Function of the Four Types of Waving Display in Uca lactea: Effects of Audience, Sand Structure, and Body Size

Multiple signals that convey different messages have been reported in many taxa, but relatively few studies have been made on such signals in invertebrates. In the present study, I investigated four types of claw-waving display used in the fiddler crab Uca lactea to test whether the displays have different functions. Three males with a sand structure beside their burrows (which can attract females) and three males without a sand structure were fenced in an opaque enclosure, and I videotaped their waving displays after releasing two burrowless males or two burrowless females to test the effects of audiences. (a) Lateral-circular waving tended to occur in enclosures with burrowless females and was performed frequently by males that had sand structures. (b) Lateral-flick waving was performed frequently by males without sand structures, and its frequency was positively correlated with the signaler's body size. (c) Rapid-vertical waving was observed frequently in enclosures with burrowless males, and its frequency was negatively correlated with the signaler's body size. (d) Circular waving tended to occur in enclosures with burrowless females and was performed frequently by males that had sand structures, and its frequency was positively correlated with the signaler's body size. In my previous study, lateral-circular waving was often seen in the breeding season and was mostly performed to female audiences, lateral-flick waving was frequently performed to neighboring resident males, rapid-vertical waving was performed mainly to intruding burrowless males, and circular waving did not have apparent audiences in most cases. Finally, I concluded that lateral-circular waving was used as a courtship display, lateral-flick waving was related to border disputes, rapid-vertical waving was used for burrow guarding, and circular waving was used to broadcast the signaler's general quality.