Denning Ecology

Brown bears hibernate in dens during winter, the time of year when their food supply is almost nonexistent. In preparation for the denning period, bears enter on a phase called ‘hyperphagia’ where the animals are heavily focused on eating and maximizing weight gain. In Scandinavia, bears have been reported to increase their body mass by over 40% during this phase (1, 2). This is important because during the denning period, they remain inactive for between 5 to 7 months relying on stored fat for energy (3, 4). Although bears are born with the instinct to den, there still appears to be a learning curve when it comes to den choice and preparation (5).

 

Den selection and preparation

 

In Scandinavia, bears hibernate in dens within their home range, making a new den each year rather than using an old one (6). New dens are often made near previous years’ den sites with the exception of subadult bears, especially males, who select dens further away from previous dens than adults (6). For example, recently independent subadult males den near their mother during their first winter, then move long distances before denning again the next winter (6). This pattern is connected to dispersal, as these young bears are in the dispersing phase, with males dispersing the furthest (6). Most bears visit and explore their denning area several times during summer, suggesting they may actively seek out and assess denning areas and choose their dens in advance (6, 7). Indeed, females have been observed visiting their next denning area almost once a month throughout the summer and fall (7). Bears must choose their den wisely, as a poor den type and location may have fitness consequences for them the following spring (e.g., via potential for disturbance and den abandonment).

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Abandoned anthills that are excavated to suit the bears needs are the most common type of den used in the Southern Study Area (6). Other den types include cavities dug into the soil (often built at or into slopes) or below uprooted trees, built in stone/rocky caves or outcroppings, or open air ‘basket dens’ which are shallow depressions in the ground that are covered with branches and twigs and the only insulation from above is the snowfall that encases them in a type of igloo. Abandoned anthills are used more often by females than males, possibly because the thick, stable, overgrown walls provide good insulation from the cold and protection from disturbance (8, 9). On the other hand, the more open basket dens are almost exclusively used by large adult males who stay in those den types almost a month less compared to bears using other dens (8). Regardless of the den type chosen, all bears make a bed consisting of tree branches, twigs, moss, and underbrush.

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Insulation is key and den attributes associated with insulative properties, such as wall and roof thickness and cavity size, can affect body condition post-hibernation (5). As one might expect, the better insulated dens are correlated with better spring body condition which is not necessarily a function of den type per se, but rather the ‘quality’ of the chosen site (5). Intuitively, we have found that bears excavate dens in relation to their size, meaning larger bears create larger cavities (5). However, there appears to be somewhat of a learning curve to den preparation. For example, older bears seem to prepare dens better-fit to their size compared to more inexperienced subadults (5). Regardless, we have found no effect of den cavity size on bear body condition after they emerge from their den (5).

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Bears of all sex and age classes tend to select for dens with similar surrounding habitats (9), although adult male den site selection does differ from females and younger bears (8). In general, bears in Scandinavia tend to den most often in open canopy forests (e.g., Scots pine) and areas with thicker vegetation, and less often in more closed canopy forests (e.g., older Norway spruce and Scots pine), younger or mountain coniferous forests, clear cuts, and bogs (9). Bears avoid denning in alpine mountain-birch or deciduous forests and areas with peat or exposed bedrock (9). They also select for steeper slopes and easterly aspects. Bears also tend to avoid humans. In general, brown bears select den sites at least 2 km from infrastructure with regular human activity, e.g., main roads and buildings (10). Bears in Scandinavia also tend to select dens that are more concealed and in more difficult to access terrain when the potential for human disturbance is higher, e.g., when they are closer to roads and human settlements (3). Furthermore, adult males tend to den further away from main roads and houses than other bears, suggesting they are less tolerant of human activities (8). Altogether, this suggests that bear den site selection and behavior are likely heavily influenced by human activities. See Human disturbance and brown bear behavior.

 

Den entry

 

Activity, heart rate, and body temperature start to drop slowly several weeks before den entry (11), with a marked reduction in activity prior to entering the den, sometimes called ‘pre-denning activity’ (12). This marked reduction in activity generally begins about a week or so before bears enter the den  (12).Many bears exhibit this marked reduction in  activity before reaching their den area (58%), while others do so after they arrive (42%) (12). Either way, bears tend to stay in the denning area for about 4-5 days before they enter the den (12). Interestingly, younger bears appear to spend more time near the den prior to den entry, possibly because they are inexperienced at building dens (12).

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The mean date of den entry for bears in Scandinavia is 27 October (range= 5 October-18 November) for males (6) and 18 October (2 October-5 November) for females (13). Most bears go into their dens at around the same time with the exception of solitary and pregnant females who may enter their dens about 10 days earlier than females with cubs of the year or yearlings (7). Den entry seems to be generally initiated by environmental cues, mainly first snow fall and when cold temperatures (≤ 0°C) arrive (11), however, bears will begin hibernation before first snow fall if the snow arrives late and the temperatures dip down (6, 13). Interestingly, pregnant bears also tend to den earlier when there was more food available in their environment throughout the active part of that year, possibly because they reached their peak fat storing capacity (13).

 

The hibernation period

 

Duration: Brown bears in Scandinavia spend between 5 to 7 months in their winter dens. In general, the length of time they stay in their winter dens decreases as they get older and as their body size increases, i.e., older, younger bears den for longer, while larger bears den for shorter periods (6, 14). Females stay in their dens longer than males, with pregnant females having the longest denning period (6, 7). Male bears spend an average of 161 days in their den, while female bears spend on average 181 days, with pregnant females spending an average of 196 days, solitary females and females with cubs 168 days (6, 7). For more on the physiology of hibernating bears and the implications of their physiology during hibernation for human health see Ecophysiology of Scandinavian brown bears and Bear physiology and human health.

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Den abandonment: Bears do occasionally abandon their dens and move to new locations for the remainder of the winter, moving up to 30 km before denning again (mean of 5.1 km) (15). In Scandinavia, around 22% of bears abandoned their dens, of which 95% of these den changes occurred before December 15, early in the hibernation period (12). Although bears will switch dens later in winter, i.e., after December 15, if the disturbance is great enough, bears that abandon their den later in spring often do not attempt to den again (2, 12). In general, females are generally less likely to abandon their dens than males (12). Interestingly, prior visitation to the denning area also appears to decrease the chances that a bear will abandon its den (12).

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Bears might change their dens for a variety of reasons, however, it has been suggested that human disturbance might the driving cause of den abandonment in Scandinavia (15), and there multiple lines of evidence to support this. For example, abandoned dens are more likely to be located closer to plowed roads, which might increase the potential for human disturbance (8). Bear often abandon dens when people visit the area, especially if they visit early in the denning period or approach within 200 m of the den site (10, 16). It is not ideal for bears to have to abandon the den and select a new one. The energy used to wake up and find and prepare a new den can decrease their body condition after den emergence as well as have fitness consequences. For example, of pregnant females that changed dens, 60% lost cubs compared to only 6% of pregnant females that stayed in the same den (15). See Effects of capture on Scandinavian brown bears for information on disturbance effects of capture during hibernation.

 

Den emergence

 

The mean date of den emergence for bears in Scandinavia is 4 April (range= 6 March–25 April) for males (6) and 20 April (6 March–14 June) for females (7). The end of hibernation and subsequent den emergence are closely linked to internal physiological cues linked with thermoregulation, rather than environmental cues such as daylength or the timing of snow melt (11). Likely, when the temperature in their dens became too warm, they moved outside to better maintain their body temperature (11). Females with newborn cubs emerge from their dens almost 1 month later than the other bears, and generally stay close to their den site in the days following, likely because the small cubs are not able to travel far, but also to decrease the risk of infanticide (7).

 

Differences with latitude

 

With a few exceptions, most research on bear denning behavior in Scandinavia has been conducted in the Southern Study Area. What we do know is that bears in northern Scandinavia hibernate longer than they do in the south; they enter their dens earlier and leave their dens later (6). On average, northern males and females spent an extra 45 and 37 days in their dens, respectively, than their southern counterparts (6). Snowfall and freezing temperatures arrive earlier and last longer in the north which both limits the growing and foraging season as well as triggers denning activity.