When we calculated the total dose rate from our data using the ERICA tool [ 33 – 35 ], total dose rate ranges of Asian black bear and wild boar in Fukushima Prefecture between 2011 and 2016 were 0.007–0.477 μGy/h and 0.010–12.700 μGy/h, respectively. From the ERICA tool database, minor decreases in body weight and moderate decreases in population density were mentioned for otter species in dose rate, similar to the maximum value of wild boar, however, radiological risk associated with the dose rate range of Asian black bear was not mentioned [ 33 – 35 ]. However, caution should be exercised when using these values because we used body geometries of both animal species from other regions [ 37 , 38 ] for the total dose rate calculation in the ERICA tool. Specially, we used the body height and body width of wild boar as a substitute for body measurements of Asian black bear because these values have not been reported for Asian black bear. In addition, this evaluation did not consider temporal and spatial differences in habitat use by animals that might have been affected on dose rate [ 48 ]. To accurately evaluate the total dose rate of these species, body measurement, particularly mass, body length, body height, and body width of the target species in the study region and consideration of habitat use by the target species is needed.

When we compared the T ag of Asian black bear in Fukushima Prefecture after the FDNPS accident with that of brown bear in Europe after the Chernobyl Nuclear Power Station accident, the geographic mean of Asian black bear (2.2×10 −3 m 2 kg -1 FM) in Fukushima Prefecture was lower than that of brown bear (7.0 x 10 −2 m 2 kg -1 FM) in Finland [ 13 ]. The same trend was found in wild boar; the geographic mean of T ag from wild boar in Fukushima Prefecture after the FDNPS accident (3.2×10 −3 m 2 kg -1 FM) was lower than that in Europe after the Chernobyl Nuclear Power Station accident (8.0×10 −3 m 2 kg -1 FM in 2003 and 6.2×10 −2 m 2 kg -1 FM in 2004) [ 25 ]. However, the range of T ag from wild boar in Fukushima Prefecture (9.2×10 −5 to 9.1×10 −1 m 2 kg -1 FM) was larger than that in Europe (4.0×10 −3 to 1.5×10 −1 m 2 kg -1 FM) [ 8 ]. These differences also might be influenced by food habits differences between Japan and Europe. For example wild boar in Europe forage fungus whereas wild boar in Japan do not. Because these studies had different protocols for sampling animal meat and soil at capture site, we need to compare radiocesium contamination of wild animals in Fukushima Prefecture and Europe using a unified method to elucidate the factor effect on the radiocesium transfer from environment to wild animal.

In our study, the muscle 137 Cs of wild boar was higher than that of bears sampled at the same location. In Europe, the muscle 137 Cs of wild boar was higher than that of ungulate species because wild boar in Europe foraged food with high 137 Cs such as fungus and food items found underground [ 7 , 8 ]. In Japan, wild boar are not known to forage fungus but they do frequently forage for food items found underground such as roots and tubers [ 9 – 11 ]. Asian black bear in Japan have been found to forage ant species found underground [ 12 , 26 , 27 ] and rarely feed on fungi and tubers [ 12 ]. Therefore, we suppose that the higher muscle 137 Cs in wild boar is a consequence of frequent foraging on roots and tubers compared to a much lower inclusion of these food items in the diet of the bear. To further identify the reasons for higher muscle 137 Cs in wild boar, examination of 137 Cs concentration in individual food items in the diet of each animal is needed.

Our study demonstrated a positive relationship between muscle 137 Cs and soil 137 Cs in Asian black bear and wild boar at each capture site. This result supports the findings of previous studies showing that radionuclide concentration in muscle was high in animals sampled from areas with high radionuclide ground deposition or high radioactive contamination level [ 6 , 20 – 22 ]. Thus, determination of radioactive contamination levels or radionuclide ground deposition at sampling sites seems to be important for analyzing radionuclide concentration in wildlife sampled from a large study area. However, muscle 137 Cs was highly varied among animals captured in areas with similar soil 137 Cs levels. As shown in Fig 2 , muscle 137 Cs varied by one order of magnitude among Asian black bears that were captured in areas with similar soil 137 Cs. In wild boar, muscle 137 Cs varied by three orders of magnitude in areas with similar soil 137 Cs. These variations suggest the importance of accounting for other factors such as season in order to understand the mechanism of radionuclide concentration in wild animals.

Seasonal variation in muscle 137Cs

Our study revealed that muscle 137Cs of wild boar and Asian black bear varied by season, and the pattern of seasonal variation differed by species. In Europe, seasonal variation in muscle 137Cs was observed in roe deer (Capreolus capreolus) and wild boar, and muscle 137Cs of those species was high from summer to autumn when fungi and underground food items accounted for a greater proportion of the diet [8,16,17], and it was lower in seasons and in regions with a higher availability of hard masts for food items [23–25]. On the other hand, seasonal variation in muscle 137Cs was not observed in red deer (Cervus elaphus), which did not forage fungi [8]. Therefore, it is possible that seasonal variation in muscle 137Cs in wild animals is affected by the food habits of the animals. In wild boar in Fukushima, muscle 137Cs was low from spring to summer and high from autumn to winter. This seasonal variation pattern supported our hypothesis 1, but it differed from seasonal patterns observed in Europe. Wild boar in Japan mainly forage the leaves of grasses and woody species in spring and summer, and mainly forage roots, tubers and hard masts in autumn and winter [9–11]. Thus, we expected that the consumption of roots and tubers would result in high muscle 137Cs in wild boar in Fukushima. However, because wild boar are omnivores, their food habits change with changing habitat conditions, resulting in seasonal variation patterns in muscle 137Cs. For example, when hard masts are abundant, muscle 137Cs decreases during part of the peak season of muscle 137Cs because wild boar forage large amount of hard masts in autumn [23,25]. Further, studies comparing 137Cs concentration in food items in the diets of wild animals in Japan are lacking. Further study of the seasonal food habits of wild boar in Fukushima and 137Cs concentrations in food items are needed to further elucidate the reasons for seasonal variations in muscle 137Cs.

In Asian black bear, muscle 137Cs increased from autumn to winter. This result did not support our hypothesis 2, but our hypothesis 3 was supported. The autumn and winter seasons coincide with large intakes of hard masts and denning [12,26,27]. Because previous studies of wild boars reported that muscle 137Cs was low when foraging on hard masts was higher because hard masts contained low 137Cs [23–25], factors other than 137Cs concentration in food items are likely to be responsible for the elevated muscle 137Cs of Asian black bear. The muscle 137Cs of roe deer increased during the season in which food intake was higher [17]. Therefore, we should not only study food habit and 137Cs concentration in food items, but also the effects of intake rate on seasonal variation in muscle 137Cs of the Asian black bear. However, the food habits of the Asian black bear in Fukushima and 137Cs concentration in food items has not been reported, and these remain important issue for understanding the seasonal variation in muscle 137Cs.

Our hypothesis 3 was supported by the study results, however, understanding the physiological characteristics of the target species is also important to revealing the mechanism of seasonal variation in radionuclide concentrations. Particularly for Asian black bear, muscle 137Cs increased in autumn when bears prepared for denning and peaked in the winter denning season. Other bear species such as the American black bear (U. americanus) and brown bear (U. arctos) also have a denning season in winter [49], and the American black bear does not urinate or void during this period [28]. Suppression of evacuation in the denning season probably accelerates the accumulation of 137Cs, and physiological changes in bear species in preparation for denning begin to take place in autumn [50,51]. Thus, if the physiological changes in the denning season of Asian black bear are the same as those or other bear species, these changes may result in an increase in muscle 137Cs from autumn to winter. To understand the mechanisms of seasonal change in muscle 137Cs in bear species, understanding physiological changes during the denning season is also important.

The muscle 137Cs of Asian black bear changed most drastically during autumn when they mainly foraged on hard masts and had the lowest muscle 137Cs in September followed by the highest muscle 137Cs of the year in November and December. It is possible that changes in the location of foraging as hard masts drop from treetops to the ground during late autumn may play a role in the increase in muscle 137Cs of Asian black bear. In the forest environment, radionuclides are more heavily concentrated at the forest floor, which is the habitat of the Asian black bear and wild boar [52,53]. Thus, it is possible that intake of radionuclides increased when the bears foraged hard masts on the ground through foraging soil and litter along with the hard masts. To reveal the mechanism of the changes in muscle 137Cs in Asian black bear in autumn, it is necessary to show the relationships between intake of radionuclides and changes in foraging location. Recently, bio-logging technology (e.g. Global Position System collar and accelerometer) to monitor animal behavior has advanced and the application of such technology to study the relationships between radionuclide intake and animal behavior, especially changes in foraging location, is promising.

In this study, no data were obtained for muscle 137Cs of Asian black bear during February to March, which is after the end of the hunting season on February 15 and when bears enter denning. Obtaining muscle samples during this period will be difficult but important to elucidating the relationships between radionuclide concentrations in the muscle of the Asian black bear and physiological changes due to denning.