Seungyun Baek, Akino Inagaki, Tomoko Naganuma, Koji Yamazaki, Shinsuke Koike

Asian black bears in Japan (Ursus thibetanus; hereafter, bears) survive winter food shortages by hibernating to suppress energy expenditure. Then, they consume large quantities of hard mast during autumn hyperphagia to accumulate energy stores before hibernation. Thus, bears presumably need to accumulate sufficient energy during this period and start hibernation at an appropriate time before food resources become depleted. Furthermore, annual fluctuations in mast production may influence the relationship between energy accumulation and the onset of hibernation.
Here, we investigated the energy balance of female bears during the hyperphagia in relation to mast productivity using movement data and video clips recorded via GPS collars (some equipped with video cameras). We estimated daily energy intake of bears by inferring behavioral states from movement and quantifying foraging amounts within each behavioral state using video clips. Daily energy expenditure was estimated by summing the basal metabolic rate and the energy costs of activities calculated for each behavioral state. We defined the daily energy balance as the difference between daily energy intake and expenditure. Total energy accumulation during the hyperphagia was estimated by summing the daily energy balance from September 1st to the onset of hibernation.
Energy accumulation during the period was significantly lower in poor mast years compared to good mast years, with a tendency for smaller inter-individual variation, suggesting that individuals initiated hibernation upon reaching a specific level of energy storage. Additionally, the daily energy balance tended to deteriorate as the onset of hibernation approached, with this deterioration occurring earlier in poor mast years than in good mast years. This implies that the trigger for the onset of hibernation may be the point at which the energy balance deteriorates and falls below a specific threshold after achieving the necessary energy accumulation.