The majority of the radioactive materials derived from the FDNPP accident were deposited in the upper soil layer and plant litter in the forest20. Because Apodemus speciosus has low body height and inhabits the contaminated ground surface, it may have suffered higher exposure to external radiation sources than other mammals living in Fukushima. The average dose rate measured on the ground was approximately 1.4-fold higher than the dose rate 1 m above the ground21. The average ambient dose rate of gamma rays at ground level in the Fukushima sites was 4.1–13.9 μSv/h. This level is the same as the “very low probability of effects” level in the ICRP criteria3. Furthermore, Apodemus speciosus, being omnivorous, could feed on soil invertebrates that showed high levels of radioactive material22. Therefore, it may have suffered from higher internal exposures than herbivores experienced.

The ambient dose rates at Site 2 decreased significantly from 2013 to 2014, but the radioactive Cs concentrations of the soil from Site 2 increased non-significantly from 2013 to 2014. Similar trends were also observed in Site 1. One might expect that ambient dose rates positively correlate with radioactive Cs concentrations of the soil. However, disparities were observed between the ambient dose rates and the soil Cs concentrations. The contamination with radioactive Cs is not uniform across the field23. Non-uniform field contamination may have affected the soil values because of small sample numbers and weights, and because the sampled points differed among years.

There are many reports of Cs contamination of wildlife in eastern Japan after the accident, and the situation is becoming clear. In some species of wild birds and mammals in Fukushima, radioactive Cs decreased from April 2011 to the end of 2013. On the other hand, the species inhabiting forests tended to retain high levels of radioactivity24. Furthermore, radioactive Cs concentration levels differed among the species. For example, omnivorous animals such as Asian black bear (Ursus thibetanus) and wild boar (Sus scrofa) showed high concentrations of Cs, while Cs concentrations in herbivorous animals such as deer (Cervus nippon) and duck (e.g., Anas poecilorhyncha) were low25. A previous report showed that the Cs concentration of wild mice captured at Fukushima ranged from 870 to 8,040 Bq/kg wet weight 7 to 9 months after the accident26. In another study, mice captured at a severely contaminated site, where the ambient dose rate measured on the ground reached approximately 60 μSv/h in December 2013, exhibited Cs concentrations of 32,700 ± 23,200 (mean ± SD) Bq/kg wet weight in a mixture of bone and muscle21. Taken together, these results in wild mice showed high variation in Cs concentration even among mice captured at the same site.

It is thought that the large variation in Cs concentration among individual mice may come from differences in food utilization14. Indeed, our preliminary observations indicated that mice that showed higher Cs concentrations preferred to eat bamboo roots rather than conifer cones (Tamaoki et al. personal communication). In addition, the large Japanese field mouse can easily migrate through areas with different contamination levels27. Their migrations are affected by food availability, population size, geography, and breeding status28,29. Further studies on the relationship between Cs contamination and food utilization in mice are warranted.

Germ cell apoptosis is an informative marker of ionizing radiation and some other toxicants30,31. Intracellular reactive oxygen species generated by radiation are major inducers of apoptosis32. Loss of male germ cells by apoptosis, which has the potential to cause infertility, could be attributed to ionizing radiation33. On the other hand, selective removal of damaged germ cells by apoptosis is a very important mechanism for preventing the transmission of genetic abnormalities to offspring34. In this study, there were no harmful effects on germ cells from environmental radiation. Indeed, few apoptotic cells were detected in animals captured at any of the sites, even the highly contaminated Site 1. There have been few studies on radiation effects on male mammal fertility following the FDNPP accident; however, no effects on bull testes and sperm were observed within a 20-km zone around the FDNPP between August 2011 and January 201235. In contrast, there are some reports about sperm morphology following the Chernobyl Nuclear Power Plant (CNPP) accident. House mice (Mus musculus) were captured between 1986 and 1994 within a 30-km zone around the CNPP, and the frequencies of abnormal sperm heads did not differ significantly among sites with different pollution levels36. Another study of 11 species of passerine birds caught in Chernobyl showed that frequencies of abnormal sperm were always higher in heavily contaminated areas of Chernobyl than in uncontaminated areas37. In laboratory mice, low-dose-rate (3.49 mGy/h) gamma ray exposure significantly decreased testes weights and seminiferous tubule diameters at 2,000 mGy total exposure, but a decrease was not observed at 20 to 200 mGy38. In addition, exposure to 25 to 250 mGy X-rays at a low-dose-rate (12.5 mGy/h) significantly increased male germ cell apoptosis, with a maximum effect at 75 mGy33. In that study, percentages of TUNEL positive germ cells and apoptotic spermatogonia were approximately 20% without irradiation, and approximately 60% with 75 mGy irradiation33. Radiation levels in these experiments were considerably higher than the ambient dose rates (under 0.02 mGy/h) at the wild mouse capture sites in Fukushima. Furthermore, it is thought that the effect of external radiation exposure on wild animals is likely to be greater than the effect of internal exposure in wild mammals39,40. Because loss of male germ cells was not detected in this study, we conclude that radiation has not caused substantial male subfertility in wild large Japanese field mice in Fukushima.

In this study, there were no significant differences in the frequency of morphologically normal sperm among wild mice captured at different sites. High dose and high-dose-rate irradiation disrupts spermatogenesis and increases the frequency of abnormal sperm41,42. Spermatozoa are produced through complex processes in the testes and epididymides, and defects in these processes can result in male infertility43. In addition, there is a positive correlation between DNA defects that might affect the next generation and altered sperm head morphology44. There have been a few reports that investigated whether low-dose irradiation causes altered sperm morphology. Following the Chernobyl accident, Møller et al. reported that the frequency of abnormal sperm in some bird species increased at highly radiation-contaminated areas37,45,46. In a laboratory test on ICR mice, low-dose-rate (0.7 mGy/h) gamma ray exposure did not affect sperm morphology at 200 to 4,000 mGy total radiation. However high-dose-rate (48 Gy/h) exposure with similar total radiation levels (from 200 to 4,000 mGy) increased sperm abnormalities significantly47. Another experiment, using Korean dark-striped field mice (Apodemus agrarius), showed similar results48. In contrast, increased rates of morphologically abnormal sperm were not observed in the wild mice captured in highly contaminated areas of Fukushima. Although the data do not indicate exactly how large a radiation dose the mice absorbed before they were captured, we conclude that low-dose-rate radiation at the environmentally observed level would not be detrimental to spermatogenesis.

Our study suggests that high levels of radioactive contamination can have surprisingly limited effects on spermatogenesis in some species. Although we expected field mice would receive higher radiation doses than most other animals, and we expected that testes would be especially sensitive to radiation, we found no evidence of detrimental effects on testes or sperm. Although our study did not measure fertility, it suggests that effects of the FDNPP accident on field mouse spermatogenesis are surprisingly weak. Given that effects were shown in previous studies on other taxa, and the high levels of within-site variation in contamination observed here, further studies are needed to investigate the roles of taxonomic differences in behavior, trophic level, and physiology on the effects of radiation exposure.