Not all sexual monomorphisms evolve equally: female and male ornamentation in Tyranni passerines

Abstract

Female animals often have ornaments such as conspicuous coloration. However, the evolution of female ornamentation and its contribution to sexual dimorphism are understudied relative to male ornamentation. We addressed this knowledge gap by investigating plumage ornamentation of Tyranni (Suboscines). We hypothesized that greater ornamentation is associated with stronger sociosexual selection in females and males, and that sexual dimorphism emerges when selection pressures differ between sexes. We tested associations of ornamentation and sexual dimorphism with territoriality, mating system, parental care, nest type, migratory behavior, and climatic seasonality. We found that females and males show similarly high ornamentation (elaborate monomorphism) in territorial, non-migratory, monogamous species with biparental care, and species nesting in cavities. In contrast, males show higher ornamentation than females in migratory and polygynous species with female-only parental care and in more seasonal climates, though these cases were less common than elaborate monomorphism. Our findings suggest that when the sexes share sociosexual selection pressures, they show similar ornamentation, whether elaborate or inconspicuous. Conversely, sexual dimorphism is greater when males face stronger sociosexual selection pressures than females. Our study provides a framework for testing how sociosexual selection shapes female traits, which is key to understanding the evolution of sex differences and similarities.