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.
Bibliographical noteFunding Information:
We thank the staff of Lien-Hua-Chih Research Centre of the Taiwan Forestry Research Institute for providing accommodations and permitting us to conduct this study and Yi-Huey Chen, Ting-Yun Stella Huang, Yi-Rou Li and Cheng-Chi Wei for field assistance. Y.-C.K. was supported with a grant from National Science Council (NSC 101-2621-B-029-002-MY3). M.A.B. was funded by a grant-in-aid of research from the University of Minnesota Graduate School and by a Land-Grant Professorship from the McKnight Foundation.
- Rana adenopleura
- acoustic communication
- individual discrimination
- individual recognition
- male–male competition
- social category