Researchers have discovered that green tree frogs in the Chernobyl Exclusion Zone have turned dark in order to better mitigate the effects of radiation.
In April of 1986, a reactor at the Chernobyl nuclear power plant in Ukraine exploded, releasing approximately 100 times the energy released by the nuclear bombs of Hiroshima and Nagasaki and drastically altering the lives of both humans and wildlife in the surrounding area. But while authorities were able to evacuate most civilians from the area closest to the nuclear disaster, the animals were left to their own devices. In the decades since, the Chernobyl Exclusion Zone has become a wildlife refuge that offers a unique view into the evolution triggered by the nuclear meltdown.
Photo: Germán Orizaola and Pablo Burraco
In 2016, a team of Spanish researchers ventured into the Chernobyl Exclusion Zone to examine and study the influence of nuclear radiation on the local flora and fauna. One of the first animals they noticed were these dark-skinned frogs that looked and sounded like eastern tree frogs (Hyla orientalis), apart from the color. Eastern tree frogs are usually bright green, only these were much darker, and some specimens were actually pitch black.
Pablo Burraco, a biologist with the Doñana Biological Station in Seville, Spain, and his colleagues wanted to understand what had caused this change in coloration, so between 2017 and 2019, they examined the coloration of Eastern tree frogs in different areas of northern Ukraine. They analyzed over 200 male frogs captured in various breeding ponds, from some of the most radioactive areas on the planet, to places outside the exclusion zone, for control.
After analyzing the data, researchers concluded that the dark skin of the usually green tree frogs is a direct consequence of their adaptation to radiation levels. Chernobyl tree frogs have a much darker coloration than frogs outside the exclusion zone, and some are completely black.
Photo: iNaturalist.org
“We consider the most plausible explanation to [why] frogs within the Chernobyl Exclusion Zone [are changing color] is that the extremely high radiation levels at the moment of the accident selected for frogs with dark skin,” the Spanish researchers wrote in the recently published study.
It is believed that the frogs underwent a process of rapid evolution in response to radiation, where the darker-skinned frogs were more resistant to the high levels of radiation around the Chernobyl reactor, and thus had a higher chance of survival. The higher levels of melanin in the frogs’ skin are believed to have shielded them against radiation.
“They actually did not ‘change color’, what changed was the proportion of dark versus normal/green frogs,” Germán Orizaola, co-author of the study, told EuroNews. “We suppose that this happened shortly after the accident (first years), when radiation levels were much higher, and radioisotopes more diverse.”
The dark frogs survived the radiation better, bred better, and now, 10-15 generations from the nuclear disaster, they make up the majority of specimens found in the Chernobyl Exclusion Zone, especially in areas known to have been affected by high levels of radiation.
“It was indeed the extraordinary selection pressure caused by ionizing radiation that directed the evolution of amphibians from green to black,” the study authors conclude.
