Drivers of hibernation in the brown bear

Abstract

BackgroundHibernation has been a key area of research for several decades, essentially in small mammals in the laboratory, yet we know very little about what triggers or ends it in the wild. Do climatic factors, an internal biological clock, or physiological processes dominate? Using state-of-the-art tracking and monitoring technology on fourteen free-ranging brown bears over three winters, we recorded movement, heart rate (HR), heart rate variability (HRV), body temperature (Tb), physical activity, ambient temperature (TA), and snow depth to identify the drivers of the start and end of hibernation. We used behavioral change point analyses to estimate the start and end of hibernation and convergent cross mapping to identify the causal interactions between the ecological and physiological variables over time.ResultsTo our knowledge, we have built the first chronology of both ecological and physiological events from before the start to the end of hibernation in the field. Activity, HR, and Tb started to drop slowly several weeks before den entry. Bears entered the den when snow arrived and when ambient temperature reached 0 °C. HRV, taken as a proxy of sympathetic nervous system activity, dropped dramatically once the bear entered the den. This indirectly suggests that denning is tightly coupled to metabolic suppression. During arousal, the unexpected early rise in Tb (two months before den exit) was driven by TA, but was independent of HRV. The difference between Tb and TA decreased gradually suggesting that bears were not thermoconforming. HRV increased only three weeks before exit, indicating that late activation of the sympathetic nervous system likely finalized restoration of euthermic metabolism. Interestingly, it was not until TA reached the presumed lower critical temperature, likely indicating that the bears were seeking thermoneutrality, that they exited the den.ConclusionsWe conclude that brown bear hibernation was initiated primarily by environmental cues, but terminated by physiological cues.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-016-0140-6) contains supplementary material, which is available to authorized users.

Human development and climate affect hibernation in a large carnivore with implications for human–carnivore conflicts

“…Indeed, temperature had twice the magnitude of effect of either natural or human food availability in decreasing the overall length of hibernation. Our results suggest that ambient temperature serves as an important trigger of hibernation behaviour in bears, and corroborates studies on marmots ( Marmota flaviventris ) and brown bears that temperature is more influential at driving changes in hibernation than snowpack (Evans et al., ; Inouye et al., ). Climate models project widespread increases in winter temperatures, while winter precipitation is predicted to be more variable, depending on location (Williams et al., ).…”

Hunting promotes spatial reorganization and sexually selected infanticide

“…This time lag in the response likely is related to the bear’s ecology. Bears den from October to April2122, shortly after the hunting season in late August—September. The size of the annual home range in our study population is mainly defined by space use during the mating season (May to mid-July), when males exhibit a roam-to-mate behavior23.…”

Physiological reactions to capture in hibernating brown bears

“…We compared the date of emergence from hibernation for the captured and the undisturbed bears in the same way. For the emergence, the day of the year when T b was above 36.7°C (Evans et al ., 2016) was the response variable. For both models, captured or not and the body mass were fixed factors, and ID and winter (to control for inter-annual variation) were random factors.…”

“…In contrast to rodents, bears exhibit a less dramatic drop in body temperature ( T b ), protein conservation, absence of urination and defaecation (Hellgren, 1998). During hibernation, both captive and wild brown bears reduce their T b by about 3–5°C from active levels of 37.0–37.5°C (Hissa, 1997; Evans et al ., 2016) and heart rate (H) from about 70–80 beats per minute (bpm) to hibernating levels of around 10–29 bpm (Nelson and Robbins, 2010; Evans et al ., 2016). These reductions in body temperature and heart rate are connected to the hibernators’ energy savings and reductions in metabolic rate (Geiser, 1988).…”