Counter shaded animal patterns: from photons to form

许多不同种类的动物都有伪装的身体。在许多情况下，肤色的特征是身体表面较暗的皮肤或皮毛更接近太阳，而另一边则有较浅的阴影。例如，鱼的背部通常是黑色的，腹部通常是浅色的。同样的道理也适用于许多其他动物，例如鹿、鸟、蜥蜴和许多昆虫。这种着色方式被称为“反着色”。在这个项目中，我们的目标是了解反阴影如何为动物提供一种有用的伪装来源，以及为什么它会进化。这是否使它们更难被发现，或者它们的外观形状，就像潜在的捕食者所感知的那样，被这种颜色形式改变了？即使是更难察觉的微小优势也会增加这种动物的存活机会，并增加它将这种颜色优势传递给下一代的可能性。为了让动物更难被发现，这种着色模式可能通过两种方式进化。首先，这只动物可能只是试图与背景相匹配。当从上面看时，动物的背部较暗，与地面匹配，而从下面看，其较亮的下腹与天空相匹配，因此伪装可能只是一种背景匹配的尝试。其次，这种图案可能代表了一种尝试，即尽量减少其表面的阴影，使其看起来比实际情况更平坦。我们从人类视觉的研究中知道，物体表面的阴影有助于我们感知物体的三维(3D)形状(称为从阴影到形状)。这解释了我们如何在黑白照片中感知3D对象，即使照片实际上只是覆盖着不同数量的黑色墨水的平面。因此，反阴影的进化可能是为了混淆大脑中的这些从阴影到形状的过程。一种感觉上更“漂亮”的猎物动物可能更难看清，或者更不想吃。我们项目的第一阶段将详细检查对被捕食动物的反阴影。On曾经测量过动物身上的精确图案，并进行了测试，看看找到的图案是否与上面给出的每一种解释所期望的图案相匹配。我们将测量单个动物的3D形状，以及它们的反阴影。使用测量结果，我们将创建动物及其阴影的3D计算机模型。在理论研究中，我们将开发数学模型来预测什么样的反阴影模式将是隐藏动物的理想模式。然后，我们将能够通过与物理测量进行比较来测试这些预测。我们的计算机模拟将向我们展示什么阴影模式对隐藏动物最有帮助。在项目的第二阶段，我们将测试“最有用的”模式是否更难检测。我们将在感知实验中使用人类和鸟类作为观察者，以测试最佳的阴影模式是否允许被捕食的动物隐藏更长时间。我们从研究人类开始，因为我们已经知道了人类视觉系统中从形状到阴影的大量知识。我们还对鸟类进行了测试，因为它们的大脑组织方式截然不同，但考虑到猎物反影的频率，人们可以预测，非哺乳动物的捕食者也应该被这种形式的伪装所愚弄。最后，我们可以利用我们从这些模拟中学到的东西，并在现实世界中测试结果。通过将食物附着在分布在真实户外环境中的印刷纸板管上，我们可以看到野生鸟类发现这些管的速度有多快。如果管子的拍摄速度较慢，那么我们可以假设阴影是更好的伪装。在我们的项目结束时，我们预计我们将更接近于解释为什么反阴影在许多类型的动物中进化。

Many different kinds of animals have camouflaged bodies. In many cases the pattern of colouration features a darker skin, or fur, on the surface of the body that is closer to the sun, and lighter shading on the other side. For example, fish are often dark along their backs and light along their bellies. The same is true for many other animals for example deer, birds, lizards and many insects. This pattern of colouration is known as 'counter-shading'. In this project we aim to understand how counter-shading might provide a useful source of camouflage for animals, and why it has evolved. Does it make them harder to detect, or is their apparent shape, as perceived by a potential predator, changed by this form of colouration? Even a small advantage in being more difficult to detect would enhance the animal's chances of survival, and increase the liklihood of its passing that colouration advantage on to the next generation. There are two ways in which this pattern of colouration might have evolved in order to make an animal harder to detect. First, the animal may simply be trying to match the background. When viewed from above the animals back is darker and matches the ground, and when viewed from below its lighter underbelly matches the sky, so the camouflage could be simply an attempt at background matching. Second, the patterning could represent an attempt to minimise the shading across its surface, so that it appears flatter than it actually is. We know from research in human vision that the shading on the surface of an object helps us to perceive the 3-dimensional (3D) shape of the object (called shape-from-shading). This explains how we can perceive 3D objects in black and white photographs, even though the photograph is actually just a flat surface covered with varying amounts of black ink. So, counter-shading may have evolved in order to confuse these shape-from-shading processes in the brain. A perceptually 'flatter' prey animal may be more difficult to see, or less desirable to eat. The first stage of our project will be to examine counter-shading on prey animals in detail. On one has ever measured the exact patterning on animals and tested to see if the patterns found match the pattern that would be expected for each of the explanations given above. We will measure the 3D shape of individual animals, and their counter-shading. Using the measurements, we will create 3D computer models of animals and their shading. In theoretical studies, we will develop mathematical models that predict what patterns of counter-shading would be ideal for hiding the animal. We will then be able to test these predictions by comparing with the physical measurements. Our computer simulations will show us what patterns of shading are most helpful in hiding an animal. In the second stage of the project, we will test whether the 'most helpful' patterns are actually harder to detect. We will use humans and birds as observers in perceptual experiments that test whether the best shading patterns allow a prey animal to remain hidden for longer. We start by studying humans, because a great deal is already known about shape-from-shading in the human visual system. We also test birds, because their brains are organised rather differently, yet given the frequency of counter-shading in prey, one would predict that non-mammalian predators should also be fooled by this form of camouflage. Finally, we can take what we learn from these simulations and test the results in the real world. By attaching treats to printed cardboard tubes that are distributed around a real outdoor environment, we can see how quickly the tubes are found by wild birds. If the tubes are taken less quickly then we can assume that the shading is much better camouflage. At the end of our project we expect that we will be much closer to an explanation of why counter-shading has evolved in many types of animal.