1 2 3 4 Room without a view -den construction in relation to body size in brown bears. 5 6 7 8 Abstract 29 Hibernation is an adaptive strategy to survive harsh winter conditions and food shortage.30 The use of well-insulated winter dens helps animals minimize energy loss during 31 hibernation. Brown bears (Ursus arctos) commonly use excavated dens for hibernation.32 Physical properties of excavated dens, such as the amount of space between a bear and the 33 inner wall, wall/roof thickness, and bedding materials, are expected to impact heat retention 34 and energy conservation of bears. The objective of this study was to examine the impact of 35 physical properties of excavated dens on energy conservation in hibernating bears. Our 36 hypothesis was that bears excavate dens in a way to minimize heat loss and optimize 37 energy conservation during hibernation. We predicted that physical properties of excavated 38 dens would significantly affect the bears' post-hibernation body condition. To test our 39 hypothesis and prediction, we analyzed data collected from brown bears in Sweden with 40 linear mixed effects models, examining (i) what factors affect den-excavation behavior and 41 (ii) if physical properties of excavated dens affect post-hibernation body condition. We 42 found that bears excavated a den cavity in relation to their body size, that older bears 43 tended to excavate better-fitting den cavities compared to young bears, and that the physical 44 properties of excavated dens did not significantly affect a bears' post-hibernation body 45 condition. Older bears excavated better-fitting den cavities, suggesting a potentially 46 experience-based shift with age in den-excavation behavior and an optimum cavity size 47 relative to a bear's body size. The strong year effect shown by the most parsimonious 48 model for post-hibernation body condition suggests that variations in physical properties of 49 excavated dens are possibly negligible, compared to the large annual variations in bioticPage 3 50 and abiotic factors affecting pre-hibernation body condition and heat loss during 51 hibernation. 52 53 Key words: body condition, brown bear, den excavation behavior, den, energy 54 conservation, hibernation, Ursus arctos, 55 Page 4 56 Introduction 57 Hibernation is a physiological and behavioral adaptation through which animals 58 survive harsh seasonal conditions, such as inclement weather or low food availability, by 59 minimizing energy loss [1-3]. Small mammalian hibernators, such as arctic ground 60 squirrels (Spermophilus parryii) and Alpine marmots (Marmota marmota), decrease their 61 body temperatures to around 0 C or even lower during hibernation to overcome their high 62 mass-specific metabolic rates and low amount of body fat stores. On the other hand, large 63 mammalian hibernators with large amount of body fat stores, such as brown bears (Ursus 64 arctos) and American black bears (U. americanus), decrease metabolic rates while 65 maintaining relatively high body temperatures [4-5]. 66 Most brown bears an...