Evolution of a multifunctional trait: shared effects of foraging ecology and thermoregulation on beak morphology, with consequences for song evolution

While morphological traits are often associated with multiple functions, it remains unclear how evolution balances the selective effects of different functions. Birds' beaks function in foraging, but also in thermoregulating and singing, among other behaviours. Studies of beak evolution abound, however most focus on a single function. Thus, we quantified relative contributions of different functions over an evolutionary time scale. We measured beak shape using geometric morphometrics and compared this trait to foraging behaviour, climatic variables, and song characteristics in a phylogenetic comparative study of an Australasian radiation of songbirds (Meliphagidae). We found that both climate and foraging behaviour were significantly correlated with beak shape and size. However, foraging ecology had a greater effect on shape, and climate had a greater effect on size. We also found that evolutionary changes in beak morphology had significant consequences for vocal performance: species with elongate-shaped beaks sang at higher frequencies, while species with large beaks sang at a slower pace. The evolution of the avian beak exemplifies how morphological traits can represent an evolutionary compromise among functions, and suggest that specialization along any functional axis may increase ecological divergence or reproductive isolation along others.尽管形态性状往往兼具多种功能，但目前学界仍未明确演化如何协调不同功能的选择效应。鸟类的喙不仅承担觅食功能，还可用于体温调节、鸣唱等多种行为活动。尽管有关喙演化的研究数量众多，但绝大多数仅聚焦于单一功能维度。因此，本研究在演化时间尺度下量化了不同功能的相对贡献程度。本研究通过几何形态测量学（geometric morphometrics）获取喙部形态数据，并针对澳大拉西亚鸣禽辐射演化类群（吸蜜鸟科Meliphagidae）开展系统发育比较研究，将喙部形态性状与觅食行为、气候变量及鸣唱特征进行关联分析。研究结果显示，气候条件与觅食行为均与喙部形态和大小存在显著相关性。不过，觅食生态对喙形的影响更大，而气候对喙部大小的影响更为显著。此外，本研究还发现喙部形态的演化变化对鸣唱表现具有显著影响：喙部修长的物种鸣唱频率更高，而喙部宽大的物种鸣唱节奏更为平缓。鸟类喙部的演化案例印证了形态性状如何在不同功能间形成演化权衡，并表明在任一功能轴上的特化，可能会提升其他功能轴上的生态分化或生殖隔离。