The evolution of the syrinx: An acoustic theory

The unique avian vocal organ, the syrinx, is located at the caudal end of the trachea. Although a larynx is also present at the opposite end, birds phonate only with the syrinx. Why only birds evolved a novel sound source at this location remains unknown, and hypotheses about its origin are largely untested. Here, we test the hypothesis that the syrinx constitutes a biomechanical advantage for sound production over the larynx with combined theoretical and experimental approaches. We investigated whether the position of a sound source within the respiratory tract affects acoustic features of the vocal output, including fundamental frequency and efficiency of conversion from aerodynamic energy to sound. Theoretical data and measurements in three bird species suggest that sound frequency is influenced by the interaction between sound source and vocal tract. A physical model and a computational simulation also indicate that a sound source in a syringeal position produces sound with greater efficiency. Interestingly, the interactions between sound source and vocal tract differed between species, suggesting that the syringeal sound source is optimized for its position in the respiratory tract. These results provide compelling evidence that strong selective pressures for high vocal efficiency may have been a major driving force in the evolution of the syrinx. The longer trachea of birds compared to other tetrapods made them likely predisposed for the evolution of a syrinx. A long vocal tract downstream from the sound source improves efficiency by facilitating the tuning between fundamental frequency and the first vocal tract resonance.

鸟类独有的发声器官——鸣管（syrinx），坐落于气管的尾端。尽管鸟类的喉头（larynx）同样存在于气管的另一端，但鸟类仅通过鸣管完成发声。为何仅鸟类演化出了位于该位置的新型发声结构，目前仍无定论，相关起源假说大多尚未得到实验验证。本研究采用理论与实验相结合的研究策略，对“鸣管相较于喉头具备发声层面的生物力学优势”这一假说进行验证。我们探究了发声源在呼吸道内的位置是否会影响发声输出的声学特征，包括基频以及气动能量向声能转换的效率。针对三个鸟类物种的理论分析与实测数据显示，发声频率受发声源与声道的相互作用调控。物理模型与计算模拟结果同样表明，位于鸣管位置的发声源可产生更高的发声效率。值得关注的是，不同物种间发声源与声道的相互作用模式存在差异，这提示鸣管发声源已针对其在呼吸道内的具体位置实现了优化适配。上述研究结果提供了确凿的证据，表明对高发声效率的强烈选择压力，或是推动鸣管演化的核心驱动力。相较于其他四足动物（tetrapods），鸟类拥有更长的气管，这使其天生具备演化出鸣管的潜在基础。位于发声源下游的长声道，可通过优化基频与第一声道共振峰之间的调谐关系，进一步提升发声效率。