We describe the vocal repertoire of the Asian cricket frog Rana nicobariensis for the first time. Three structurally-distinct call types exist: advertisement calls, aggressive calls and encounter calls. Compound calls consisting of both advertisement and aggressive elements were also recorded. A remarkable feature of communication in this species is the highly variable advertisement call which shows a 20-fold variation in duration, comprising 1-25 notes. In natural choruses, the duration of the advertisement call is inversely related to the distance between a focal male and its closest calling neighbour. In playback experiments, males clearly responded to stimuli containing different numbers of notes by adjusting the number of notes emitted. However, they did not match the stimulus precisely, and were inhibited from calling by a stimulus consisting of a continuous train of notes. Different proportions of each call type were recorded during playbacks at different distances; aggressive and encounter calls were increasingly emitted at closer distances. Our results suggest that a graded communication system has evolved in Rana nicobariensis, in which the best call for a male to use at any moment depends on the type of calls being emitted by other males in the chorus.
Rana nicobariensis, Malaysia, acoustic communication, graded call variation