Treefrogs adjust their acoustic signals in response to harmonics structure of intruder calls

Spectral properties of animal acoustic signals may help individuals to assess the characteristics of rivals and to adjust their competitive strategies in territorial disputes. Thus, we hypothesize that the distribution of energy across frequency bands in anuran calls determine behavioral responses in male-male competition. Using playback experiments, we investigated the relevance of the harmonic calls in the acoustic communication of the treefrog Dendropsophus minutus. We exposed territorial males to three synthetic acoustic stimuli composed of aggressive notes: (1) standard call (all harmonics and peak frequency corresponding to the second band); (2) inverted-energy call (all harmonics and peak frequency corresponding to the first band); and (3) concentrated-energy call (all energy contained in the second harmonic). Males responded aggressively to all stimuli, mainly by increasing the rate and duration of their aggressive notes. However, when exposed to stimuli with different harmonic configurations, males changed the harmonic structure of their own calls, emitting more A- and B-notes with peak power in the fundamental frequency, particularly when exposed to the concentrated-energy call. Our results suggest that male frogs may use the harmonic structure of calls to assess opponents and modulate territorial and aggressive behavior, triggering complex acoustic adjustments. This study contributes to our knowledge about the functions of acoustic traits in amphibian social interactions, and particularly of the presence of harmonics that has received less attention compared to other acoustic properties in the study of animal acoustic communication.动物声学信号的频谱特征，能够帮助个体评估竞争者的各项特质，并在领地争端中灵活调整自身竞争策略。据此，我们提出研究假说：无尾类（anuran）鸣叫声的跨频带能量分布格局，将决定雄性间竞争场景下的行为响应模式。 本研究采用声学回放实验，探究了小点滴树蛙（Dendropsophus minutus）声学通讯中谐波鸣唱的相关功能。我们将领地内的雄性个体暴露于三种由攻击性鸣唱音节构成的合成声学刺激物：1. 标准鸣唱（包含全部谐波，峰值频率对应第二频带）；2. 能量倒置鸣唱（包含全部谐波，峰值频率对应第一频带）；3. 能量集中鸣唱（全部能量集中于第二谐波）。 雄性个体对所有刺激均表现出攻击性响应，主要通过提升攻击性鸣唱音节的发放速率与时长来实现。但当接收不同谐波配置的刺激时，雄性个体自身鸣唱的谐波结构会发生改变，会发出更多基频处具有峰值功率的A、B音节，该现象在接收能量集中鸣唱刺激时尤为显著。 本研究结果表明，雄性蛙类可通过鸣唱的谐波结构评估对手，并调控领地行为与攻击性表现，进而触发复杂的声学调整行为。本研究有助于深化我们对两栖动物社会互动中声学特征功能的认知，尤其针对在动物声学通讯研究中相较其他声学属性关注度较低的谐波存在现象，具有重要的补充价值。