Counter-shading camouflage: shape-from-shading in nature?

Presenter: Professor Julie Harris, The School of Psychology and Neuroscience at the University of St Andrews                  

Shape-from-shading is a powerful source of visual information about depth and shape. It has been argued that some animals have evolved a form of camouflage to counter the effects of shape from shading. This is called countershading: a darker colour on the back and lighter on the belly, and it appears in many taxa and environments. The countershading pattern has been proposed to contribute to visual camouflage by counterbalancing the gradient of illumination on the body to deliver reduced shading. But the actual counter-shading pattern on animals has never been measured in this context and empirical studies that explore it have not attempted to predict optimal shading patterns. We have examined the camouflage counter-shading hypothesis in a number of ways: (i) by developing a physical light model of counter-shaded animals in realistic environments and using the model to make specific predictions, (ii) by testing empirically the effect of different countershading patterns on visual detection performance, and (iii) measuring the three-dimensional form and the reflectance of counter-shaded species of caterpillar to determine how the real pattern could contribute to concealing shape. My talk will review this ongoing research to demonstrate how counter-shading can provide a form of camouflage that conceals three-dimensional shape, and how vision science can successfully intersect with behavioural ecology.