Assessment of repeated displays: a test of possible mechanisms

Many animals signal their resource holding potential (RHP) to deter competitors from engaging them in potentially costly fights. Studies of this opponent assessment function have generated important insights into signal design and evolution. In the case of sounds, rate of production is often a salient feature. We used digital video playback to conduct analogous experiments exploring the importance of temporal variation in visual signals. Our study focused on the push-up display of male Jacky dragons Amphibolurus muricatus, an Australian agamid lizard. This stereotyped movement-based signal is commonly performed during male-male contests. A previous study has shown that Jacky dragons responses are influenced by the overall display rate of a video conspecific. We built upon this finding by investigating the effect of short-term variation in display rate. Each playback sequence varied systematically across a different combination of display parameters, while keeping the total number of push-ups constant. Other potential cues, such as morphology and the characteristics of individual motor patterns, were precisely controlled. The aggressive signalling and locomotor behaviour of subject males varied significantly between sequences. Most notably, performance of throat expansions, a typical agamid threat posture, was suppressed by video sequences with temporal clumping of displays. These results show that lizards are sensitive to differences in the temporal fine structure of display sequences and suggest that display concentration is an important assessment cue.