-winged blackbirds were compared over a 6-yr period. Overall, there was no evidence of higher overwinter mortality among larger males. Among adult (ASY) males, large individuals appeared to have higher survival than small individuals, although among subadult (SY) males, large size may have been disadvantageous. Weak evidence of stabilizing selection on female body size was found. Among adults, sexual size dimorphism seemed more pronounced after winter than before winter. Our results do not support the hypothesis that body size in male red-winged blackbirds is limited by selective mortality outside the breeding season. It is possible that size selection occurs earlier in life, when males are still in the nest. Our results suggest that caution should be exercised when interpreting interspecific evidence showing higher adult male than female mortality in sexually dimorphic species. Such patterns could arise as a cost to males of sexual selection and yet provide no insight into how natural selection opposes sexual selection for increased male size.     

Breeding synchrony and extrapair fertilizations in two populations of red-winged blackbirds

We tested the relationship between synchrony of breeding andthe frequency of extrapair fertilizations (EPFs) in two populationsof red-winged blackbirds known to differ in female extrapairbehavior. We found no association between the number of simultaneouslyfertilizable females (temporal neighbors) and EPF rate in eitherpopulation, although a significant difference between populationsin the direction of this relationship (positive where femalesinitiated extrapair copulations and negative where males initiatedthem) suggested a modest difference in the influence of synchrony.Males losing offspring to EPFs tended to have more fertilizablefemales at that time than the actual sires in some analysesbut not in others. We also tested several assumptions underlyingtwo competing hypotheses for the effects of synchrony. We foundno evidence that females pursued extrapair copulations moreoften when other females were synchronous. Rather, females weremore likely to gain EFFs with exirapatr males whose social mateswere not yet building their nests. Synchrony also did not consistentlyaffect male pursuit of exirapair copulations or achievementof EPFs. These results suggest that timing of breeding has someeffects on extrapair activity, but that those effects are bothrelatively weak and influenced by other factors that vary betweenyears or populations.     

Experimental analyses of intra- and intersexual competition in red-winged blackbirds

The roles of intra- and intersexual selection in determiningharem size for many polygynous birds have proved difficult toseparate. We designed experiments to feed only one sex to distinguishbetween intra- and intersexual selection in polygynous red-wingedblackbirds. We then tested whether choices made by females benefitfemales. Our experiments were conducted on a number of differentmarines, so we could evaluate whether females were choosingmales within marshes or whether they are choosing their breedingsituation primarily by making choices between different marshes.Females choosing between marshes strongly preferred marsheswith males that had supplemental food and experimentally increaseddisplay rates. At the marsh level, some male displays were predictorsof larger harem size and of greater nesting success. But whilefemale choice was very strong among marshes, we detected onlyweak female preference for individual male traits within marshes.These results suggest that problems of scale probably affectstudies of mate choice in avian breeding systems in ways that,to the best of our knowledge, have not previously been investigatedby behavioral ecologists     

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.     

SEXUAL DIMORPHISM,MATING SYSTEM AND BODY SIZE IN NEW WORLD BLACKBIRDS (ICTERINAE)

Although sexual selection is widely accepted as a primary functional cause of sexual size dimorphism in birds and mammals, results from some comparative studies have cast doubt on this conclusion. Chief among these contradictory results is the widespread association between body size and size dimorphism—large species tend to be more dimorphic than small species. This correlation is not directly predicted by the normal sexual selection scenario, and many hypotheses have been advanced to explain it. This paper reviews these hypotheses and evaluates them using data for the New World blackbirds (Icterinae). In this avian subfamily, (1) body size correlates with the intensity of sexual selection (as measured by mean harem size), and (2) size does not correlate with dimorphism if the effects of mating system are removed. Similar results are obtained when controlling for the confounding influence of phylogeny. Further, body size and mating system are associated with nesting dispersion. These results strongly argue that sexual dimorphism is a product of sexual selection in this subfamily, and suggest that either: (1) large body size itself, or the ecology of large species, promotes the development of coloniality and a polygynous mating system; or (2) polygyny and/or coloniality lead to the evolution of large size in both males and females. None of the other hypotheses examined predict an association between size and mating system, and all predict that size will correlate with dimorphism after the effects of mating system are removed. Thus, none of the other hypotheses seem applicable in this case. These results are compared to those obtained for other avian and mammalian taxa. Difficulties of analysis present in previous studies are discussed. I argue that it is inappropriate to assume that associations between a trait and body size or phylogeny are evidence of nonadaptive evolutionary “constraints.”     

Spatiotemporal variation in selection on body size in the dung fly Sepsis cynipsea

Standardized measures of the strength of selection on a character allow quantitative comparisons across populations in time and space. Spatiotemporal variation in selection influences patterns of adaptation and the evolution of characters and must therefore be documented. For the dung-breeding fly Sepsis cynipsea, we document patterns of variation in sexual, fecundity and larval and adult viability selection on body size at several spatiotemporal scales: between-populations, over the season, over the day and between dung pats. Adult viability selection based on residual physiological survivorship in the laboratory was nil or weakly negative. In contrast, larval viability selection in two laboratory environments was weakly positive for males at low competition and females at high competition. Fecundity selection was positive and strong at all times and in all populations. Sexual selection reflecting pairing success was overall strongly positive (about three times stronger than fecundity selection), while selection reflecting male reproductive success via the clutch size of his mate (i.e. assortative mating) was essentially nil. Only sexual selection varied significantly at coarse (between populations and seasonally) but not at fine (within a day or between pats on a pasture) spatial and temporal scales. Quadratic and correlational selection differentials were low and inconsistent in all episodes except for fecundity selection, where there was some evidence that clutch size reaches an asymptote at large body sizes, implying weaker selection for large size as females get bigger. Implications of these results for the evolution of body size and body size dimorphism are discussed.     

Reproductive consequences of bilateral asymmetry for individual male red-winged blackbirds

We used morphological and breeding data from a 2-year fieldstudy of red-winged blackbirds (Agelaius phoeniceus) to testthe hypothesis that males characterized by low levels of bilateralasymmetry (i.e., high developmental competence) realize a reproductiveadvantage. Specifically, we evaluated each of several distinctcomponents of male reproductive success relative to asymmetrymeasures made on five bilaterally paired characters. Resultsof a male removal experiment generally failed to support theprediction that symmetry would be associated with success incompetition for access to breeding territories: establishedterritory owners and nonterritorial replacement males were effectivelyindistinguishable in this regard. Similarly, there was no indicationthat symmetrical males were more likely to establish territoriesin high-quality marsh habitat than in marginal upland fieldhabitat. Finally, monitoring of breeding activity in high-qualityhabitat revealed that male symmetry was generally unrelatedto recruitment of social mates (i.e., harem size), the productivityof those mates (average female reproductive success), withinpairpaternity (assessed using DNA-based analysis of parentage),or extrapair mating success. Collectively these results indicatethat symmetry is not an important determinant of reproductivesuccess among individual male red-winged blackbirds. This observation,in combination with the results of several other recent investigations,suggests that the fitness consequences of subtle departuresfrom perfect symmetry may be less significant and/or less ubiquitousthan initially suggested.     

Sex-limited expression of ornamental feathers in birds

Extravagant secondary sexual characters show sexual size dimorphismin some species but are completely sex limited in others. Sexualornamentation has been hypothesized to benefit mainly malesthrough sexual selection, but the costs of secondary sexualcharacters initially would be experienced by both sexes. Theevolution of sexual size dimorphism of ornaments and, eventually,the complete sex-limited expression of these characters, willdepend on the effects of sexual and natural selection on thetwo sexes. A phylogenetic analysis controlling for similaritiesdue to common ancestry of 60 independent evolutionary originsof feather ornamentation in birds was used to investigate ecologicalfactors correlated with sexual size dimorphism and sex-limitedexpression of secondary sexual characters. When the size ofan ornament is large relative to body size, the trait willbe particularly costly for females, resulting in selectionfor increased sexual size dimorphism of the ornament. Indeed,sexual size dimorphism of ornaments was positively relatedto the relative size of male ornaments but was unrelated torelative size of female ornaments. Species with polygynousand lekking mating systems with little or no male parentalcare (in particular nest building and incubation) demonstratedsex-limited expression of ornaments as compared to monogamousspecies. Species with no food provisioning of offspring by themale showed a trend for increased sexual size dimorphism ofornaments. Therefore, large natural selection costs duringreproduction imposed by the expression of secondary sexualcharacters are related to the evolution of sexual size dimorphismof ornaments and eventually their complete loss from females.     

Macroevolutionary patterns of bumblebee body size: detecting the interplay between natural and sexual selection

Bumblebees and other eusocial bees offer a unique opportunity to analyze the evolution of body size differences between sexes. The workers, being sterile females, are not subject to selection for reproductive function and thus provide a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other natural selection. Using a phylogenetic comparative approach, we explored the allometric relationships among queens, males, and workers in 70 species of bumblebees (Bombus sp.). We found hyperallometry in thorax width for males relative to workers, indicating greater evolutionary divergence of body size in males than in sterile females. This is consistent with the hypothesis that selection for reproductive function, most probably sexual selection, has caused divergence in male size among species. The slope for males on workers was significantly steeper than that for queens on workers and the latter did not depart from isometry, providing further evidence of greater evolutionary divergence in male size than female size, and no evidence that reproductive selection has accelerated divergence of females. We did not detect significant hyperallometry when male size was regressed directly on queen size and our results thus add the genus Bombus to the increasing list of clades that have female-larger sexual size dimorphism and do not conform to Rensch's rule when analyzed according to standard methodology. Nevertheless, by using worker size as a common control, we were able to demonstrate that bumblee species do show the evolutionary pattern underlying Rensch's rule, that being correlated evolution of body size in males and females, but with greater evolutionary divergence in males.     

Directional changes in sexual size dimorphism in shorebirds, gulls and alcids

The Charadrii (shorebirds, gulls and alcids) are one of the most diverse avian groups from the point of view of sexual size dimorphism, exhibiting extremes in both male-biased and female-biased dimorphism, as well as monomorphism. In this study we use phylogenetic comparative analyses to investigate how size dimorphism has changed over evolutionary time, distinguishing between changes that have occurred in females and in males. Independent contrasts analyses show that both body mass and wing length have been more variable in males than in females. Directional analyses show that male-biased dimorphism has increased after inferred transitions towards more polygynous mating systems. There have been analogous increases in female-biased dimorphism after transitions towards more socially polyandrous mating systems. Changes in dimorphism in both directions are attributable to male body size changing more than female body size. We suggest that this might be because females are under stronger natural selection constraints related to fecundity. Taken together, our results suggest that the observed variation in dimorphism of Charadrii can be best explained by male body size responding more sensitively to variable sexual selection than female body size.