Duško Ćirović, University of Belgrade Faculty of Biology

Hibernation occurs as a unique strategy enabling some mammals to cope with severe environmental conditions. Bears are considered facultative hibernators and also the only mammal species in which females give birth during hibernation period, meaning that for pregnant females, hibernation is obligatory. However, it has been shown that bears, mostly males, might change their winter behavior, causing them to remain occasionally or constantly active during winter. We analyzed winter behavioral patterns of 8 brown bears monitored in Serbia between 2021 and 2026, resulting in a total of 15 bear winters. Previous study suggested four different hibernation strategies: an obligate hibernation strategy in females, a facultative hibernation where individuals hibernate all winter, a facultative intermittent hibernation and a strategy where individuals remain active for the entire winter. However, new data revealed a remarkable finding; one female with yearlings hibernated for only 41 days, which is substantially shorter not only compared to previously obtained results for this reproductive category (136 days), but also to the overall denning duration range for females in general (75-172 days). In addition, we identified a long-distance excursion made in late October (pre-hibernation period) while accompanied by four COY. Total length of excursion was ~53 km, calculated as the furthest point from the centroid of the 99% dBBMM UD. Over 49 days, she covered a total distance of 303 km, traveling on average 6 km/day. She was moving from Varda Mountain (BIH), across the Tara Mountain (SRB), crossing the Drina River, all the way to Srebrenica, before she returned to Varda, using almost the same route. There is evidence that she was disturbed by hunting activity, highlighting the serious impact that human-related disturbances can have on bears. This may pose an increased risk for females with COY, as their movement is constrained by the limited mobility of their offspring.