Overwintering foraging tactics of sedentary sika deer in a region prone to heavy snow in northern Japan

Large-sized herbivores in cold regions often suffer from deep snow, which sharply increases their physio-behavioral costs. Among such animals, sika deer (Cervus nippon) have recently expanded their distribution into northern mainland Japan, one of the snowiest regions on Earth. In this study, we aimed to reveal the overwintering foraging tactics adopted by sika deer in these regions. We recorded the feeding marks of sedentary deer for 3 years with different snow accumulations by surveying transects established in southwestern Fukushima (total lengths: 145.5, 133.6, and 172.9 km in 2021–2023, respectively) and analyzed the composition and selectivity of food plants. Our key findings are as follows: (1) sedentary deer fed on the branch tips and bark of 112 woody plant species; (2) although the expected species richness of plants consumed by the deer differed only slightly between different snow conditions based on rarefaction/extrapolation analysis, food plant composition varied widely across the years (Bray–Curtis index >0.5 between every pair of years); (3) Manly's selectivity index indicated that the compositions of positively and negatively selected plant species were sensitive to different snow conditions; and (4) deer relied heavily on bark when snow depth increased; specifically, the probability of debarking when deer fed on plants in the heavy snow year was seven times higher than in the light snow year. These findings reveal that sedentary deer adopt euryphagous and opportunistic feeding tactics by switching their dietary composition to readily available foods in response to different snow conditions.