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.
Reflectance spectra of the foot webbings of Staurois parvus (n = 16, dashed line) and Micrixalus kottigeharensis (n = 13, solid line). Gray lines represent respective standard errors.
Foot movements of Staurois parvus (gray) and Micrixalus kottigeharensis (black) differ during respective foot flagging displays. Visual mean shape differences of the foot flag trajectory (A) and S. parvus and M. kottigeharensis occupy different areas in principal component (PC) space (B). Points represent individual foot flag trajectories. Shape approximations of PC loadings are plotted in the background.
Foot flagging trajectory shapes separate Staurois parvus (gray) and Micrixalus kottigeharensis (black). Scores of linear discriminant analysis (LDA) based on trajectory shapes (A). Particular principal components (PC) of shape information that best distinguish between S. parvus and M. kottigeharensis (B). The solid line and dotted line for each PC represent the upper and lower extremes of the PC axis, respectively. The overlap of the two lines show what shape information is loaded into each PC. The asterisk denotes significant differences (P < 0.05).
Frame-by-frame stills that show Micrixalus kottigeharensis (A) and Staurois parvus (B) performing a foot flag, while tracking foot trajectory (white line and blue dot) through space and time (1–10 stills). The red arrows indicate the direction of foot movement. Note that M. kottigeharensis faces directly toward the camera, where S. parvus faces directly away from the camera. Both points of view produce the same 2D shape of the foot flag signal as it is traced in the air (see Methods); thus, we provide both views to illustrate this point, even though we collected data from each species from both of these ‘front and back' positions.
Contributor Notes
Associate Editor: C. Bevier.