Vocal Communication in a Neotropical Treefrog, Hyla Ebraccata: Aggressive Calls

We studied the aggressive calls of Hyla ebraccata in Panama by recording natural interactions in the field and by playing recordings of natural and synthetic calls to males. Aggressive calls of this species had higher pulse repetition rates than advertisement calls and often were much longer. Aggressive calls also were much more variable in introductory note duration, pulse rate, rise time, and the number and structure of click notes. Pulse repetition rate was negatively correlated with the duration of the introductory note, as were the number of click notes in the call and the number of pulses in each click. In natural encounters, the duration of introductory notes increased as the distance between interacting males decreased. Males gave aggressive calls in response to playbacks of both advertisement and aggressive calls, but aggressive calls were much more effective in eliciting such responses. The proportion of aggressive calls and the duration of introductory notes generally increased with increasing playback intensity. Playbacks of synthetic calls which varied in pulse repetition rate and rise time showed that a high pulse repetition rate was the most important feature in eliciting aggressive responses from males, but rise time also was important when calls had fast pulse rates. The aggressive calls of this species appear to be a graded signaling system, the first to be documented in amphibians.