Why signal if you are not attractive? Courtship synchrony in a fiddler crab

Synchronised male courtship signals are puzzling because males generally compete with each other for females. Male Austruca mjoebergi fiddler crabs wave in synchrony to attract females, but, all else being equal, females have a strong preference for ‘leader’ males that can produce waves before other males (‘followers’). So why do followers participate in synchrony? Here, we experimentally investigate three explanations for why followers might wave in synchrony: 1) followers obtain a small proportion of matings, 2) followers are more likely than a leader to attract females if they are positioned closer to her than is the leader, and 3) synchrony functions as a long-distance visual signal that attracts females so there is a net benefit to synchrony for all males. Using robotic male crabs, we found that females show a strong preference for leading males, but followers obtain a ‘better-than-nothing’ proportion of mates. We also showed that closer proximity of a follower to the female did not affect her preference for leaders, although being a leader increased a male’s success when he was further from the female than were rival males. Finally, females were more likely to approach a distant group if there was a leader present, suggesting that followers do benefit from participating in synchrony.

Methods
Data was collected in the field using wild-caught female Austruca mjoebergi fiddler crabs and robotic crabs mimicking male A. mjoebergi crabs.

For experiments 1-2, we conducted female mate choice trials and recorded female preferences for individual robotic male crabs based on their wave timing role (i.e. leader or follower).

For experiment 3, we recorded whether females approached a group of males based on 1) her distance from the group and 2) the wave rhythm of the group of males (synchrony, asynchrony or leader-follower).

Provided dataset is the raw data collected from the field. For more detailed experimental methods, please see the Methods section of the research article.