Frogs have no pigment cells. Why then do they dazzle in all sorts of shades of green and are perfectly adapted to their flora? This is exactly what frog researchers want to find out.
Actually, frogs shouldn’t be green at all. Their skin is usually transparent and contains no pigment cells. The fact that the animals still cover a wide range of shades of blue and green is apparently due to certain proteins from the family of the so-called Serpin.
This is reported by a team led by Luca Chemesk and Sara Baric from the Universidad Nacional de San Martin and the Universidad de Buenos Aires in the Proceedings of the National Academy of Sciences of the United States of America (PNAS). The serpins bind to a dye molecule that is found in large amounts in the blood, lymph and bones of the amphibians. On the one hand, this obviously means that the dye is not excreted so quickly. On the other hand, the binding changed its reflective spectrum, as the team determined with the help of photometric measurements on their experimental subject.
For this they had chosen the Polka-dot tree frog (scientifically known as Boana punctatus), a yellow-green tree frog from South America. The serpin proteins gives the frog a color similar to that of the plants in its environment. So it is perfectly camouflaged. The dye in the frog’s bodily fluids is biliverdin, which is known to have a bluish green color. Until now, however, it was unclear how the different shades of color, which are perfectly matched to the plant environment of different frog species and range from green to reddish, are created.
Other researchers, for example, had already shown that brown frogs do not become greener when they injected them with biliverdin. The animals simply eliminated the dye.
The team also went in search of serpins in eight other frog species – and found them. All frogs have representatives of the protein family, some even several, such as one in the lymph and another in the bone. Apparently, serpins that binds biliverdin have emerged several times independently of each other in the course of evolution. Because the proteins have slightly different reflection spectra, they could be responsible for the individual coloring of the frog species.
In the case of Boana punctatus, for example, the area of light shifts through the serpin further into the red area, which is why the animal appears greenish yellow and has small red speckles. However, the scientists do not rule out the fact that other molecules play a role in the frog’s coloring.
In addition, the newly discovered frog proteins may also have other tasks, such as acting as radical scavengers or antioxidants, they write. Not only frogs, but also fish, lizards and mammals like us have biliverdin in the body: it is produced when the red blood dye hemoglobin is broken down. This can sometimes even be seen: when a hematoma, i.e. a blue spot, slowly turns green. It then turns yellow – bilirubin, another degradation product of hemoglobin, is responsible for this.
In people with liver or bile problems, biliverdin and bilirubin are found in larger amounts in the blood. People with liver cancer, for example, cannot properly break down or excrete the red blood dye. As a result, their eyes and skin turn yellow. According to the researchers, green frogs have at least four times as much biliverdin in their blood as people suffering from jaundice.