Sexual selection for structure building by courting male fiddler crabs: an experimental study of behavioral mechanisms

Males of the fiddler crab Uca musica sometimes build sand hoodsat the entrances of their burrows, to which they attract femalesfor mating with claw waving and other displays. Females significantlymore often approached males with hoods than males without hoods,but once at a burrow, they were just as likely to stay andmate whether the male had a hood or not. To determine how hoodsaffect male attractiveness, we conducted experiments that controlledfor other differences in courtship behavior between buildersand nonbuilders; we removed hood builders' hoods and we addedhood models to nonbuilders' burrows. We then measured the attractivenessof hood builders and nonbuilders with and without hoods. Neithermanipulation measurably affected male courtship behavior. Thepresence of a hood did not increase male—female encounterrates, suggesting that hoods do not attract distant femalesinto a male's courtship range. However, once a male courteda female, she was significantly more likely to approach ifhe had a real or model hood. We obtained direct evidence thatfemales orient to hoods by replacing them with hood modelspositioned about 3 cm away from the openings to males' burrows.Females approached the models, not the courting males, about27% of the time. We conclude that hood building is sexuallyselected because courted females differentially approach hoods,not because hoods attract distant females and not because femalesprefer to mate with hood builders.