High Contrast Markings Can Negate the Benefits of Transparent Camouflage.

Transparency is, in theory, the ultimate form of concealment allowing for perfect background matching camouflage regardless of the environment. In nature, despite some remarkable examples of highly transparent organisms, physiological constraints mean that transparency is often partial or imperfect. This raises the question of how deviation from true transparency may affect detectability and how camouflage functions. Indeed, it has recently been suggested that partial transparency may function as disruptive camouflage as adjacent transparent and opaque patches differentially blend into the background. Differential blending may therefore offer a route by which obligate opaque structures may be concealed. The glass frogs (Centrolenidae) are a classic example of transparency with ventral skin that allows for a view of the internal organs. However, although the ventral skin is transparent and the frogs appear translucent, the internal organs are still largely opaque. Here we performed visual modelling and a field predation study with model frogs to ask how the degree of translucency and the arrangement of opaque structures affects detectability and survival. We predicted that greater translucency would improve concealment by facilitating more effective differential blending and that opaque elements which highlighted the recognisable shape of the frog would suffer increased predation risk. We found that greater translucency did improve background colour matching, but the presence of salient opaque patterns negated this camouflaging effect. However, the effect of salient patterning differed depending on model opacity, with outer markings receiving fewer attacks than central stripes for opaque but not translucent models. Here, survival was best explained by the distance at which the opaque patterning could be resolved, with thinner outer stripes receiving fewer attacks than thicker central stripes. Our data suggest that transparency may facilitate effective differential blending, but the efficacy of camouflage may be reduced by the presence of obligate opaque structures. These limitations to the efficacy of transparent camouflage may favour the evolution of translucency and explain why, despite having transparent ventral skin, glass frogs retain sparse green pigmentation in their dorsal skin.