No preference for exaggerated courtship signals in a sensory trap

Abstract

Courting male fiddler crabs, Uca terpsichores, construct unusually large sand hoods at the entrances to their burrows to which they attract females for mating. Females sequentially visit several courting males before they stay with one and mate in his burrow, and they preferentially approach burrows with hoods. While moving between burrows, crabs are at risk of predation by birds and they sometimes run to objects that provide temporary cover. Thus, the female preference for burrows with hoods may help females to avoid predators. Could selection for predator avoidance produce a directional preference for especially large hoods? To examine this possibility, we made multiple replicas of two kinds of hood models with exaggerated dimensions (super models and wall models) and a single model with average dimensions (average models). Super models were four standard deviations taller and two standard deviations wider than average-size natural hoods. Wall models were of average height but near the maximum width of natural hoods; when males 'overbuild', they construct wall-like hoods. We replaced males' hoods with these models and measured their effects on male attractiveness. Males with exaggerated models neither encountered nor attracted more females than did males with average models or natural hoods. Females did not show a directional preference for larger hoods. The attractiveness of hoods may plateau with increasing size because discrimination between average and larger hoods may result in fatal hesitation, preventing the evolution of a directional preference for an exaggerated form of this courtship signal. Males build hoods from pure sand, so males may build unusually large hoods because they are more durable, not because they are better signals.