Animal theory is no longer in the shade

SCOTTISH scientists have helped to prove a theory that so many animals are darker on their backs than their bellies in order to disguise their shape.

The theory, first proposed by American artist Abbot Thayer more than a century ago, has been tested by scientists from the universities of St Andrews and Abertay, together with colleagues at the University of Bristol.

Experts say cues to shape that come from light and shading are what the brain relies on to determine 3D form. As light comes from above, a uniformly-coloured animal would appear lighter on top and darker below.

Thayer’s idea was that by having pigment gradients opposite to those created by illumination, an animal could remove the shape-from-shad- ing cues and thus be harder to spot.

The latest research involved using mathematical models of the effects of the type of illumination to generate predictions that could be tested with artificial prey.

The “prey” were caterpillar-sized tubes of paper printed with different gradients of leaf-green shading and containing a dead mealworm that birds would happily eat if they spotted the cylinder.

Hundreds of these paper caterpillars were attached to bramble bushes in parks and their “survival” in the face of bird predation was recorded.

Prof Innes Cuthill, of Bristol University, said: “Ours is the first study to show the match between the type of countershading and the lighting is important for effective camouflage.”