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

Abstract

Males of the fiddler crab Uca musica sometimes build sand hoods at the entrances of their burrows, to which they attract females for mating with claw waving and other displays. Females significantly more often approached males with hoods than males without hoods, but once at a burrow, they were just as likely to stay and mate whether the male had a hood or not. To determine how hoods affect male attractiveness, we conducted experiments that controlled for other differences in courtship behavior between builders and nonbuilders; we removed hood builders’ hoods and we added hood models to nonbuilders’ burrows. We then measured the attractiveness of hood builders and nonbuilders with and without hoods. Neither manipulation measurably affected male courtship behavior. The presence of a hood did not increase male–female encounter rates, suggesting that hoods do not attract distant females into a male’s courtship range. However, once a male courted a female, she was significantly more likely to approach if he had a real or model hood. We obtained direct evidence that females orient to hoods by replacing them with hood models positioned about 3 cm away from the openings to males’ burrows. Females approached the models, not the courting males, about 27% of the time. We conclude that hood building is sexually selected because courted females differentially approach hoods, not because hoods attract distant females and not because females prefer to mate with hood builders.