On the northern edge of Ukraine, inside the 30-km (19-mile) exclusion zone surrounding the abandoned Chornobyl (commonly known as Chernobyl) nuclear plant, thousands of animals now roam freely through forests, abandoned towns and decaying industrial estates. Among them are the stray dogs – around 900 descendants of the pets left behind, now living in a landscape shaped by the world’s worst nuclear disaster.

Recently, three of them were spotted with unusually blue fur, prompting speculation that either radiation had turned their coats that color, or they’d undergone some kind of mutation that had altered their phenotype. (Or the photos captured were actually AI.) The story of these blue dogs has now been unearthed thanks to the Clean Futures Fund’s Dogs of Chornobyl Program, an organization that’s been providing food and vet care, as well as deploying scientists to research the population, since 2017.

“The blue dye likely came from a tipped over port-a-potty where the dogs were rolling around in the poop as dogs are prone to do (think cat litter box!),” said Timothy A. Mousseau, Scientific Advisor for the Dogs of Chornobyl Program and biologist at the University of South Carolina, in a social media post this month. “The blue coloration was simply a sign of the dog’s unsanitary behavior. As any dog owner knows, most dogs will eat just about anything, including feces!”

Essentially, their fur acted like a sponge, picking up the blue-tinted contaminants it had come in contact with. Subsequent vet checks found no radiation-related illness, no structural abnormalities and no indication of genetic damage.

While these blue dogs have captured the public’s imagination, there’s an even more fascinating scientific story emerging from the semi-feral dog communities of the Exclusion Zone – which includes the abandoned city of Pripyat, around 16 km (10 miles) north of the sarcophagus that covers the radioactive reactor four.

A team of researchers led by Mousseau has found that the dog populations in the Exclusion Zone are genetically distinct from domestic populations elsewhere in Ukraine and Europe. In the study, the scientists examined the genetic structure of 302 dogs, which made up three free-roaming groups living on the power plant’s grounds, as well as animals 15 to 45 km ( (9 to 28 miles) from the site. They compared this data with purebred and free-breeding dogs from around the world and discovered that the isolation had gradually changed their genetic makeup.

“Analysis of shared ancestral genome segments highlights differences in the extent and timing of western breed introgression,” the researchers noted. “Kinship analysis reveals 15 families, with the largest spanning all collection sites within the radioactive exclusion zone, reflecting migration of dogs between the power plant and Chernobyl City. This study presents the first characterization of a domestic species in Chernobyl, establishing their importance for genetic studies into the effects of exposure to long-term, low-dose ionizing radiation.”

The researchers found that certain families of dogs were associated with different areas of the zone – some near the plant itself, others near checkpoints or abandoned villages –indicating that micro-habitats and human food sources have also played a part. The dogs living inside the power plant/industrial area were genetically distinct from those living outside, in the town/urban area, even though the distance between some groups is only around 16 km (10 miles). And dogs from the power-plant zone had increased genetic similarity within their group, reflecting their isolation.

In the study, the team identified 391 “outlier loci” (genome regions) where the two dog populations differed more than expected by chance. And more than 50 candidate genes lie in or near those regions. Some of those genes are involved in DNA repair, immune function, and stress response – changes that could potentially help an animal cope with environmental stressors, including radiation or chemical contamination. However, a following study by Mousseau and team found no evidence of increased overall mutation rates in the power-plant dogs compared with the outer-Exclusion Zone dogs, indicating there’s no real evidence of genetic mutation driven by the animals’ adapting to living in the most radioactive parts of the region.

So, despite speculation that the dogs – and wolves in the surrounding forests – have become genetically more resistant to radiation exposure, the research is preliminary and any differences are most likely due to isolation.

Interestingly, the study found that there were distinct breed differences separating the power-plant and city populations. Around the plant, 9% of their chromosomes could be traced back to shepherds. More than half of all the shepherd-type genetic markers here appeared at higher frequencies (5–10% of chromosomes), while those same markers were seen in just 1–5% of chromosomes in the city population.

The dogs also were found to have low levels of “pinscher-clade” DNA – genetic segments associated with breeds like dobermans and miniature pinschers. Unlike the shepherd ancestry, these pinscher markers appear at similar low frequencies in both the power-plant and city populations, suggesting more recent mixing or shared ancestry, rather than the long-term isolation seen in the shepherd descendants.

And why shepherds? The researchers believe that it’s consistent with Soviet-era use of German Shepherds and East-European Shepherds as guard dogs, in military units and as security at industrial sites. These working dogs would have been prominent in the animals left behind when the area was evacuated.

CFF/Dogs of Chornobyl

The team identified 15 genetically distinct family groups among the 302 sampled dogs – from units of just two closely related animals (usually a parent and a pup) to larger clusters of more than 10. Some of the larger families were spread across multiple locations, suggesting that dogs move through the Exclusion Zone to find mates. That movement has helped maintain a degree of genetic diversity within what is otherwise an isolated population.

It’s also worth noting that scientists have found no evidence of new additions to the dog community since 1986, and the current population is most likely as large as it will get, as Dogs of Chornobyl have undertaken widespread sterilization (and vaccination) programs since 2022.

However, the ways in which the radiation exposure is impacting the dogs is still largely unknown and the focus of ongoing studies.

Of course, the dogs are only one piece of the Chornobyl animal community. In the absence of high numbers of humans, wolves, lynx, wild boar, moose, deer, foxes, European bison and even the endangered Przewalski’s horse can now be found in the area.

A 2004 study looked into how radiation might be impacting four small mammals common to the Exclusion Zone – bank voles (Myodes glareolus), striped field mice (Apodemus agrarius), yellow-necked mice (Apodemus flavicollis) and wood mice (Apodemus sylvaticus). Tracking population sizes across areas with different levels of radiation, the researchers found no evidence that higher contamination resulted in fewer rodents. And while radiation can be detrimental to individual animals, its impact on population sizes appeared to be offset by advantages – the absence of humans, predators and agricultural disturbances. Meanwhile, a 2021 study looking at gut bacteria and fungi changes in the same four species found that local environments shaped small-mammal gut microbiomes far more than radiation, and any radiation-related microbial changes were subtle and inconsistent across species.

Looking at the data that has been gathered – particularly in the last 20 years – the Chornobyl disaster has not created a habitat of mutants but a surprisingly abundant wildlife (and dog) sanctuary. While there is some evidence that radiation exposure is having a detrimental impact – Mousseau identified the rise of cataracts in birds in 2015 – there’s still a lot we don’t know about how it’s effecting life in the region.

“The majority of studies investigating populations of plants, animals and microbes in the Chornobyl Zone have not found any evidence of signs of adaptive evolution,” Mousseau noted. “There is only one study that shows what might be adaptation to radiation and it is for bacteria living on the wings of birds. This is not surprising given that bacteria can reproduce very quickly with thousands of generations since the disaster, allowing for adaptive evolution whereas dogs and most of the other plants and animals often have only a single chance to reproduce per year, which dramatically slows evolutionary response to change.”

Across many long-term studies, mammals in particular have shown surprising resilience, however, just as much research has found negative health outcomes across a broad range of species. While radiation can cause measurable biological damage to individuals, the removal of humans – including farming, hunting and habitat disturbance – has also had a huge impact.

“In general, the majority of published scientific studies show that many of the organisms surveyed show significant negative impacts of the radiation in the areas of the zone where radiation is high, but are largely unaffected in areas that are relatively ‘clean’ (i.e. not radioactive),” Mousseau said. “Most people do not realize that within the 2,600-km2 (1,000-miles2) Chornobyl Exclusion Zone (CEZ), perhaps only 30% of the land area would be considered hazardous (i.e. significantly radioactive), while the remainder is relatively ‘cold’ (i.e. not radioactive). The CEZ is actually a kind of quilt work or mosaic of radiation levels that reflects the patterns of wind direction and rainfall at the time of the accident. It is not uniformly radioactive.”

And the dogs remain the most visible and emotionally relatable inhabitants, helping to reshape how we think of the area surrounding the disaster zone – once considered to be uninhabitable for any life.

“Contrary to some reports in the media, the Chornobyl dogs show no signs of elevated tumor (i.e. cancer) rates, but also show no signs of reduced cancer rates,” Mousseau wrote. “The truth is that cancers are generally a disease of old age (in both dogs and humans) and most dogs in the harsh conditions of Chornobyl do not live long enough to express cancers, even if they were predisposed to do so.”

CFF/Dogs of Chornobyl

As for the wolves being immune to cancer –news that was widely circulated in 2024 on the back of this conference abstract from Princeton University biologists – at best it’s not confirmed, with their rise in numbers also most likely due to the absence of humans.

“In truth, there is no report published in the scientific literature to support this claim,” Mousseau said. “A few scientists have behaved irresponsibly by promoting this idea in the absence of peer reviewed scientific data to support their claim. In addition, given the very small size of the Chornobyl wolf population (i.e. a few dozen individuals), an epidemiological study demonstrating an association between radiation, cancers, and immune system genetic changes would be impossible. Such studies usually require millions of observations (and certainly minimally tens of thousands) as even when cancer rates are high, they are still relatively rare, making statistical associations very challenging. And, as stated above, cancers are usually a disease of the old, further reducing the likelihood of seeing them in a natural population where life spans tend to be relatively short.

“More generally, the growth of the wolf population in Chornobyl has been often cited as an example of re-wilding and used as evidence that radiation may not be that dangerous, and that hunting is the main reason that wolves were absent from this region prior to the disaster,” he continued. “Although the former (i.e. hunting) is likely true, there is no evidence to suggest that wolves are not being negatively impacted by the radioactive contaminants. All we can say for sure is that hunting is likely a more important factor affecting wolf populations than radiation, which is not really that surprising.

“There is only one clear scientific study showing signs of adaptive evolution to radiation at Chornobyl, and this was for bacteria which have had thousands of generations of selection and thus time to evolve,” he concluded.

Sources: Dogs of Chernobyl (Facebook), Clean Futures Fund, Science Advances