acase@tulane.edu / A comprehensive understanding of sexual dichromatism and sexual selection depends on understanding selective pressures on females, which may differ from those experienced by males. Conventional theory suggests that ornamentation in females evolves as the byproduct of selection pressures on males, and is non-adaptive. My dissertation challenges this assumption through a series of linked studies related to female ornamentation in a species of tropical passerine bird, the White-shouldered Fairywren (Malurus alboscapulatus), of New Guinea. The White-shouldered Fairywren is ideally suited to evaluate the evolution of female ornamentation, because populations are characterized by divergence in female plumage coloration from brown (unornamented) to black-and-white (ornamented), with no variation in males, which are uniformly black- and-white. My thesis research employed field-based observation and experimentation with contemporary genomic, endocrine, and microscopy techniques to identify proximate mechanisms, current adaptive function, and evolutionary history of female ornamentation in this system. / 1 / Erik Enbody

Ornamental traits in male birds have been the subject of much research effort, and sexual selection is recognized as the leading explanation for their evolution. The expression of ornamental traits in females has received little study until recent decades. Female colouration has been considered a non-adaptive, correlated response to selection on males. However, models predict that male mate choice, female competition, and the evolution of honest signals could help explain female ornamentation, especially where male investment in offspring and variation in female quality are high. I investigated this in the American robin (Turdus migratorius), a socially monogamous species with bi-parental care and variable female ornamentation. Female robins display conspicuous red breast plumage, bright yellow bills, and achromatic ornamentation. Female ornamentation is similar to males, but is subdued to varying degrees across individuals. Female colouration could function as a useful criterion in mate selection by males if it is correlated with aspects of female quality important to producing viable offspring. I assessed whether female ornamentation in robins might act as an honest signal by relating variation in female colour to measures of individual quality and reproductive investment. To assess ornamentation, I took colour measurements of the bill, crown and breast of male and female robins in the field using reflectance spectrometry. Female bill, breast and crown traits reliably predicted age, crown and bill colour traits were related to better body condition, and bill colour decreased seasonally as well as with ectoparasite load. I found evidence of assortative mating based on crown UV reflectance and bill colour. To assess reproductive investment, I measured egg size, yolk proportion, and deposition of yolk testosterone and carotenoids. Females with brighter (lighter) carotenoid-based bill colour laid larger eggs, and females with yellower bills laid eggs with higher yolk proportions and more total yolk carotenoids. Yolk testosterone level was associated with redder female breasts. These results support the hypothesis that female colour may be a reliable indicator of individual quality and capacity for reproductive investment. / Thesis (Master, Biology) -- Queen's University, 2014-01-30 15:09:57.797

Elaborate morphological ornaments can evolve if they increase the reproductive success of the bearer during competition for mates. However, ornament evolution is incredibly rare in females, and the type and intensity of selection required to develop female-specific ornamentation is poorly understood. The main goals of my thesis are to clarify the relationship between the type and intensity of sexual selection that drives the evolution of female ornamentation, and investigate alternative hypotheses that might be limiting or contributing to the development of female ornaments. I investigated the ecology and evolution of female-specific ornaments within and between species of dance flies from the subfamily Empidinae (Diptera: Empididae). The dance flies display incredible mating system diversity including those with elaborate female-specific ornaments, lek-like mating swarms, aerial copulation and nuptial gift giving. To elucidate the form of sexual selection involved in female-ornament evolution, I experimentally investigated the role of sexual conflict in the evolution of multiple female- specific ornaments in the species Rhamphomyia longicauda. Through manipulative field experiments, I found that variation in the attractiveness of two ornaments displayed by females indicates that sexual conflict, causing a coevolutionary arms race, is an important force in the evolution of multiple extravagant female ornaments. Using R. longicauda again, I tested for a role of functional load-lifting constraints on the aerial mating ability of males who paired with females displaying multiple large ornaments. I found no evidence of functional constraints influencing the mating opportunities of elaborately ornate females, but instead discovered a relationship consistent with positive assortative mating for mass. Biased sex ratios are predicted to increase the intensity of sexual selection in a population, which in turn, is predicted to influence the evolution of ornamentation. I measured the incidence and prevalence of vertically transmitted symbiotic bacteria that has been observed to distort the sex ratio in other Dipteran hosts. While my survey revealed that symbionts occur at high incidence and variable prevalence across dance fly hosts, I found no effect of symbiont infection levels on population sex ratios, or female- specific ornament evolution. Further investigation into the relationship between sex ratios and female-ornament evolution using the comparative method revealed that the operational sex ratio (OSR) of a population did not predict continuous measures of female ornamentation across species. However, female-ornament evolution did predict male relative testis investment across species indicating that female ornaments likely indicate increased levels of polyandry. My thesis reveals that sexual selection theory developed to describe male-specific ornament evolution cannot readily be translated to apply to females. I show that male mate choice, rather than functional constraints or ecological associations with bacteria, is likely driving the evolution of female-specific ornaments. I also identify sexual conflict as an important selective force in the evolution of female-specific ornaments.

Sexual selection was defined by Darwin in 1871 as selection acting solely on reproduction success. It is known to act on males resulting in extravagant ornamentations or other attributes, but in recent years more studies have shown that sexual selection also act on females. There is empirical evidence in several taxa that the secondary sexual ornament also acts as a measurement on the females’ reproductive quality, it is an honest trait. In Gallus gallus the comb has been found to be an honest ornament. Quantitative Trait Loci have been found on chromosome 1 and 3 for comb- and egg size. The honesty of the comb is hypothesized to be caused by either a pleiotropic effect gene linkage. In this study an 8th generation Advanced Interline Cross was used to guarantee maximum recombination of alleles to observe phenotypic effects. 177 females were detained during 4 weeks to measure fecundity. Egg number, mean egg weight and total egg size were correlated to comb size: length, area and weight. Correlation between comb size and total egg weight were found to be negative, suggesting the comb to be a dishonest signal. The phenotypic measurements observed in this study suggest that the genes for egg production and comb size are linked, and this linkage has been broken in the F8 analysed in this study.