The role of skin colour in camouflage: experiment with green plasticine models of the European tree frog Hyla arborea

The role of skin colour variations in the survival of European tree frogs is unknown. We presumed that green frogs become more noticeable to predators in the autumn when the colour of the leaves changes. For the study of their survival chance, we made plasticine models of European tree frogs with the same shape and the same dominant colour spectrum of 550 nm. The experiment was conducted in autumn 2018 in three vegetation types with varied backgrounds (vineyard, bulrush belts and reed bed) and in the habitat with a permanent background (concrete wall). In our experiment lasting more than a month we used 50 green coloured tree frog plasticine models in every habitat. Only 10% of the models were damaged by predators. Daily survival rates were high in all habitats (above 99%), and predation pressure on the concrete wall was significantly higher than in the different vegetation types. This was caused by the fact that the colour contrast of frog models on concrete walls (13.3) was twice as high as on grape (6.8), bulrush (4.2), or reed (5.0) leaves. Predators in the vineyard, bulrush belts and reed bed were mostly birds which left traces primarily on the heads of the models, while on the models displayed on the wall most of the traces (on the trunk and limbs) were left by mammals (small mammals, red foxes, and domestic cats). The colour spectrum of the vegetation background of the European tree frog models changed to brownish in autumn, but predation pressure did not increase. The constant green colour of the European tree frog models in natural habitats did not affect predation, from which we conclude that the colour variants may have other functions besides camouflage.