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，颜色模式的特征是身体表面较暗的皮肤或皮毛，靠近太阳，而另一侧的阴影较浅。例如，鱼的背部通常是深色的，腹部是浅色的。许多其他动物也是如此，例如鹿、鸟、蜥蜴和许多昆虫。这种着色模式被称为“反阴影”。在这个项目中，我们的目标是了解反遮阳如何为动物提供一种有用的伪装来源，以及它为什么会进化。是让它们更难被发现，还是它们的外观形状，被潜在的捕食者所感知，被这种形式的颜色改变了？即使是微小的难以被发现的优势也会增加动物的生存机会，并增加其将颜色优势传递给下一代的可能性。为了使动物更难被发现，这种颜色模式可能通过两种方式进化而来。首先，动物可能只是试图与背景相匹配。从上面看，它的背部颜色较深，与地面相匹配，从下面看，它的腹部下部颜色较浅，与天空相匹配，所以这种伪装可能只是为了与背景相匹配。其次，这种图案可能是为了尽量减少其表面的阴影，使其看起来比实际更平坦。我们从人类视觉的研究中知道，物体表面的阴影帮助我们感知物体的三维（3D）形状（称为shape-from-shading）。这就解释了我们如何在黑白照片中感知3D物体，尽管照片实际上只是一个覆盖着不同数量黑色墨水的平面。所以，反阴影的进化可能是为了混淆大脑中这些形状-阴影的过程。感知上“平坦”的猎物可能更难以看到，或者更不受欢迎。我们项目的第一阶段将是详细检查猎物动物的反阴影。没有人曾经在动物身上测量过精确的模式，并测试了发现的模式是否与上述每种解释所期望的模式相匹配。我们将测量单个动物的三维形状，以及它们的反阴影。使用测量，我们将创建动物和它们的阴影的3D计算机模型。在理论研究中，我们将开发数学模型来预测哪种反阴影模式最适合隐藏动物。然后，我们将能够通过与物理测量相比较来检验这些预测。我们的计算机模拟将向我们展示哪种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“最有帮助”的模式是否真的更难被发现。我们将使用人类和鸟类作为感知实验的观察者，测试最佳阴影模式是否能让猎物隐藏更长时间。我们从研究人类开始，因为我们已经对人类视觉系统中的阴影形状有了很多了解。我们也对鸟类进行了测试，因为它们的大脑组织结构与鸟类截然不同，然而考虑到猎物反遮光的频率，我们可以预测，非哺乳动物的捕食者也应该被这种形式的伪装所欺骗。最后，我们可以从这些模拟中学到什么，并在现实世界中测试结果。通过将食物贴在印刷纸板管上，这些纸管分布在真实的户外环境中，我们可以看到野生鸟类找到这些纸管的速度有多快。如果用得慢一些，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，我们希望我们将更接近于解释为什么在许多类型的动物中进化出反阴影。

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.