Data from: A quantitative test of the predicted relationship between countershading and lighting environment.

Countershading, a vertical luminance gradient from a dark back to a light belly, is perhaps the most common coloration phenotype in the animal kingdom. Why? We investigated whether countershading functions as self-shadow concealment (SSC) in ruminants. We calculated “optimal” countershading for SSC by measuring illumination falling onto a model ruminant as a function of time of day and lighting environment. Calibrated images of 114 species of ruminant were compared to the countershading model, and phylogenetic analyses were used to find the best predictors of coats’ countershading characteristics. In many species, countershading was close to the model’s prediction of “optimal” countershading for SSC. Stronger countershading was associated with increased use of open lighting environments, living closer to the equator, and small body size. Abrupt transitions from dark to light tones were more common in open lighting environments but unassociated with group size or antipredator behavior. Though the SSC hypothesis prediction for stronger countershading in diurnal species was not supported and noncountershaded or reverse-countershaded species were unexpectedly common, this basic pattern of associations is explained only by the SSC hypothesis. Despite extreme variation in lighting conditions, many terrestrial animals still find protection from predation by compensating for their own shadows.

反荫蔽（Countershading），即从深色背部到浅色腹部的垂直亮度梯度，或许是动物界中最为普遍的体色表型。其普遍性背后的原因何在？本研究针对反刍动物中反荫蔽是否具有自身阴影遮蔽（SSC）功能展开了探究。我们通过测量照射在反刍动物模型上的光照随一日时段与光照环境的变化情况，计算出了适配自身阴影遮蔽的“最优”反荫蔽模式。我们将114种反刍动物的校准图像与反荫蔽模型进行比对，并通过系统发育分析，筛选出预测动物皮毛反荫蔽特征的最佳影响因子。在诸多物种中，反荫蔽程度与模型预测的适配自身阴影遮蔽的“最优”反荫蔽模式高度吻合。反荫蔽程度更强的物种，更倾向于栖息于开阔光照环境、更接近赤道，且体型更小。体色从深色到浅色的急剧过渡在开阔光照环境中更为常见，但与种群规模或反捕食行为并无关联。尽管自身阴影遮蔽假说关于昼行性物种反荫蔽程度更强的预测未得到验证，且非反荫蔽或反相反荫蔽的物种意外地较为常见，但这类关联的基本模式仅能通过自身阴影遮蔽假说得到解释。尽管光照环境存在极大差异，诸多陆生动物仍可通过抵消自身阴影的方式，获得捕食防御的保护。