Wind- and rain-induced vibrations impose different selection pressures on multimodal signaling

The world is a noisy place, and animals have evolved a myriad of strategies to communicate in it. Animal communication signals are, however, often multimodal; their components can be processed by multiple sensory systems, and noise can thus affect signal components across different modalities. We studied the effect of environmental noise on multimodal communication in the túngara frog (Physalaemus pustulosus). Males communicate with rivals using airborne sounds combined with call-induced water ripples. We tested males under control as well as noisy conditions in which we mimicked rain- and wind-induced vibrations on the water surface. Males responded more strongly to a multimodal playback in which sound and ripples were combined, compared to a unimodal sound-only playback, but only in the absence of rain and wind. Under windy conditions, males decreased their response to the multimodal playback, suggesting that wind noise interferes with the detection of rival ripples. Under rainy conditions, males increased their response, irrespective of signal playback, suggesting that different noise sources can have different impacts on communication. Our findings show that noise in an additional sensory channel can affect multimodal signal perception and thereby drive signal evolution, but not always in the expected direction.

自然界本就充斥着各类噪声，动物已演化出多种多样的通信策略以适应这一环境。然而，动物的通信信号往往属于多模态信号（multimodal signal）：其组成成分可被多套感知系统处理，因此噪声可对不同模态下的信号成分产生影响。我们以泡蟾（*Physalaemus pustulosus*）为研究对象，探究了环境噪声对其多模态通信的影响。雄性泡蟾会结合空气传播的鸣叫声与鸣响诱发的水面波纹，与同类雄性竞争者进行通信。我们分别在对照条件与模拟降雨、风力诱发的水面振动噪声的条件下，对雄性泡蟾进行了测试。相较于仅播放声信号的单模态信号（unimodal signal）回放刺激，雄性泡蟾对结合了鸣叫声与水面波纹的多模态回放刺激的响应更强，但该现象仅在无降雨与风力的对照条件下出现。在风力噪声条件下，雄性泡蟾对多模态回放刺激的响应有所降低，这表明风力噪声会干扰其对同类竞争者产生的水面波纹的感知。而在降雨噪声条件下，无论回放的刺激类型如何，雄性泡蟾的响应均有所提升，这说明不同来源的噪声对动物通信的影响存在差异。本研究结果表明，额外感知通道中的噪声可影响多模态信号的感知过程，进而推动信号演化，但该影响并非总是符合预期方向。