Sabrina Manchi, Andreas Zedrosser, Andrea Friebe, Jon Swenson, Fanie Pelletier

Anthill dens are the most common (53%) winter den type used by brown bears in central Sweden, i.e., a den dug under or into a large, inactive and typically overgrown anthill originally built by red wood ants Formica spp. Especially female brown bears commonly use abandoned anthills as winter dens, as it appears to improve their reproduction success. However, anthills likely require several decades to reach the size large enough to serve as a bear den after the colony has abandoned the site. At the same time, reuse of anthill dens by bears is very rare and red wood ants are an important food source for brown bears. This feeding behavior reduces the abundance and size of potential den sites, especially in areas with high bear densities. Because the bear population has increased substantially in the study area, we suspected that the availability of inactive anthills suitable for dens has declined over time.
The aim of this study was to determine temporal changes in the use of anthills as winter dens by brown bears. We documented 1215 dens used by 270 bears monitored by telemetry from 1986 to 2023. We predicted that a) the use of inactive anthills as winter dens has decreased during the study period, b) this decline is less pronounced in females than in males, and c) the average size of anthills used as dens has decreased over time. Our results supported all predictions. Because anthill denning likely is associated with improved reproductive success in females, a reduction in the use of such dens may negatively affect reproductive rates and ultimately population growth. However, the availability of large, inactive anthills for denning may have been unnaturally high at the beginning of the study, as bears had started to recolonize the study area 50-60 years ago.