Vocal attributes of rivals and territory holders interact to drive aggression in a tropical frog

Territory defence is an important aspect of animal behaviour and tightly linked to rival assessment and species recognition. 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 created synthetic rocket frog, Silverstoneia flotator , calls to simulate rivals with exceptionally fast and slow note repetition rates relative to a note repetition rate limit present in natural calls, and tested the aggressive responses of males to these stimuli in the field. We found that aggressive males approached the slow call 2.4 times faster than the fast stimulus. The probability of aggression depended on the interaction between the stimulus treatment and the peak frequency of the territorial males relative to the stimulus. Low-frequency males were more likely to respond aggressively to the fast call, whereas high-frequency calling males showed a higher probability of aggression towards the slow stimulus. Males increased the peak frequency of their calls in response to the playbacks, but only if they called with lower values than the stimulus. Body weight was not correlated with the peak frequency or the internote interval, and therefore males could not assess the size of rivals through these variables. We argue that low-frequency males were not singing at their maximum capacity, and they increased their vocal output (that is, their frequency and likely also amplitude) when challenged by a fast-calling rival. High-frequency males, however, were likely close to or at their maximum vocal output. We showed that rocket frog aggression depends not only on the temporal parameters of the rival’s call but also on how territory holders compare their own calls to those of challengers in terms of their frequency. Rocket frogs, therefore, pay attention to different aspects of rival signals during aggressive decision-making. • We tested rocket frog aggression towards fast- and slow-synthetic calls. • The probability of aggression depended on the temporal structure of the stimulus. • Aggression also depended on frequency content of male calls relative to stimulus. • Low-frequency males increased frequency of their calls in response to the playback. • Neither frequency nor temporal parameters were associated with body weight.