Evolution of Acoustic and Visual Signals in Asian Barbets

The study of animal communication systems is an important step towards gaining greater understanding of the processes influencing diversification because signals often play an important role in mate choice and can lead to reproductive isolation. Signal evolution can be influenced by a diversity of factors such as biophysical constraints on the emitter, the signalling environment, or selection to avoid heterospecific matings. Furthermore, because signals can be costly to produce, trade-offs may exist between different types of signals. Here, we apply phylogenetic comparative analyses to study the evolution of acoustic and visual signals in Asian barbets, a clade of non-Passerine, forest-dependent birds. Our results suggest that evolution of acoustic and visual signals in barbets is influenced by diverse factors, such as morphology and signalling environment, suggesting a potential effect of sensory drive. We found no trade-offs between visual and acoustic signals. Quite to the contrary, more colourful species sing significantly longer songs. Song characteristics presented distinct patterns of evolution. Song frequency diverged early on and the rate of evolution of this trait appears to be constrained by body size. On the other hand, characteristics associated with length of the song presented evidence for more recent divergence. Finally, our results indicate that there is a spatial component to the evolution of visual signals, and that visual signals are more divergent between closely related taxa than acoustic signals. Hence, visual signals in these species could play a role in speciation or reinforcement of reproductive isolation following secondary contacts.

动物通讯系统的研究是深入理解物种分化影响过程的重要环节——因为通讯信号往往在配偶选择中发挥关键作用，并可导致生殖隔离（reproductive isolation）的产生。信号演化可受多种因素影响，例如信号发送者的生物物理限制、信号传导环境，或是为避免异种交配而产生的选择压力。此外，由于信号产生需付出代价，不同类型的信号之间可能存在权衡（trade-off）关系。本研究采用系统发育比较分析（phylogenetic comparative analyses）方法，对亚洲拟啄木鸟（Asian barbets）——一类非雀形目、依赖森林生境的鸟类演化支——的声学信号与视觉信号演化展开研究。研究结果表明，拟啄木鸟类的声学与视觉信号演化受形态特征、信号传导环境等多种因素影响，这提示感官驱动（sensory drive）可能发挥了潜在作用。我们未发现视觉信号与声学信号间存在权衡关系；恰恰相反，体色更为艳丽的物种，其鸣唱时长显著更长。鸣唱特征的演化呈现出截然不同的模式：鸣唱频率很早就发生了分化，且该性状的演化速率似乎受体型大小的制约。而与鸣唱时长相关的特征，则显示出较晚近的分化迹象。最后，本研究结果显示视觉信号的演化存在空间维度特征，且亲缘关系较近的分类单元（taxa）间，视觉信号的分化程度要高于声学信号。因此，这些物种的视觉信号可能在物种形成，或是次级接触（secondary contacts）后生殖隔离的强化过程中发挥了作用。