Yellow tree frogs are amphibians

Lots of colorful tree frogs

Tree frogs are usually known as green frogs that rest on reed stalks or the like at some height above the ground. Why are they sometimes ash gray or extremely dark?

The pond in north-eastern Poland was on a country road, and on the edge of the road there were several smaller burned areas with heaps of branches. The branches were gray in color. But not just them! Some tree frogs that sunbathed on the upper parts of the branches were also ash gray and thus well camouflaged. Upon closer inspection of the location, very dark, almost brown-green specimens of these small frogs were found in the tangle of branches. Next to it, normal light green tree frogs shone out of the branches, just as one generally knows them and sees them in most of the pictures. So many unusually colored tree frogs in one place - what had happened there? One of the participants suspected that it was a consequence of Chernobyl. But the color deviations apparently have nothing to do with mutations, i.e. genetic changes. Rather, it is a question of reversible color changes that occur through the migration of pigment particles in the frogs' skin.

Physiological color change

Animals that can change their color always cause astonishment. Squids, for example, can change the color or pattern of their skin in a flash. Especially when excited, these intelligent sea creatures show a lively play of colors, which is influenced by the inner mood as well as by the colors of the environment. To a certain extent, the squids are masters of the quick-change artists of color. In her skin there are several layers of different pigment-bearing color cells that evoke the play of colors. These color cells are provided with stretchable membranes that can be moved by muscle fibers. If a color cell is pulled apart, the pigment contained in it is distributed over a larger area, and a spot of the corresponding color is created. The muscle fibers are partly controlled directly by the nervous system, which enables quick reactions.
In the tree frog, the color change is much simpler and slower. Like other frogs, it has three different types of color cells in its skin. First of all are the xanthophores, which are provided with yellow pigments. Underneath there is a somewhat thicker layer of guanophores, which contain small platelets that have a strong refractive effect. At the bottom lie the melanophores with long appendages. These cells contain the pigment melanin, which appears black in high concentrations and brownish in thinner layers. The extensions of the melanophores reach up into the upper pigment layers and partially cover the yellow-bearing xanthophores.

Migratory pigments

How does the color change in the tree frog come about? Unlike the squid, it cannot pull the color cells apart with lightning-fast muscle movements. Instead, stimulated by hormonal influences, the melanin pigments are scattered to different extents in their cells. If they are concentrated around the cell nucleus, the color brightens from yellow to yellow-green. If the melanin granules migrate to the beginning of the long cell processes, a dark pigment screen is created under the guanophores. The refraction of light by the guanophores creates blue, which, together with the yellow of the xanthophores above, turns into green. If the melanin pigments also get into the cell processes between the guanophores, the frog becomes darker. And when they get into the processes that lie above the xanthophores, they cover the yellow pigments and the frog appears gray.
The change in color takes at least a few minutes in the frog. Within a quarter of an hour a gray tree frog can turn dark green if you place it on a rough, dark tree bark. In addition to the nature of the surface, temperature and humidity or the internal mood of the animal can cause a change in color. The color change is technically referred to as physiological, as nothing changes in the actual structure of the pigment cells.

Rare pigment disorders

Tree frogs are well known for their color changeability: The physiological color change is not so clearly expressed in any other domestic frog. Genetic mutations that cause certain color cells to fail and thus cause a different appearance, on the other hand, seem to be rare in wild tree frogs. Blue tree frogs have been found here and there. These are obviously animals that lack the yellow pigment cells. In nature, blue tree frogs are undoubtedly disadvantaged by the loss of their camouflage.
So far, only one albino of the European tree frog has been discovered, as amphibious specialist Dr. Wolf-Rüdiger Grosse reports in his new book. Due to the lack of melanin, this animal appeared very light, slightly yellowish. Albinos were found somewhat more frequently in tadpoles of the tree frog, but examined individuals did not survive to conversion. However, not every yellow tree frog is an albino: When it is very hot, tree frogs can turn a strong yellow to protect themselves from overheating when in the sun. Then their dark side stripes are retained and the eyes also remain dark. Unfortunately, the tree frog has become very rare in Switzerland due to the loss of natural habitats.

Published in: Tierwelt Nr. 22, June 4th 2010

© E. Wullschleger Schättin