Attracting mates or suppressing rivals? Distance-dependent calling strategy in Hainan frilled treefrogs

Abstract

Acoustic communication is prevalent in a broad range of taxa, playing an important role in a variety of scenarios. Acoustic signals attenuate with distance, allowing individuals to assess proximity, which is one of the crucial factors affecting animal decision making in intraspecific competition. Female choice and male–male competition in anurans (frogs and toads) are highly dependent on acoustic signals, and numerous studies have shown that males adjust their call effort, call complexity or call type according to competitive contexts. However, there is less empirical work focused on the effects of spatial distance between calling males on the calling strategy. In the present study, we used playback experiments to investigate whether and how male Hainan frilled treefrogs, Kurixalus hainanus, adjust their calling strategies based on spatial distance. We found that compared to the spontaneous period, focal males produced significantly fewer advertisement calls but more suppression calls and encounter calls when presented with conspecific advertisement calls, regardless of the distance from the simulated rival. We also found that the number and proportion of advertisement calls and corresponding note type increased with distance. In contrast, the number and proportion of suppression calls, encounter calls and their corresponding note types decreased with distance. Interestingly, the descending trends of these two types of aggressive calls were not quite the same. These findings suggest that male K. hainanus can alter call types and their frequency of occurrence based on the distance from a calling rival, demonstrating a graded aggressive interaction. Our study provides evidence for a distance-dependent competitive strategy in vocal interaction and contributes to a better understanding of the reproductive trade-off in male–male competition.