(ORDO NEWS) — Researchers at McMaster University have found that the transparent skin of the lower body makes glass frogs indistinguishable on leaves, creating an optical illusion. An article about this is published in the publication Proceedings of the National Academy of Sciences .
Glass frogs living in Central and South America got their name because of their unique appearance: their bones, intestines and beating hearts can be perfectly seen through the skin. Until now, it was unknown why such an unusual sign could have been evolutionarily entrenched in these small animals, but the new work seems to offer an explanation.
Transparency seems like the perfect form of camouflage. But, as the leading author of the work, James Barnett, unfortunately for terrestrial animals, notes, such a trick works mainly under water, where the characteristics of the refraction of light in a liquid help such an animal to hide, not being calculated from the visible parts of internal organs. That is why tissue transparency is found among the inhabitants of the waters, and a terrestrial animal, such as a glass frog, surprises scientists.
In fact, if you look at them carefully, the glass frogs are not completely transparent: their backs are painted in a cheerful green color, as it should be for respectable frogs, and their sides and bellies are transparent. This “gradient” aroused the interest of biologists and served as the key to unraveling the mystery of the glass frog.
To explore their camouflage, scientists photographed 55 glass frogs on green leaves and on a white background. Then the researchers conducted three experiments. First, they used computer analysis to evaluate the color and shape of the frog in each photograph.
The authors found that the frogs always looked green, but they became lighter or darker depending on the background on which they were sitting. This “twisting” of brightness allows these amphibians to more closely match the shade of the sheet on which they are currently sitting. At the same time, their barrel, tummies and paws are transparent – the more “peripheral” the body part, the more transparent. In combination, this creates the effect of blurring the contours of a sitting animal, which helps to ideally merge with the environment and become practically invisible to a predator.
Researchers also showed a group of 25 volunteers 125 images of frogs modeled on a computer and varying in degree of transparency. They asked the subjects as soon as possible to find frogs, and the most difficult to find were those whose transparency was characteristic of a glass frog.
In the third series of experiments, scientists made 360 fake gelatin frogs and left them for three days in the Ecuadorian rainforest. One half of the artificial frogs was opaque, and the other half was translucent, and the researchers found that birds, natural predators of local amphibians, attacked opaque frogs more than twice as often.
Scientists explain that visual systems are very sensitive to the presence of clear contours, which are formed where one color changes sharply to another. Many species of animals have a color that, in various ways, eliminates the appearance of contours. The two most common strategies are a pattern that repeats elements of the environment, and a “destructive coloring” in which patterns with high contrast create unrecognizable shapes that hide the outlines of animals.
Glass frogs use a fundamentally different technique: their blurry gradient helps to ideally merge with the background, not marking the boundaries of the body on it, while adjusting the transparency of the tissues, and not, for example, the brightness of the color. Researchers have called this new camouflage technique “edge diffusion.”
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