Department of Biology, Wake Forest University, Winston-Salem, NC 27101, USA.
Department of Biology, Wake Forest University, Winston-Salem, NC 27101, USA
Proc Biol Sci. 2018 Feb 28;285(1873). doi: 10.1098/rspb.2017.2628.
Sexual selection drives elaboration in animal displays used for competition and courtship, but this process is opposed by morphological constraints on signal design. How do interactions between selection and constraint shape display evolution? One possibility is that sexual selection continues exaggeration under constraint by operating differentially on each signal component in complex, modular displays. This is seldom studied on a phylogenetic scale, but we address the issue herein by studying macroevolutionary patterning of woodpecker drum displays. These territorial displays are produced when an individual rapidly hits its bill on a hard surface, and drums vary across species in the number of beats included (length) and the rate of drumbeat production (speed). We report that species body size limits drum speed, but not drum length. As a result of this biomechanical constraint, there is less standing variation in speed than length. We also uncover a positive relationship between sexual size dimorphism and the unconstrained trait (length), but with no effect on speed. This suggests that when morphology limits the exaggeration of one component, sexual selection instead exaggerates the unconstrained trait. Modular displays therefore provide the basis for selection to find novel routes to phenotypic elaboration after previous ones are closed.
性选择驱动着动物展示行为的复杂化，这些行为既用于竞争，也用于求偶，但这种过程受到形态约束的限制。选择和约束之间的相互作用如何影响展示行为的进化？一种可能性是，性选择通过在复杂的模块化展示中对每个信号成分进行差异化作用，继续在约束下夸大展示。这在系统发育尺度上很少被研究，但我们通过研究啄木鸟鼓鸣声展示的宏观进化模式来解决这个问题。这些领地展示是当个体快速地用喙敲击坚硬表面时产生的，而不同物种的鼓鸣声在包含的击鼓声数量（长度）和击鼓速度（速度）上存在差异。我们报告说，物种的体型限制了鼓的速度，但不限制鼓的长度。由于这种生物力学的限制，速度的固定变异比长度小。我们还发现了性二型性与不受约束的特征（长度）之间的正相关关系，但对速度没有影响。这表明，当形态限制了一个成分的夸大程度时，性选择会转而夸大不受约束的特征。因此，当以前的途径受到限制时，模块化展示为选择提供了寻找新途径来实现表型复杂化的基础。
