Synchronous waving in a dotillid crab Ilyoplax pusilla: behavioral analyses using a robotic model

The waving behavior exhibited by some species of male mudflat-dwelling crabs is characterized by rhythmic claw movement, which is considered to be a form of courtship toward females and agonism toward males. A fascinating aspect of the waving behavior is that its rhythm is synchronized with that of the surrounding males. To elucidate the behavioral mechanism underlying the synchronization of the waving rhythm in the dotillid crab (Ilyoplax pusilla), we conducted a laboratory experiment using a waving-mimicking robotic model. When a single robot was operated continuously at a certain frequency (0.91-1.66 Hz) in front of a burrow-holding male, the crab responded to the robot with corresponding waving. In cases of relatively slow operating frequencies (≤ 1.11 Hz), the crabs tended to perform synchronous waving at a timing that preceded the robot. In contrast, when the operating frequency was relatively high (≥ 1.25 Hz), they waved synchronously with the robot with almost no time lag. The crabs tended not to wave at the same frequency as the high-frequency robot but waved once every two or three movements of the robot. These results suggest that synchronous waving without delay occurs in situations where the waving frequency increases competitively among neighboring males. The crabs may also adjust their waving frequency to half or one-third of that of high-performance opponents while maintaining their waving to avoid delays.