Phenotypic plasticity drives a depth gradient in male conspicuousness in threespine stickleback, Gasterosteus aculeatus

Signal evolution is thought to depend on both a signal's detectability or conspicuousness (signal design) as well as any extractable information it may convey to a potential receiver (signal content). While theoretical and empirical work in sexual selection has largely focused on signal content, there has been a steady accrual of evidence that signal design is also important for trait evolution. Despite this, relatively little attention has been paid to spatial variation in the conspicuousness of a given signal, especially over small spatial scales (relative to an organism's dispersal distance). Here, we show that visual signals of male threespine stickleback vary in conspicuousness, depending on a male's nest depth within a given lake. Deeper-nesting males were typically more chromatically conspicuous than shallow nesting males. This trend is partly because all male stickleback are more conspicuous in deep optical environments. However, deep males are even more conspicuous than environmentally-driven null expectations, while shallow males tend to be disproportionally cryptic. Experimental manipulation of male nesting depth induced plastic changes in nuptial color that replicated the natural gradients in conspicuousness. We discuss a number of potential mechanisms that could produce depth gradients in conspicuousness in male stickleback, including concomitant depth gradients in diet, predation pressure, male/female density, female preference and opportunity for sexual selection.

信号演化通常被认为同时取决于信号的可检测性或醒目度，即信号设计（signal design），以及其向潜在接收者传递的可提取信息，即信号内容（signal content）。尽管性选择（sexual selection）领域的理论与实证研究大多聚焦于信号内容，但越来越多的证据表明，信号设计对性状演化（trait evolution）同样至关重要。尽管如此，针对特定信号的醒目度空间变异的研究仍相对较少，尤其是在相对于生物扩散距离而言的小空间尺度上。本研究发现，雄性三刺鱼（threespine stickleback）的视觉信号醒目度会因个体在特定湖泊中的筑巢深度而异：筑巢深度较深的雄性，其色彩醒目度通常高于筑巢较浅的雄性。这一趋势部分源于所有雄性三刺鱼在深水光学环境中的醒目度均更高，但相较于环境驱动的基准预期，筑巢较深的雄性醒目度更高，而筑巢较浅的雄性则往往异常隐蔽。对雄性筑巢深度进行实验操控后，其婚姻色（nuptial color）发生了可塑性变化（plastic changes），这一变化复现了自然状态下醒目度的梯度特征。本研究还讨论了可能导致雄性三刺鱼醒目度随深度产生梯度变化的多种潜在机制，包括饮食、捕食压力（predation pressure）、雌雄种群密度、雌性偏好以及性选择机会等随深度的协同梯度变化。