Denning in brown bears

Blog written by Enrique González-Bernardo. Read the full article here.

Hibernation represents an adaptation for coping with unfavorable environmental conditions. For brown bears (Ursus arctos), hibernation coincides with winter in seasonal environments generally associated with low temperatures and low food abundance during which pronounced temporal reductions in several physiological functions occur. It is a critical period because at that time: (a) pregnant females give birth and undergo lactation while in dens; (b) energy savings during hibernation can be substantial; and (c) premature exit can negatively affect energy conservation and cub survival.

Brown bears are widely distributed around the northern hemisphere, with populations of different characteristics inhabiting very diverse habitats. Thus, the management and conservation of this species requires detailed knowledge about denning ecology of different populations, identifying patterns or gradients of denning behavior throughout the distribution range of the species and highlighting possible differences between populations. It is reasonable to expect that: (1) den chronology may vary by sex and bear reproductive status; (2) the duration of hibernation also depends on environmental factors, i.e. snow, temperature and food availability; (3) a relationship may exist between denning period and latitude, longitude and altitude; and (4) although the choice of a den and den surroundings may be variable, some differences may still exist by class, age and sex.

There are differences between sexes and ages in relation to denning chronology, with pregnant females enter the den earlier and exit later than other bear classes to take care of new-born and males and subadults generally having shorter denning periods. Hibernation in brown bears is mostly governed by a trade-off between environmental conditions metabolic dietary-related and energy conservation needs, with the exception of pregnant females which ultimate reasons are reproductive needs. As a general rule, the duration of hibernation in different brown bear populations seems to be conditioned by both snowfall/snow depth and snowmelt in spring. Snowfall can act as a major impetus to begin hibernation, as well as the ambient temperature. Warmer winters seems to be associated with a decrease in the length of the hibernation period and the postponement of den entry, while low autumn temperatures may cause early den entry. Den emergence seems to be regulated in some way with an increase in temperature, but not depending on the exact ambient temperature on the day of emergence (probably because den emergence is a longer process), but by the temperature the previous weeks. However, the hibernation period is primarily affected by a decrease in food availability due to the snow cover, which guides den entry and exit. Continuous availability of food and mild climates may prompt individuals to spend the winter outside dens, with bears in more northern areas spending more time hibernating than bears in the southernmost latitudes. There is an evident gradient between den chronology and latitude, but it seems to affect primarily to adult bears. Similarly, altitude has been described as influencing denning dates, with bears denning at lower altitudes emerging earlier than those hibernating at higher altitudes. Den abandonment occurs naturally, but human activities, such as industrial and forestry activity, hunting, transit of people or even research activity have been reported as the main cause. It may have negative consequences for populations, such as increases in cub mortality. In some populations where there are natural food sources during the winter, part of the population may not hibernate. This phenomenon is especially intense where supplementary food is provided during winter, which may alter the chronology of hibernation or winter.

The choice of a den and its surroundings may affect individual fitness, for example, loss of offspring and excessive energy consumption. Den selection is the result of broad- and fine-scale habitat selection, mainly linked to den insulation, remoteness, and availability of food in the surroundings of the den location. The most common den types are those excavated in the ground or located inside natural caves, but other types of dens such as depressions under rock shelters, nest dens and tree cavities have been reported. The typical conformation of a den consists of an entrance, a tunnel and the chamber where the bear builds the nest. Brown bears select sunny slopes and sites with deeper snowpack, which have insulating properties that help maintain a constant temperature protection. Other selected features are the slope of the hillside (simpler digging, stability, solar radiation and protection) and the distance and altitude (more permanent snow cover and further from human disturbances). Differences have been described in terms of sex, with pregnant females hibernating higher because they have a longer hibernation and need more isolation, while adult males hibernate in lower areas, which would provide them with greater food availability at den emergence, rapidly increase body mass and thus improve breeding success.

Brown bears hibernate for several months during which they do not eat, drink, defecate or urinate. This huge metabolic challenge has demanded a series of physiological adaptations in tissues and organs, many of them unique, enabling bears to survive under nutritional deprivation and to overcome anuria, hyperlipidaemia and immobilization, preserve muscles and bones avoiding osteoporosis or sarcopenia and prevent diseases such as metabolic syndrome, diabetes and cardiovascular pathologies. During hyperphagia, brown bears becoming temporary insensible to leptin which allows a constant appetite. Bears exhibit an increase in body mass of around 40% during the pre-hibernation hyperphagia, mainly fat. During hibernation, the mass loss could reach 4 kg per day. The metabolic rate drops to a quarter of the active state level and the temperature drop is about 3-5 °C, starting days before the den entrance. As a result, heart rate begins to decrease up to 80%. During hibernation, brown bears exhibit blood parameters that would be indicators of pathology in humans and as a consequence of fat-based metabolism, maintaining concentrations of phospholipids, cholesterol and free fatty acids much higher than healthy human values. However, atherosclerosis, fatty streaks, foam cell infiltration and inflammation have not been reported in hibernating bears. The bladder becomes permeable and both water and nitrogenous substances re-enter the blood. This and other adaptation, such as a reduction of urea synthesis and reincorporating urea into skeletal muscle and other proteins, allow recycling of nitrogenous substances and prevent renal complications or azotemia. Organs involved in digestive, metabolic and excretion processes, such as the liver and kidneys, have their activity decreased compared to the active period decrease in function. Brown bears do not appear to suffer from skeletal muscle and bone dysfunction or bone loss during hibernation, despite immobilization and lack of food. Brown bears are able to maintain both muscle and bone mass and function by reducing catabolic processes and maintaining a certain level of mechanical activity, such as shivering. Brown bears give birth during hibernation, around January-February, when there is no intake of food and water. Pregnant females appear to have delayed implantation of the embryo. In older females, as well as in females with high body fat content, den entry dates and birth dates tend to be earlier and the lactation period may be longer than in other females. In addition, in females with a higher percentage of fat and lean mass, implant embryos and cub birth occur earlier and the mortality of their cubs during the first summer is lower than in other females.

Concerning the hibernation of brown bears it is important to correctly distinguish between actual drivers of hibernation (temperature, snow, food availability) and their correlates (physiological changes). We suspect that the correlates of proximate factors might fluctuate according to current environmental variations. Predicted variations in air temperatures generally point towards an increase in temperature and precipitation variability has already affected biological systems by altering the phenology. Inter-annual fluctuations in hibernation chronology are expected to occur due to inter-annual variations in climate, extreme climatic events and temperature anomalies resulting from climate change. These fluctuations are probably unpredictable for many organisms, including bears. It has been reported how brown bears advance the den exit by increasing spring temperature, and how the total duration of hibernation is reduced. Brown bears have been reported showing a noticeable plasticity when hibernating, adapting their denning behavior to environmental factors, availability of food during hyperphagia or changing snow conditions during the winter. Despite this, energy demands of hibernating mammals would increase with higher winter temperature, and mismatches could be expected between the den exit dates and food resources, in addition to a reduction in their abundance or availability. Altering hibernation conditions could have a negative effect on reproductive success and cub survival. Such increased mismatches might also increase the likelihood of bear-human conflicts if bears emerge earlier. It is important to highlight here the potential effect of anthropogenic food, and especially supplementary feeding (provided with hunting, eco-tourism and the mitigation of human-bear conflicts purposes), on the chronology of hibernation, being able to halve the denning period or increase den abandonments. For all this, future research is important to know how changes in climatic factors might affect the ability of bears to face global climate change and the human-mediated changes in food availability. In addition to this, a better understanding of how hyperphagia, predenning and denning periods, including for those populations in which bears do not hibernate every year, and how to approach the study of bear denning merging insights from different perspectives, that is, physiology, ecology, and behavior.

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