Story behind the frogs that turned black to survive Chernobyl radiation

On April 26, 1986, reactor four at the Chernobyl Nuclear Power Plant failed, releasing a vast burst of radioactive material into air, soil, and water, a record noted by UNSCEAR.

The ionizing radiation released at Chernobyl is high-energy radiation that can strip electrons from atoms, damage DNA, and disrupt cells.

Nearly four decades on, the Exclusion Zone is also a sprawling refuge where large animals roam and breed.

This trend is visible in long term wildlife surveys that track stable or rising mammal numbers inside the zone. That backdrop sets the stage for a striking observation about small amphibians living there.

Changes in Chernobyl frogs

This work was led by Pablo Burraco, Doñana Biological Station, Spanish National Research Council (CSIC).

His team focused on Chernobyl’s Eastern tree frogs, Hyla orientalis, and sampled males from breeding ponds across northern Ukraine.

In a field study that ran from 2017 to 2019, the researchers found that frogs from within or near the most contaminated sites from 1986 were much darker than frogs from control ponds outside the zone.

Today’s individual dose rates did not line up with how dark any given Chernobyl frog was.

The dataset spanned 12 ponds and close to two hundred males, and some individuals were pitch black. That pattern demanded a closer look at the biology behind color.

Melanin’s role in Chernobyl’s frogs

Melanin is the pigment that darkens skin, hair, and eyes, and experiments show it can buffer cells against radiation by soaking up energy and managing reactive molecules in the cell.

That makes melanin a plausible shield in a place laced with radionuclides.

“Melanin is known to reduce the impact of radioactive particles on cells by dissipating the energy of the radiation and avoiding damage,” said Burraco.

The team also checked physiology. They looked at oxidative stress, the cell level imbalance caused by reactive oxygen that can damage lipids and proteins, and did not find that darker frogs carried extra oxidative costs.

Radiation exposure links

The key result is about timing. Darker coloration mapped onto proximity to the hotspots from the spring of 1986, not to the dose each frog absorbs now.

That points to rapid natural selection, where rare dark individuals had a survival edge when radiation was intense and diverse shortly after the accident.

Their offspring inherited those traits as the population rebounded in the decades that followed.

Ten to fifteen generations have passed since 1986, which is enough time for a pigment linked advantage to spread. The black morphs that were once uncommon are now common inside the zone.

What the data say

Color was measured as dorsal skin luminance, and frogs from inside the Exclusion Zone averaged 43.6 percent lower luminance than frogs from outside, which means they were much darker.

That contrast held even after accounting for location and body condition.

Short term tests showed no shift in color after two days under dark or light backgrounds. That stability supports the idea that the darker tone is not a quick, reversible response.

Body condition did not predict which frogs were darker. The trait stood on its own rather than being a side effect of health.

Life under lower radiation today

Radiation levels in the area have fallen sharply since 1986, and recent work suggests current conditions may not be enough to speed aging or elevate stress hormones in these frogs.

A newer paper reports no effect of present day exposure on age, telomere length, or corticosterone levels in Chornobyl tree frogs.

That does not contradict the color results. It simply says the selection event likely happened early, and the trait persisted as radiation waned.

The frogs offer a clear case to examine how a trait born of crisis can remain common when the crisis fades. That story matters for other polluted landscapes where conditions also change over time.

Learning from Chernobyl’s frogs

Researchers will want to test whether specific pigment genes or regulatory regions carry signatures of selection in these populations.

Work can also probe whether internal organs show similar pigment shifts, since dark skins sometimes match darker internal tissues in amphibians.

Materials science is taking note too. The protective properties of melanin in lab organisms raise questions about how that pigment, or molecules like it, might be used in radiation shielding for equipment and habitats, including in spaceflight.

Fieldwork will keep refining dose reconstructions and life history measures as access and safety allow. The frogs are not just surviving, they are teaching.

The study is published in Evolutionary Applications.

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