Eastern tree frogs sampled in the Chornobyl Exclusion Zone had darker skin on average than frogs sampled outside it. A 2022 study found that darker coloration was associated with being closer to places that had high radiation levels after the 1986 accident—not with the frogs’ current individual dose rates. The researchers suggest historical radiation may have favored darker frogs through natural selection, but their study does not prove how the difference arose.
What the study found
The 2022 study examined male Eastern tree frogs (Hyla orientalis) at 12 localities in northern Ukraine: eight inside the Chornobyl Exclusion Zone and four outside it, about 40 km east. Researchers sampled reproductive adults during three breeding seasons, from 2017 to 2019. They measured dorsal skin luminance, a measure of lightness. Lower luminance means darker skin.
On average, frogs sampled inside the zone had 43.6% lower dorsal skin luminance than those sampled outside it. This is a difference between sampled groups; it does not mean every frog in the zone was black, or that researchers watched individual frogs change color over time. The study’s PubMed record summarizes the sampling and result.
Why might the frogs be darker?
Across the sampled locations, frogs tended to be darker nearer areas that had high radiation levels after the 1986 accident. Their coloration was not associated with their measured current individual dose rate. That distinction matters: the geographic pattern points toward historical exposure as a possible influence, rather than showing that present-day radiation levels caused the coloration.
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What the findings do—and do not—say
- They show a population-level difference. The sampled frogs inside the zone were darker on average than those outside it; individual coloration can vary.
- They do not document frogs “turning black.” Researchers compared samples from different places, rather than tracking the same frogs as their color changed.
- Historical geography fit the pattern better than current dose. Coloration was associated with distance from historically highly irradiated areas, but not with measured current individual dose rates.
- Other measured factors did not account for the pattern. The researchers found no association between darker coloration and measured body condition or oxidative status. A short-term background experiment also detected no color change. Those results narrow some possible explanations but do not rule out every environmental or physiological influence.
How a separate genetic study fits in
A 2021 study looked at genetic markers in 19 Eastern tree frog populations in the Chernobyl region, around three decades after the accident. It reported no genetic erosion in the sampled Exclusion Zone populations and found higher mitochondrial diversity than in comparison European populations. The authors described a distinct recent evolutionary history in the haplotype network. This offers context about the populations, but it is separate evidence and does not show that radiation caused the darker coloration. The genetic study is available in PubMed Central.
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