Combat, display and ritualization in Fiddler Crabs (Ocypodidae, genus Uca )

Abstract

The principal elements of fighting and display in the genus Uca are surveyed from the point-of-view of their apparent evolutionary origins. The components include combat behaviour between males, threat postures, acoustic signals and visual displays. The latter are characterized by rhythmic motions of the large cheliped and other appendages. Combat between males is highly ritualized, with morphological and behavioural deterrents to maximum intensity. When the deterrents are effective the fights are usually without noticeable results. Although serious injury virtually never occurs, when the deterrents are inadequate the loser sometimes gives up his burrow and occasionally does not court for varying periods thereafter. Combat seems to have evolved directly from the decapod motion of grasping combined at low intensities with an appeasement element in which the major cheliped—a releaser of aggressive behaviour—is turned away from the opponent. Threat postures are primarily intention motions of fighting. Both in the burrows and occasionally on the surface stridulation and other acoustic signals are used in threat, courtship or both. Unlike combat behaviour and threat postures, visual display is species-specific. The twenty-odd elements most often occurring in both acoustic and visual display seem clearly to be derived chiefly from feeding, cleaning and threat movements, usually through the intermediary of displacement activities; sometimes the display elements apparently evolved from conflict between feeding and threat tendencies and sometimes from intention motions. Even in species with the most advanced displays, ritualization of some elements is often only partly or temporarily achieved, while the corresponding displacement motion, unaltered and uncomplicated, is frequently elicited. Parallelisms are evident between the courses of evolution in the social behaviour of fiddler crabs and vertebrates.