Fishing For The Evolutionary Bases Of Female Ornamentation

Abstract

Sexual dimorphism, or phenotypic differentiation of the sexes, is widespread amongst animals. It is understood mainly in the context of sexual selection, i.e. selection on mating success, with such selection typically acting more strongly on males. In some cases however, conspicuous ornaments similar to those observed in males are expressed in females—but the causes for their evolution have often been elusive. In my dissertation I examine the behavioral, hormonal, and genetic correlates of female throat coloration—a male typical trait—in the threespine stickleback fish (Gasterosteus aculeatus), to test whether the evolution of the female trait might be attributed to adaptive or nonadaptive mechanisms. My results indicate that while male-typical throat color in females is associated with some components of fitness, e.g. body size and growth, it does not reflect reproductive readiness or body condition. Also, red-throated females are not socially more dominant or aggressive than dull-throated females in either dyadic or socially complex contexts. Further, the masculinized female trait is not associated with variation in fish-specific androgen mechanisms, 11-ketotestosterone and its cognate receptor, suggesting that the female trait is not under the principal male-specific endocrine control. Investigating the genetic basis of throat coloration I find that that the same genomic regions are associated with the presence of red throat coloration in both sexes, suggesting that female throat coloration might be a genetically correlated byproduct of selection on males. In sum, using an interdisciplinary approach to studying the evolutionary causes of female ornaments, I find that the red throat in female sticklebacks is potentially a nonadaptive trait that has arisen due to a genetic correlation between the sexes; thus female conspicuous traits do not necessarily result from adaptive processes related to sexual selection on females.