Factors affecting body weight fluctuation in free-ranging Asian black bears

The Asian black bear (Ursus thibetanus) shows seasonal changes in body weight, activity level, and range use. However, there is little information available on these factors in individuals in the wild. We documented the body weights of free-ranging Asian black bears from spring to autumn, 2017 to 2019, in the Nikko-Ashio Mountains, Tochigi Prefecture, Japan, by using a newly noninvasive body weighing system (NIBWS) that we developed. We also fitted Asian black bears with global positioning system (GPS) collars with built-in activity sensors to evaluate the relationship between body weight, behavioral factor, and activity level. Bears visited the NIBWS 88 times in 2017, 176 times in 2018, and 321 times in 2019. We recorded the body weights of 4 bears in 2017, 15 bears in 2018, and 11 bears in 2019. Specifically, we consecutively recorded the body weight of an adult female from 2017 to 2019 and collected GPS location data from 2017 to 2018. The adult female was solitary in 2017, and she was accompanied by 2 cubs in 2018 and the yearlings of those cubs in 2019. Her body weight was lowest in 2018, likely because she had spent a large amount of her energy expenditure on nursing her cubs, compared with the year when she was solitary or nursing her yearlings. The GPS data showed that no significant relationship was found between body weight and any behavioral factors. Although her activity level decreased drastically in late August, her body weight did not decrease accordingly. In contrast, a subadult female that we tracked in 2019 increased her body weight during May to August, likely because she was solitary and required high intake of energy and nutrition for growth. Although our results provide only a snapshot, we established NIBWS and this is the first record of body weight changes over multiple years in free-ranging Asian black bears to our knowledge.