Aggressive responses to rivals depend on the interaction between the vocal traits of territory-holder and mimicked intruders in a miniature tropical frog

Territory defence is an important aspect of animal behaviour, and tightly linked to rival assessment and species recognition processes. An important component of these two processes is signalling, and animals must decide how to respond to a challenge based on the signals they perceive from their rivals. Here we manipulated the note repetition rate of rocket frog calls to simulate rivals with high and low vocal performances, and tested the aggressive responses of males in the field. We found that the probability of aggression depended on the interaction between the stimulus treatment and the peak frequency of the territorial males. Low-frequency calling males were more likely to respond aggressively to the fast call, while high-frequency calling males showed a higher probability of aggression towards the slow call. Frogs that responded aggressively approached the slow call 2.4-times faster than the fast call. Low-frequency calling males also increased their peak frequencies in response to the playbacks, while high-frequency calling males did not modify their vocalizations. Body weight was not correlated with the peak frequency or the inter-note interval of males. Peak frequency and amplitude (loudness) are positively interrelated in frogs, and are potential indicators of the motivational state of the signaller. We argue that low-frequency males were not singing at their maximum capacity, and they increased their vocal output when challenged by a rival, especially when the rival was a high-performance individual. We show that rocket frog aggression depends on the temporal parameters of the rival, but also on how territory-holders compare their own calls to those of challengers in terms of their frequency, and therefore demonstrate that frogs pay attention to different aspects of signals during aggressive decision-making.