The strength of a female mate preference increases with predation risk.

When females search for mates and their perceived risk of predation increases, they less often express preferences for males that use conspicuous courtship signals, relaxing sexual selection on production of these signals. Here, we report an apparent exception to this general pattern. Courting male fiddler crabs Uca beebei sometimes build pillars of mud at the openings to their burrows in which crabs mate. Females visit several males before they choose a mate by staying and breeding in their burrows, and they preferentially visit males with pillars. Previous studies suggested that this preference is based on a visual orientation behaviour that may reduce females' risk of predation while searching for a mate. We tested this idea by determining whether the female preference for males with pillars increases with perceived predation risk. We attracted avian predators to where crabs were courting and measured the rates that sexually receptive females visited courting males with and without mud pillars. Under elevated risk, females continued to search for mates and they showed a stronger relative preference for males with pillars. Thus, when predation risk is high, females may continue to express preferences that are under natural selection because they help females avoid predation, strengthening sexual selection for use of the preferred signal.