The seasonal sensitivity of brown bear denning phenology in response to climatic variability

Abstract

BackgroundFor brown bears (Ursus arctos), hibernation is a critical part of the annual life cycle because energy savings during hibernation can be crucial for overwintering, and females give birth to cubs at that time. For hibernation to be a useful strategy, timing is critical. However, environmental conditions vary greatly, which might have a negative effect on the functionality of the evolved biological time-keeping. Here, we used a long-term dataset (69 years) on brown bear denning phenology recorded in 12 Russian protected areas and quantified the phenological responses to variation in temperature and snow depth. Previous studies analyzing the relationship between climate and denning behavior did not consider that the brown bear response to variation in climatic factors might vary through a period preceding den entry and exit. We hypothesized that there is a seasonal sensitivity pattern of bear denning phenology in response to variation in climatic conditions, such that the effect of climatic variability will be pronounced only when it occurs close to den exit and entry dates.ResultsWe found that brown bears are most sensitive to climatic variations around the observed first den exit and last entry dates, such that an increase/decrease in temperature in the periods closer to the first den exit and last entry dates have a greater influence on the denning dates than in other periods.ConclusionsOur study shows that climatic factors are modulating brown bear hibernation phenology and provide a further structuring of this modulation. The sensitivity of brown bears to changes in climatic factors during hibernation might affect their ability to cope with global climate change. Therefore, understanding these processes will be essential for informed management of biodiversity in a changing world.Electronic supplementary materialThe online version of this article (10.1186/s12983-018-0286-5) contains supplementary material, which is available to authorized users.

Denning in brown bears

“…However, no significant correlations were detected between den entry/exit and snow depth (Delgado et al, 2018;Judd et al, 1986).…”

“…Moreover, the increased climatic variability could make the weather patterns that govern the seasonality of animal life cycles to some extent more unpredictable for many organisms, including bears (Weiskopf et al, 2020). Actually, these interannual fluctuations in hibernation chronology due to climatic conditions have already been reported in both American black bears (Miller, Smith, Auger, Black, & Allphin, 2017) and brown bears (Delgado et al, 2018;Evans et al, 2016;Friebe et al, 2014;McLoughlin, Case, et al, 2002;. reported that for each 4°C increase in spring temperature brown bear den exit occurs 10 days earlier.…”

“…Thus, changes in climate could reduce the duration of hibernation in bears and lead to advanced den exit (Berman, Coops, Kearney, & Stenhouse, 2019;Johnson et al, 2017;. It has been reported how brown bears emerge from winter dens when the ambient temperature reaches a certain level (3.7 ± 1.3 C in Evans et al, 2016), and since warmer springs may promote earlier first den exits (e.g., Delgado et al, 2018;González-Bernardo et al, in press), bears are expected to emerge from dens earlier as the climate continues to warm (Johnson et al, 2017;. Although few data are available, it has been suggested that:…”

“…Another important proximate factor affecting den entry/exit is ambient temperature (Craighead & Craighead, 1972;Delgado et al, 2018;Evans et al, 2016;Friebe et al, 2014;Manchi & Swenson, 2005;McLoughlin, Case, et al, 2002;. In Scandinavia, Evans et al (2016) observed that the average (mean ± SE) daily temperature when bears enter dens is 1.03 ± 0.95°C and that ambient temperature is associated with a decrease in body temperature which, consequently, results in a change in heart rate (Evans et al, 2016).…”

“…In Poland, brown bear winter activity has been positively related to ambient temperature (Bojarska et al, 2019). However, brown bear sensitivity to changes in climatic conditions varies as a function of den entry and exit dates (Delgado et al, 2018). Indeed, brown bears are most sensitive to climatic variations around first exit and last entry dates, that is, a change in ambient temperature in periods closer to the average date that bears first enter/exit their dens has a greater influence on denning dates than during other periods (Delgado et al, 2018).…”