Aggressive Waves in the Lemon-Clawed Fiddler Crab (Uca perplexa): A Regional “Dialect” in Fiji

A population of the lemon-clawed fiddler crab (U. perplexa) in Fiji (island of Vanua Levu) was studied for types of communication (i.e., signaling via waving the male’s larger claw). Two types of signals were observed. In addition to the expected territorial display of a large and complex vertical wave that conveys its message over a typical distance of 10–40？cm (with large males signaling to other large males over the greatest distance), a short, rapid, and horizontal wave was typically directed over a much shorter distance, rarely exceeding 10？cm. This latter wave type, seemingly of an aggressive nature, differs from the vertically directed aggressive signal observed in an Australian population of this species and thus appears to be a regional “dialect” for this mode of communication. 1. Introduction Animal communication signals are crucial to many species, as they may convey information to conspecifics about the identity, location, and motivation of the sender. This information is often valuable for reproductive success or survival. In the genus Uca (fiddler crabs), the male’s enlarged claw is waved in order to attract females for mating, although it may also be used in territorial and agonistic displays and may be exhibited in the absence of a target. Different species have different patterns of claw waving [1, 2], helping females to find conspecific males [3, 4]. Waves have been classified as “vertical” and “lateral” [1]. Vertical waves are described as simple lifts of the claw with little horizontal movement, while lateral waves include complicated lateral components (extending and flexing), in addition to vertical components, generally accompanied by walking leg and body movements. Since waving can have two very different functions, as a signal for mating or for conflict, the purpose of a given waving signal may be uncertain and confusing to both crustacean and human observers. The courtship waving pattern of the lemon-clawed fiddler crab (U. perplexa) in Japan and Australia has been described [1, 5–7]. The typical wave, which has been termed a “lateral” wave, involves extending the claw, then raising it, and then lowering it rapidly to the resting position, taking 1–1.5 seconds. At the same time, the minor claw and front walking legs are also raised and lowered along with the major claw. This is a conspicuous signal that can be detected at a considerable distance, and it has been associated with courtship [1, 5, 8, 9]. How et al. [6] described a second, totally different, type of wave in Australian U. perplexa, a vertical wave in which the claw