Data from: Bill morphology and neutral genetic structure both predict variation in acoustic signals within a bird population

Adaptive evolutionary divergence within a population can be facilitated by associated divergence in mating signals. Acoustic signals are often involved in mate choice, and are also known to diverge spatially in response to a variety of processes. In birds, for instance, variation in bill size and shape can result in correlated changes in vocalizations due to functional constraints on sound production. Acoustic signals can also vary spatially in relation to neutral genetic structure (due to cultural drift) and/or habitat structure (due to acoustic adaptation for optimal sound transmission). Here we test these alternative hypotheses as causes of variation in acoustic signal structure in the Island Scrub-Jay (Aphelocoma insularis), a species that is restricted to one small island (Santa Cruz Island, California, USA) and exhibits spatial genetic structure and microgeographic divergence in bill morphology across short distances and habitat types. We find that bill morphology is related to the structure of the female “rattle” call, a vocalization associated with territorial disputes and male-female interactions. Females with longer, shallower bills produced calls that were more rapid, and those with shallower bills also produced calls that were lower in frequency. In addition, rattle rapidity varied across the island in accordance with neutral genetic structure. Vocal characteristics were not related to habitat structure, suggesting that variation in rattle calls is unlikely to reflect optimization for sound transmission. Our findings indicate that selection on bill morphology and cultural drift can jointly shape variation in acoustic signal structure, even at fine spatial scales within populations.

种群内的适应性进化分化，可通过交配信号的协同分化得以促进。声学信号常参与配偶选择，且已知其会因多种过程而发生空间分化。以鸟类为例，由于发声的功能限制，喙的大小和形状变化会导致鸣叫产生相应改变。声学信号的空间差异还可能与中性遗传结构（源于文化漂变）以及/或栖息地结构（源于实现最优声音传播的声学适应）相关。本研究以岛丛鸦（Island Scrub-Jay，*Aphelocoma insularis*）为对象，检验上述替代假说，探究其声学信号结构变异的成因。岛丛鸦仅分布于美国加利福尼亚州圣克鲁兹岛这一小岛之上，且在短距离内和不同栖息地类型间存在空间遗传结构以及喙形态的微地理分化。研究发现，喙形态与雌性"咔哒鸣唱"（rattle call）的结构存在关联——这类鸣唱用于领地争端以及雌雄个体间的交流。喙更长且更浅的雌性个体，其鸣唱速率更快；而喙更浅的雌性个体，其鸣唱频率也更低。此外，全岛范围内的咔哒鸣唱速率变化与中性遗传结构相契合。鸣唱特征与栖息地结构并无关联，这表明咔哒鸣唱的变异不太可能是为了优化声音传播而产生的适应性改变。本研究结果表明，即使在种群内的精细空间尺度下，对喙形态的选择作用与文化漂变也可共同塑造声学信号结构的变异。