Countershading enhances camouflage by reducing prey contrast

A 3D body shape is problematic for camouflage because overhead lighting produces a luminance gradient across the body’s surface. Countershading, a form of patterning where animals are darkest on their uppermost surface, is thought to counteract this luminance gradient and enhance concealment, but the mechanisms of protection remain unclear. Surprisingly, no study has examined how countershading alters prey contrast, or investigated how the presence of a dorso-ventral luminance gradient affects detection under controlled viewing conditions. It has also been suggested that the direction of the dorso-ventral luminance gradient (darkest or lightest on top) may interfere with predators’ abilities to resolve prey 3D shape, yet this intriguing idea has never been tested. We used live fish predators (western rainbowfish, Melanotaenia australis) and computer-generated prey images to compare the detectability of uniformly pigmented (i.e. non-countershaded) prey with that of optimally countershade...