Three-dimensional camouflage: exploiting photons to conceal form

Many animals have a gradation of body color, termed “countershading,” where the areas that are typically exposed to more light are darker. One hypothesis is that this patterning enhances visual camouflage by making the retinal image of the animal match that of the background, a fundamentally two-dimensional theory. More controversially, countershading may also obliterate cues to three-dimensional (3D) shape delivered by shading. Despite relying on distinct cognitive mechanisms, these two potential functions hitherto have been amalgamated in the literature. It has previously not been possible to validate either hypothesis empirically, because there has been no general theory of optimal countershading that allows quantitative predictions to be made about the many environmental parameters involved. Here we unpack the logical distinction between using countershading for background matching and using it to obliterate 3D shape. We use computational modeling to determine the optimal coloration for the camouflage of 3D shape. Our model of 3D concealment is derived from the physics of light and informed by perceptual psychology: we simulate a 3D world that incorporates naturalistic lighting environments. The model allows us to predict countershading coloration for terrestrial environments, for any body shape and a wide range of ecologically relevant parameters. The approach can be generalized to any light distribution, including those underwater.

许多动物的体色存在梯度变化，该现象被称为‘反荫蔽（countershading）’，即通常受光照更强的区域体色更深。有假说认为，这种体色模式可通过使动物的视网膜成像与背景相匹配来强化视觉伪装，这本质上是一种二维理论。更具争议的是，反荫蔽还可能消除由光影所传递的三维（3D）形状线索。尽管这两种潜在功能依赖于截然不同的认知机制，但迄今为止学界仍将它们混为一谈。此前尚无通用的最优反荫蔽理论能够针对涉及的诸多环境参数做出定量预测，因此无法通过实证手段验证这两种假说中的任意一种。本研究厘清了利用反荫蔽进行背景匹配，与利用其消除三维形状线索之间的逻辑差异。我们通过计算建模，确定用于隐藏三维形状的最优体色配置。我们的三维隐藏模型基于光学原理构建，并结合了感知心理学的研究结论：我们模拟了包含自然光照环境的三维世界。该模型能够针对陆地环境、任意躯体形态，以及一系列生态相关参数，预测出反荫蔽的体色分布。该研究方法可推广至任意光照分布场景，包括水下环境。