Differences in context and function of two distinct waving displays in the fiddler crab, Uca perplexa (Decapoda: Ocypodidae)

To respond appropriately to communication signals, animals must have the ability to decipher signal meaning. At a basic level, interpreting the difference between territorial and courtship signals can be vital for the survival and reproduction of social animals. Male and female fiddler crabs communicate with claw-waving displays, but the function of these waves remains uncertain. Species differ in the context in which they wave: Some wave during courtship, some during territorial defence and some during both. In this paper, we provide evidence that males of an Australian species of fiddler crab, Uca perplexa, use two different types of claw waving display, lateral and vertical. Lateral waves are employed solely in a courtship context, whilst vertical waves are employed during courtship as well as territorial interactions. Using video recordings, we show that lateral waves were produced spontaneously (broadcast), and their frequency increased only in the presence of female wanderers. Vertical waves were not broadcast but were elicited by male wanderers during agonistic interactions and female wanderers during close range courtship. Male resident U. perplexa were able to discriminate the sex of wandering crabs on the mudflat at distances of 32 cm. During all resident–wanderer interactions, residents attempted to maintain a position directly between the wanderer and the home burrow and orient themselves to face females and to present the major claw towards males. To our knowledge, this is the first demonstration of the multiple use of waving displays in a fiddler crab species.