Room without a view—Den excavation in relation to body size in brown bears

Abstract

Hibernation is an adaptive strategy to survive harsh winter conditions and food shortage. The use of well‐insulated winter dens helps animals minimize energy loss during hibernation. Brown bears ( Ursus arctos ) commonly use excavated dens for hibernation. Physical attributes of excavated dens are expected to impact the bear's heat retention and energy conservation. The objective of this study was to examine the determinants of cavity size of excavated dens and the impact of physical attributes of excavated dens on energy conservation in hibernating bears, hypothesizing that bears excavate dens in a way to minimize heat loss and optimize energy conservation during hibernation. We predicted that den cavity size would be determined by the bear's body size and that older bears would excavate better‐fitting cavities to minimize heat loss, due to their previous experience. We further predicted that physical attributes of excavated dens would affect the bears’ posthibernation body condition. Our results revealed that bears excavated a den cavity in relation to their body size, regardless of sex, and that older bears tended to excavate better‐fitting den cavities compared to young bears, as we expected. Older bears excavated better‐fitting den cavities, suggesting a potentially experience‐based shift with age in den‐excavation behavior and an optimum cavity size relative to a bear's body size. Our key finding is that insulation of excavated dens provided by wall/rood thickness and bedding materials had a significant positive effect on bears’ posthibernation body condition. We believe that our study provides new insight into how not only the quality of denning habitat, but also the quality of dens may affect hibernating animals, by presenting a potential adaptive aspect of den preparation (age effect on efficiency in den excavation) and effect of den attributes on the posthibernation body condition of brown bears.

Seasonality in Biological Rhythms in Scandinavian brown Bears

“…For future studies, adding light sensors to the collars of hibernating brown bears could add valuable information about the environmental cues bears perceive in their dens. Additionally, Shiratsuru et al, 2020 found that older bears tend to choose den sites with a better fit to their individual body shape and better insulation properties, which in turn had a positive effect on their post-hibernation body condition index. The choice of den site and den cavity size/shape based on individual experience by the bear may have indirectly influenced the expression of biological rhythms, even though we did not find any effect of age and did not test for specific den cavity to bear-body shape-ratio.…”

Selection of den sites and chronology of denning by black bears in the eastern Sierra Nevada and western Great Basin

“…Additionally, factors that have the greatest effect on fitness in terms of selection should occur at greater spatial scales and lesser fitness limiting factors should occur at smaller spatial scales (McLoughlin et al., 2002 ; Rettie & Messier, 2000 ; Waller et al., 2012 ). Nevertheless, characteristics immediately around the den, such as concealment, and the structure and type of den affect energy efficiency and concealment, which also have direct effects on fitness, through survival of offspring and body condition of individuals over winter (Baldwin & Bender, 2008 ; Shiratsuru et al., 2020 ). Our data indicated that including field‐based measurements with the remotely sensed data at the smallest scale resulted in a model that showed strong selection for concealment of the den site in addition to being located in rugged terrain, which was the only variable that entered the remotely sensed model at that scale.…”

“…Bears cannot readily flee dens if disturbed without substantial energetic costs, and disturbance also can result in lowered body condition of adults and reduced survival of young (Baldwin & Bender, 2008 ). Dens that were more energetically efficient resulted in brown bears emerging from dens in better body condition than those occupying dens that were less energy efficient (Shiratsuru et al., 2020 ). Time spent in dens during winter may vary across a species range or annually with fluctuations in climate and availability of resources (Amspacher et al., 2023 ).…”

“…Dens that were more energetically efficient resulted in brown bears emerging from dens in better body condition than those occupying dens that were less energy efficient (Shiratsuru et al, 2020). Time spent in dens during winter may vary across a species range or annually with fluctuations in climate and availability of resources (Amspacher et al, 2023).…”

Extrinsic and intrinsic factors drive the timing of gestation and reproductive success of Scandinavian brown bears

“…Furthermore, the characteristics of the den could influence the reproductive success. Older bears excavate better-fitting and more energy-efficient dens compared to younger and less experienced bears (Shiratsuru et al, 2020), and the probability for a pregnant female to use an excavated anthill, which provides a better thermal insulation than other den types, increases with age (Mannaart, 2016). Therefore, adult females could have reduced heat and fat losses during hibernation compared to young adults, leading to a better body condition at den emergence (Shiratsuru et al, 2020) and therefore a higher reproductive success.…”