Variable female preferences and the evolution of complex male displays in satin bowerbirds (Ptilonorhynchus violaceus)

Abstract

Models of sexual selection suggest that females benefit from assessing male display traits to choose their mates, but little is known about how individual females use particular male traits in mate choice. Here I show age-specific use of male display traits by female satin bowerbirds. Male satin bowerbirds build specialized stick structures - bowers - for courtship and copulation, and decorate their bowers with objects collected from the environment. When a female arrives at a bower for courtship, males produce intense behavioral displays that can threaten females. Using a decoration augmentation experiment, I find that young females emphasize blue decorations in mate choice decisions, while old females emphasize male display intensity. These variable preferences support a novel hypothesis for the evolution of multiple male display traits. I find that age-specific preferences reflect age-related differences in female tolerance for intense male displays: young females do not tolerate high-intensity displays and are frequently startled during courtship, while old females actively solicit high-intensity displays, and are rarely startled by these displays. I find that the presence of blue decorations calms females, especially young females, though the reason for the calming effect remains unclear. I find age-related improvement in females' abilities to discriminate among males in mate choice resulting in young females choosing males with higher parasite loads than those males chosen by old females. Finally, I find that juvenile males assess the quality of adult male tutors' displays when deciding which adult males to visit for male-male courtship - the behavioral context in which young males learn their displays. This finding supports a novel hypothesis for display trait learning, and shows that juvenile males have evolved preferences used in tutor choice that parallel female mate choice preferences. Overall, my work shows that (i) multiple male display traits may evolve via variable female preferences, (ii) variable female preferences may reflect interactions between female psychology and the male display traits available for assessment, and (iii) preferences for exaggerated male display traits may be shared by both sexes, but expressed in sex-specific roles.