期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：46
页码：13093-13097
DOI：10.1073/pnas.1611589113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceBecause the sun and sky are above us, natural illumination is directional and the cues from shading reveal shape and depth. However, many animals are darker on their backs and, over 100 years ago, it was proposed that this phenomenon was camouflage: countering the cues to shape that directional illumination creates. However, does this camouflage work in practice? We predicted the optimal countershading for different lighting conditions and tested this possibility with correspondingly patterned model "caterpillars" predated by birds in the wild. Predation rates varied with coloration and lighting in exactly the manner predicted. Such subtlety in the effects of countershading vindicates conclusions from prior evidence demonstrating stronger countershading in animals in more brightly lit habitats. Countershading, the widespread tendency of animals to be darker on the side that receives strongest illumination, has classically been explained as an adaptation for camouflage: obliterating cues to 3D shape and enhancing background matching. However, there have only been two quantitative tests of whether the patterns observed in different species match the optimal shading to obliterate 3D cues, and no tests of whether optimal countershading actually improves concealment or survival. We use a mathematical model of the light field to predict the optimal countershading for concealment that is specific to the light environment and then test this prediction with correspondingly patterned model "caterpillars" exposed to avian predation in the field. We show that the optimal countershading is strongly illumination-dependent. A relatively sharp transition in surface patterning from dark to light is only optimal under direct solar illumination; if there is diffuse illumination from cloudy skies or shade, the pattern provides no advantage over homogeneous background-matching coloration. Conversely, a smoother gradation between dark and light is optimal under cloudy skies or shade. The demonstration of these illumination-dependent effects of different countershading patterns on predation risk strongly supports the comparative evidence showing that the type of countershading varies with light environment.
