Harsh vocal elements affect counter-singing dynamics in male rock hyrax

Morton's motivation-structural (MS) rules suggest that the acoustic structure of a signal can reflect the caller's motivation and internal state. In many species, low-frequency and noisy (harsh) sounds have been found to comply with MS rules, accompanying agonistic interactions and functioning as a badge of aggression and dominance. Male rock hyraxes (Procavia capensis) often produce long and complex advertisement calls (songs) both "spontaneously" and in counter-singing sessions with other males. Hyrax songs include a "snort" vocal element, which is a harsh sound produced only by mature, dominant males. We predicted that the number of snort elements in the song would affect the dynamics of male hyrax counter-singing. We performed 3 series of playback experiments of natural and artificially manipulated songs on a wild hyrax population. We found that the probability of initiating counter-singing by nearby males increased together with the number of snorts in the stimulus song. Furthermore, the receivers replied to the synthetic "snort-only" vocal sequences at an equal rate as to their origin song as long as the snort elements maintained their original position within the signal. Our findings suggest that the snort component is one of the main information transfer channels in male hyrax singing and can elicit conspecific singing even when isolated from other vocal elements. In addition, the position of snort elements (their temporal pattern and rhythm) bears a possible significance in keeping the overall signal meaningful. Finally, our findings support previous claims that harsh sounds constitute one of the key components in vocal communication.