Attraction waves of male fiddler crabs: A visual display designed for efficacy

Animal signals can be very complex, particularly those used in mate attraction. They can contain multiple elements that have different functions- some based on signal efficacy and some on signal content. In this study, we show that the fiddler crab, Austruca lactea, produces a complex mating signal with three sequential wave types, each given in a different context. We suggest that the first two wave types are alerting signals that are designed for signal efficacy. They function to draw female attention before the male switches to the content-carrying signal that is used for female mate choice. We show that males produce ‘broadcast waves’ when no female is visible to them. These waves are given steadily, but at a slow rate, throughout the diurnal low tide mating period. They are effective at drawing the attention of the rare and unpredictable mate-searching females. Once a distant female becomes visible to the male, he switches to producing ‘directed waves’; they are given at a faster rate and their temporal distribution matches the abundance of mate-searching females. Both broadcast and directed waves appear to be designed for signal efficacy rather than signal content. They are alerting signals that draw female attention to the presence and location of the male. The third wave type is the courtship wave; it is given when the female is close to the male and is equivalent to the typical information-carrying mate choice wave found in other fiddler crab species.