Context-dependent frequency alteration in a treefrog.

Acoustic communication is widespread across various taxa, playing crucial roles in intrasexual competition, social interaction, and territorial defense. For anurans (frogs and toads), female choice and male-male competition heavily rely on acoustic signals. It has been demonstrated that males of many species alter the spectral traits of their calls to enhance competitiveness during vocal interaction. However, how male frogs adjust the spectral traits of their calls in different competitive contexts remains unclear. Using playback experiments, we investigated how male Hainan frilled treefrogs (Kurixalus hainanus) adjust the dominant frequency of their advertisement calls in response to different call types from conspecifics. We found that males with higher dominant frequencies significantly decreased their frequency, whereas males with lower frequencies increased their frequency, with the greatest magnitude of frequency adjustment occurring during exposure to aggressive calls. Further analysis revealed that the dominant frequency during the playback of aggressive calls shifted closer to the population average compared to the spontaneous (no experimental manipulation) period. This pattern was not observed during the playback of advertisement or compound calls, indicating that males selectively alter spectral traits in response to perceived competition. These findings suggest that frequency adjustments facilitate the ability of male K. hainanus to maintain female attraction by suppressing their vocalizations. This study supports that male K. hainanus use frequency alteration to dishonestly signal body size and has an implication for understanding how male frogs enhance competitiveness during vocal interaction by adjusting the spectral traits of their calls.