Graded Aggressive Calls in the African Painted Reed Frog Hyperolius marmoratus (Hyperoliidae)

The African painted reed frog, Hyperolius marmoratus, has a potentially complex communication system. Advertisement calls and aggressive calls, although distinct from each other, are in fact two ends of a continuum of graded calls. Playback experiments using standard advertisement calls showed that males increased the proportion of aggressive calls as the stimulus intensity was increased. In addition, three characteristics of the aggressive calls changed in response to higher playback levels. Males increased the number of pulses/call, increased call duration, and decreased dominant frequency. Aggressive calling occurred primarily during the early hours of the night, with considerable overlap with times when females were searching for mates in the chorus. Females tested in two-choice arena trials discriminated against aggressive calls in favor of advertisement calls. It is suggested that aggressive calls reduce a male's ability to attract a female and that a graded signalling system may enable males to escalate agonistic encounters with other males without rendering calls completely unattractive to females.