Four decades after reactor number four at the Chernobyl nuclear power plant exploded on April 26, 1986, the exclusion zone surrounding the site remains one of the most closely studied landscapes in the world for understanding how wildlife responds to long-term radioactive contamination.The explosion, regarded as the world’s worst nuclear disaster, released radioactive material across large parts of Europe.

Winds carried radioactive dust as far as the United Kingdom, Norway and parts of North Africa. The immediate area surrounding the plant in northern Ukraine received the heaviest contamination, prompting the evacuation of tens of thousands of residents and the establishment of a 60-kilometre-wide exclusion zone where human activity sharply declined.

In the decades since, scientists have documented a landscape transformed not only by radiation but also by the near-total absence of people. Wolves, bears, bison, deer, wild boar and elk now move through forests and abandoned settlements that were once heavily populated.

Yet researchers say the question of whether radiation itself has changed wildlife through adaptation, mutation or selective survival remains unresolved.Pablo Burraco, an evolutionary biologist at Doñana Biological Station, part of Spain’s National Research Council, has spent years studying tree frogs in and around Chernobyl.

During his first field trip in 2016, he captured a male tree frog near the abandoned reactor site and noticed it was darker in colour than similar frogs found farther from the exclusion zone.That observation led to broader fieldwork involving more than 250 tree frogs.

In 2022, Burraco and his colleagues published findings showing that frogs inside the exclusion zone were, on average, darker than those outside it, particularly in areas that experienced the highest radiation exposure immediately after the 1986 accident.

The researchers proposed that the darker colour, linked to higher levels of melanin, may offer some protective advantage against ionising radiation. Melanin is known to play protective biological roles in many organisms, and the team suggested that darker frogs may have had better survival rates after the disaster.

Burraco has stressed that this remains a hypothesis rather than established proof. He argues that radiation levels today differ significantly from those immediately after the explosion, and that the frogs were sampled across habitats that were otherwise comparable.

Timothy Mousseau, a biologist at the University of South Carolina who has conducted extensive research in Chernobyl, has questioned the strength of that conclusion. He argues that the frog sampling was not broad enough to establish a clear distinction between frogs inside and outside the exclusion zone and says melanisation does not clearly correlate with present-day radiation levels.

Carmel Mothersill, professor emeritus of radiobiology at McMaster University, described the 2022 study as methodologically sound and noted that its authors were careful not to overstate their conclusions. She said the disagreement reflects a broader scientific challenge in Chernobyl research: separating the direct effects of radiation from other environmental pressures.

Heavy metals and other pollutants also remain present in the area, complicating efforts to isolate radiation as the sole cause of unusual biological traits. Similar debates surround studies of feral dogs living near Chernobyl, where researchers have observed genetic differences but have not established definitive evidence linking those changes directly to radiation exposure.

Bank voles have also become a focus of study. Research has shown that voles living in contaminated areas carry higher levels of genetic diversity in their mitochondria compared with those from uncontaminated regions. Scientists say these differences may reflect mutations caused by radiation exposure, though other ecological factors may also contribute.

Mothersill notes that the landscape itself changed dramatically after the accident. Pine forests, which are highly sensitive to radiation, suffered extensive die-off following fallout exposure. In some areas, birch trees replaced them, creating different habitats and altering the ecological balance.

“It’s teeming with trees and wildlife but it’s not the same as it was before the accident,” she has said, arguing that species responses may reflect habitat transformation as much as radiation exposure.The absence of people has also played a major role.

Species that were once rare or absent have returned. Brown bears, not recorded in the region for more than a century, were captured on camera traps inside the exclusion zone in 2014. Eurasian lynx have reappeared after disappearing long before the nuclear disaster.Wolf populations are estimated to be significantly higher inside the exclusion zone than in nearby protected reserves, likely supported by abundant prey and reduced human disturbance.

Groups of dogs descended from pets abandoned during the evacuation also continue to live in the area, often cared for informally by security personnel stationed around the zone.The question of whether some organisms have evolved true adaptations to survive radiation remains one of the most contested areas of research.

A 2012 study found evidence that soybeans grown in contaminated parts of Chernobyl had adapted to cope better with both radioactivity and heavy metal stress. Bank voles have also shown greater resistance to DNA damage, raising the possibility of inherited protective traits.Mousseau points to the black fungus growing inside the damaged reactor building as one of the strongest examples supporting this theory.

The fungus appears to benefit from increased melanin, which may provide resistance to ionising radiation.He says this supports the idea that melanin offers biological protection, though he rejects claims made by some researchers that the fungus has evolved to use radiation itself as an energy source for growth.

Experiments conducted aboard the International Space Station have also shown that some fungi become darker in response to radiation exposure, reinforcing the idea that melanisation may be adaptive.For Mothersill, the critical issue is whether mutations triggered immediately after the disaster have persisted across generations even as environmental radiation levels declined.

A 2006 study found that chromosomal abnormalities in bank voles continued through successive generations, even after the animals were moved to contamination-free laboratory conditions for reproduction.Not all species have benefited. Recent research suggests that barn swallows living around Chernobyl face increasing strain from the combined effects of radioactive heat exposure and rising global temperatures linked to climate change, reducing their resilience.

The radioactive legacy of Chernobyl also extends far beyond Ukraine. Small amounts of radionuclides linked to the disaster have been detected in edible mushrooms in Poland, blueberries sold in the United States and firewood burned in Greece, demonstrating the long reach of contamination decades after the explosion.

Jonathon Turnbull, a geographer at Durham University, says the exclusion zone should not be viewed simply as either a thriving wildlife refuge or a damaged wasteland.

He argues that the reality is more complex, shaped by radiation, ecological succession and the disappearance of human pressure.The Chernobyl zone, he says, is not evidence that nature has fully recovered or collapsed, but a place where multiple forces continue to reshape life long after the reactor fire was extinguished.