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Overwintering is important for the survival of northern mammals, and northern ungulates deposit body fat. Researchers have interpreted fat deposition as an adaptation to food shortages during winter. However, it is likely that reproductive activities also affect fat deposition. We compared kidney fat indices (KFIs) of sika deer (Cervus nippon) between the northern and southern populations in Japan. In the northern population, the KFIs of adult males and females peaked during summer and autumn, respectively. In adult males, the KFIs abruptly declined during the rut (October), while in yearling males and fawns, they were low in summer. In the southern population, KFIs of adult males were high in summer but decreased in October. KFIs of adult females were low and increased in December–January. KFIs of fawns were low. These results suggest that most classes of the southern population did not deposit much fat during the summer except for adult males. The result that southern adult males displayed as much fat deposition as the northern males suggests that body fat was deposited not only for overwintering but also for reproductive activity of adult males. [ABSTRACT FROM AUTHOR]