Wild monkeys in the Fukushima region of Japan have blood abnormalities linked to the radioactive fallout from the 2011 nuclear power plant disaster, says a scientific study.
The Japanese macaques were found to have low white and red blood cell levels and low haemoglobin, which the researchers say could make them more prone to infectious diseases.
But critics of the study say the link between the abnormal blood tests and the radiation exposure of the monkeys remains unproven and that the radiation doses may have been too small to cause the effect.
The scientists compared 61 monkeys living 70km from the Fukushima Daiichi nuclear power plant with 31 monkeys from the Shimokita Peninsula, over 400km from Fukushima.
The Fukushima monkeys had low blood counts and radioactive caesium in their bodies, related to caesium levels in the soils where they lived.
Professor Shin-ichi Hayama, at the Nippon Veterinary and Life Science University in Tokyo, said during Japan's snowy winters the monkeys feed on tree buds and bark, where caesium has been shown to accumulate at high concentrations.
"This first data from nonhuman primates - the closest taxonomic relatives of humans - should make a notable contribution to future research on the health effects of radiation exposure in humans," he said.
The work, which ruled out disease or malnutrition as a cause of the low blood counts, is published in the journal Scientific Reports. White blood cell counts were lowest for immature monkeys with the highest caesium concentrations, suggesting younger monkeys may be more vulnerable to radioactive contamination.
Hayama noted: "Abnormalities such as a decreased blood cell count in people living in contaminated areas have been reported from Chernobyl as a long-term effect of low-dose radiation exposure."
But Professor Geraldine Thomas, at Imperial College London, said the Chernobyl studies were "not regarded as scientifically validated" and that the correlations between the caesium and low blood counts were not statistically strong.
"Unfortunately this is yet another paper with insufficient power to distinguish real effects and relevance to human health," she said.
"We know that one of the most damaging health effects comes from fear of radiation, not radiation itself."
She also noted that people, unlike the wild monkeys tested, could easily avoid eating contaminated food from the Fukushima region.