Fukushima’s boar-pig hybrids reveal how nature can heal after humans

On March 11, 2011, a magnitude 9.0 earthquake — the largest ever recorded in Japan — generated a tsunami that devastated the island nation. The tsunami also triggered an accident at a nuclear reactor in the Fukushima region of Japan, leading to the evacuation of 164,000 residents within a 20-kilometer radius of the reactor.

Among the evacuees were pig farmers who left their swine behind — fleeing from the threat of nuclear radiation.

According to research published Wednesday in the journal Proceedings of the Royal Society B, the absence of humans and the sudden release of pigs into the wild led to new boar-pig hybrids that are now reclaiming Fukushima — though researchers don’t know if these hybrids will last in the long run.

“While humans cannot return, wildlife did return and even thrived in this human abandoned landscape,” co-author Donovan Anderson, a Ph.D. student at Fukushima University, tells Inverse.

How animals reclaim lost habitat

In the early days of the pandemic, several reports emerged of animals reclaiming urban spaces, ranging from coyotes that traipsed around San Francisco to goats overtaking a Welsh town. Animal lovers cheered that nature had finally “healed” after pandemic-related lockdowns.

However, the history of animals reentering natural spaces or expanding their range begins pre-pandemic — and often comes in the wake of disasters. The most famous example is the 1986 Chernobyl nuclear disaster, considered the world’s worst nuclear reactor accident.

After the Chernobyl nuclear reactor accident, which forced the evacuation of human residents from the area, wolves and elk populations grew in the so-called “exclusion zone,” thriving in the absence of humans. Animals sometimes can adapt to changed environments in a way humans cannot.

“Wildlife, especially large mammals, can genetically mix with new populations that they could not before,” Anderson explains. “Maybe they can roam to new areas, or as a result of increased population densities, they must disperse greater distances to search for food resources.”

In recent years, plants at Chernobyl have even flourished — regrowing new cells and adapting to a changing environment. A number of species threatened across wider Europe now live in the Chernobyl Exclusion Zone, including a wild species known as Przewalski’s horses.

The boar-pig study authors note that wild boar also increased in Chernobyl in the wake of the accident. This population eventually stabilized then declined — possibly forecasting the future of the boar-pig hybrids in Fukushima as well.

What’s new — After the Fukushima disaster, wild boars who lived in nearby hills moved into evacuated towns. Research suggests these wild boars are “relatively” radioactive, and there have been efforts to kill and remove them from the region.

Some of these boars, this new study shows, encountered abandoned pigs and mated.

The researchers analyzed the boar populations in the evacuation zone of Fukushima, finding that 31 wild boar, or 16 percent of the population, were hybrids of pure wild boar and domestic pigs.

While these hybrid boars reclaimed the natural areas surrounding Fukushima, they didn’t stray too far. Roughly 75 percent of the hybrid boars were located within 20 kilometers of the nuclear reactor site. Researchers identified only one female hybrid outside the initial 20-kilometer evacuation zone.

Moreover, although the pigs interbred with the wild boars, their genes didn’t seem to last across successive generations. Researchers found that boars with pig ancestry had only 8 percent of pig DNA on average.

“In case of the invasive species — pigs that were released from the [Fukushima] event — we think they had poor adaption or survival characteristics in the wild,” Anderson says.

Theoretically, increased genetic diversity from pigs could help these boars combat swine fever and other diseases, but that doesn’t seem to be the case with these boar-pig hybrids.

“Our supplemental results suggest boars have very low genetic diversity in this area compared to other areas of Japan,” Anderson says.

However, Anderson adds that while the pigs failed to adapt to the natural environment, the “boars’ resilience” may have aided them as they roamed evacuated towns and farmlands, which were suddenly absent of human life.

How they did it — The researchers analyzed the muscle samples of 243 wild boars, pigs, and boar-pig hybrids from the initial 20-kilometer area around the Fukushima reactor as well as the expanded evacuation zone up to 40 kilometers away.

With some help from pre-existing literature on the topic, the researchers used the mitochondrial DNA of the boars to identify animals that carried pig genes from their maternal lineage.

In other words: if a female pig mated with a male boar, the researchers would be able to identify these hybrids.

These methods have some limitations. For example, the researchers suggest that male pigs may better survived in the wake of the Fukushima disaster than female pigs. So if the male pigs mated with female boars, the researchers may not be able to identify these hybrids as well.

Why it matters — The findings point to the complex ways that animals adapt to their environments in the wake of human disasters. Fukushima is an especially interesting, albeit tragic, case study because humans were absent for many years. To this day, some areas remain empty.

“Humans and human activities shape landscapes and the wildlife within.”

As a result, it’s a unique place to study how animal populations can evolve in these areas that are suddenly devoid of human life.

“Humans and human activities shape landscapes and the wildlife within,” Anderson says. “When there is an environmental disaster like the Fukushima events, there are mandated evacuations of people for a period of time.”

While the wild boar had already been expanding its range to northern regions of Japan due to global warming and the climate crisis, the Fukushima disaster likely accelerated their growth, Anderson explains.

“While we suggest from the genetic data that the boar expanded north as temperatures warmed and predators went extinct in Japan, we also believe that the wild boar expansion increased after the Fukushima event,” Anderson says.

What’s next for the boars — Due to the low percentage of pig DNA in these boar hybrids, the researchers aren’t very hopeful that these individuals will remain hybrids in the long run. The researchers’ DNA findings suggest that the proportion of pig genes will decline over time.

“We think the pigs' genes and lineages will disappear in our local populations,” Anderson says.

The eventual return of humans to Fukushima will likely challenge the boars’ ability to roam freely as well, potentially putting an end to the boars’ free reign over the land.

“We do not expect these adaption changes in boar, likely caused from the absence of people, to maintain in populations especially as [human] disturbance returns,” Anderson says.

For now, this sounder will continue to pillage and roam — as humans consider whether or not they are ready to return to the region they were forced to abandon.