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.

虽然形态特征通常与多种功能相关，但进化如何平衡不同功能的选择效应仍不明确。鸟类的喙不仅用于觅食，还参与体温调节、鸣唱等多种行为。关于喙进化的研究虽多，但大多聚焦于单一功能。因此，我们量化了不同功能在进化时间尺度上的相对贡献。我们采用几何形态测量学（geometric morphometrics）测量喙的形态，并在Australasian鸣禽（吸蜜鸟科，Meliphagidae）辐射类群的系统发育比较研究中，将该特征与觅食行为、气候变量及鸣唱特征进行对比。研究发现，气候和觅食行为均与喙的形态和大小显著相关；但觅食生态对形态的影响更大，而气候对大小的影响更显著。此外，我们还发现喙形态的进化变化对鸣声表现具有显著影响：喙形细长的物种鸣唱频率更高，而喙较大的物种鸣唱节奏更缓慢。鸟类喙的进化案例表明，形态特征可作为不同功能间的进化妥协，且沿任一功能轴的特化可能会加剧其他轴上的生态分化或生殖隔离。