Ecophysiology of
Scandinavian Brown Bears

Understanding the fundamental baseline physiology of brown bears allows us to assess how physiological parameters can vary between seasons, life stages and events, and potential stressors such hunting. Over the last 14 years, we have applied heart rate loggers developed for humans and we adapted them for bears (1) and used internal temperature loggers for body temperature. We then explored variation in these physiological measures with respect to season, hibernation periods, and stressors. In general, bear physiology changes at seasonal and daily scales and can also depend on a variety of environmental factors (2). The use of biologgers has allowed us to fill in important knowledge gaps in the basic ecology and physiology of free-ranging brown bears while also providing a solid foundation for exploring further details on conservation, management, and implicates of climate variability on bear biology.

 

Physiology during the active phase

 

Body temperature: During the active period, the body temperature of bears generally ranges between about 37-38 °C, depending on their body size. For example, we have also observed consistently higher body temperature with sequentially smaller bears during summer n(3). Body temperatures follow a diel rhythm and are higher from sunset to late morning compared to midday, which corresponds to the time when they are the most active (2).

 

Heart rate: During the active period, the baseline heart rate of bears generally ranges between about 65±20 beats per minute (bpm), but can overall be quite variable, depending on the time of day and year as well as body size of the bear (3, 4). Similar to body temperatures, heart rates are the highest when they are the most active, commonly during and between sunset and sunrise (2) and during the late summer (August) compared to early spring (May) (4). Body size also affects heart rates, but only during summer; smaller bears have higher daytime heart rates than larger bears (3). Bear heart rates also increase when they travel long distances relatively quickly, i.e., >50 m in 1 hour (4). Furthermore, bears that had cubs generally had a lower heart rate than those that had no cubs, especially during May, likely because of the decreased movement and activity associated with caring for young cubs (4). However, after August, bears with cubs generally have higher rates than solitary bears (4). In general, bear heart rates increased with metabolic rates, which are both highest during the hyperphagia period before denning (4). The hyperphagic shift in brown bears does not increase to the same extent as seen in Asiatic black bears (5).

 

Heart rate variability and stress: Heart rate variability (HRV) can be considered as an indicator of stress. Stress produces changes in HRV rates and although heart rates are highly affected by movement while HRV is less so (6). In this context, low HRV indicates high stress, or a “fight or flight” response, while high HRV indicates low stress and a “rest and digest” response (6). Our research suggests that bears are more stressed (lower HRV) when they are near human settlements, especially during the fall berry season when more people tend to be in the forests harvesting berries and hunting (6).

 

The physiology of hibernation

 

Northern mammals show a host of behavioral, ecological, and physiological strategies for survival in harsh climates. Bears are unique in that they hibernate six months of the year without eating, drinking, urinating, or defecating, and the females give birth in their dens. For more information on denning patterns see Denning ecology. Although there are numerous previous studies on the physiology and ecology of hibernation in bears, many gaps in our knowledge remain. Also, most physiological studies have been conducted in laboratory conditions, independently from the bear’s natural ecology. Captures during the winter hibernation period and the deployment of biologging devices have allowed for addressing fundamental questions about the basic biology of the bear’s annual cycle, including hibernation. For information on how capture procedures, including the implantation of biologgers, affects brown bears see Effects of capture on Scandinavian brown bears.

 

Activity, heart rate and body temperatures begin to drop slowly several weeks before bears entre their den (2). Denning appears to be tightly coupled with metabolic suppression and slowing down in biological rhythms (2). During arousal, body temperature begins to rise almost two months before den exit (2). An initial spike in body temperature is driven by ambient temperature, independently of autonomic nervous system activity which only became active three weeks before den exit (7). The difference between body temperature and ambient temperature decreased gradually. Although the sympathetic nervous system began to restore euthermic metabolism three weeks before den exit, it was not until ambient temperature reached the bear’s lower critical temperature that bears exited the den (7).

 

Body temperature: During the core hibernation period, bear body temperatures remain low, averaging 33.6 °C (ranging between 30.2-37.3 °C) (3). Furthermore, the body temperature of hibernating bears is consistently lower with sequentially smaller bears (3). Interestingly, observed lowered levels of leukocytes during hibernation were, when compared across species, explained by the decline in body temperature during hibernation, suggesting that body temperature is the main driver of immune function regulation during hibernation (3). Furthermore, pregnant females have higher body temperatures during pregnancy, which then drops back down after they have given birth in the den (8).

 

Heart rate: Heart rates also remain lower during the main hibernation period, averaging 16 bpm (ranging between 7-70 bmp (3)). Unlike their active period, however, heart rates during hibernation are not associated with bear body mass (3).

 

Hematological and biochemical parameters: In blood samples taken during winter, analyzed together with similar sized bears in spring and summer, we found significant shifts in hematological and biochemical measurement in winter (February-March) compared to in spring and summer (April-July) (9). This reflects the lowered metabolic, kidney, and liver activity and a shift to a lipid-based (fat-based) metabolism during hibernation (9). As a previous study has shown that metabolic rate and oxygen consumption are independent from lowered body temperature, we investigated changes in oxygen binding affinity of red blood cells from hibernating bears, which consistently showed higher oxygen affinity than their summer counterparts. This likely maintains a relatively constant tissue oxygen tension during hibernation.