Territorial olive frogs display lower aggression towards neighbours than strangers based on individual vocal signatures

Some territorial animals display a form of social recognition in which they direct low levels of aggression towards established neighbours, but maintain greater readiness to respond aggressively towards unfamiliar individuals. In many taxa, such as songbirds, this so-called ‘dear enemy’ effect involves discrimination between neighbours and strangers based on individually distinctive vocal signatures. Although most anuran amphibians (frogs and toads) are highly vocal, and many are also territorial, we know very little about neighbour–stranger discrimination in this group. In the present study of the olive frog, Babina adenopleura (Ranidae), we show that the vocal signals of males are individually distinct, and that territory holders use this information to direct lower levels of aggression towards their nearby neighbours. Analyses of individual variation in advertisement calls revealed many individually distinctive spectral and temporal acoustic properties, with spectral properties contributing most towards statistical discrimination among individuals. In a field playback experiment that simulated territorial intrusions, territorial males had higher thresholds for producing aggressive calls in response to the advertisement calls of their nearby neighbours compared with those of strangers. A simple model based on sound attenuation due to spherical spreading estimated that males responded aggressively to strangers at distances that were approximately twice as far away as for neighbours and that were similar to intermale distances recorded in the field. Together, results from this study indicate that territorial male olive frogs develop vocally mediated dear enemy relationships with their nearby neighbours. These results highlight the potential for convergence in social recognition systems across diverse taxa.