Insight into the Evolution of Anuran Foot Flag Displays: A Comparative Study of Color and Kinematics

Understanding how complex animal displays evolve is a major goal of evolutionary organismal biology. Here, we study this topic by comparing convergently evolved gestural displays in two unrelated species of frog (Bornean Rock Frog, Staurois parvus, and Kottigehara Dancing Frog, Micrixalus kottigeharensis). This behavior, known as a foot flag, is produced when a male ?waves' his hindlimb at another male during bouts of competition for access to mates. We assess patterns of variation in the color of frog feet and the kinematics of the display itself to help pinpoint similarities and differences of the visual signal elements. We find clear species differences in the color of foot webbing, which is broadcast to receivers during specific phases of the display. Analyses of foot-trajectory duration and geometry also reveal clear species differences in display speed and shape - S. parvus generates a faster and more circular visual signal, while M. kottigeharensis generates a much slower and more elliptical one. These data are consistent with the notion that color, speed, and shape likely encode species identity. However, we also found that foot flag speed shows significant among-individual variation, particularly the phase of the display in which foot webbings are visible. This result is consistent with the idea that frogs alter temporal signal components, which may showcase individual condition, quality, or motivation. Overall, our comparative study helps elucidate the variability of foot flagging behavior in a manner that informs how we understand the design principles that underlie its function as a signal in intraspecific communication.