Counter shaded animal patterns: from photons to form

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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.