Hibernation: News from the Bear’s Den

Do bears really hibernate or do they enter a shallow form of torpor called winter sleep? True hibernation is particularly well understood in small mammals (i.e. ground squirrels), where core body temperatures drop to near freezing for several days or weeks during profound hibernation. Metabolic activity also declines precipitously during hibernation to roughly 5% of their non-hibernating basal metabolic rate (BMR). Ground squirrels also exhibit periodic arousals from hibernation that typically last for up to 24 hours.

In contrast, bears hibernate for up to 7 months continuously without eating, drinking or excreting wastes (Figure 1). Unlike smaller mammalian hibernators, core body temperatures for hibernating bears fall only a few degrees Celsius, from 37
oC to approximately 33oC. At the same time, the bear’s heart rate slows to only 8 beats per minute. This decoupling of body temperature and heart rate has led to confusion over the nature of hibernation in bears.
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Figure 1. A mother and her two cubs hibernating in a den (From National Park Service)

To help clarify the issue Øivind Tøien, from the Institute for Arctic Biology in Alaska, and colleagues from Stanford University built special hibernation “dens” in outdoor enclosures where several black bears (
Ursus americanus) were held (Figure 2). The hibernation boxes were fitted with video cameras and instruments to measure oxygen consumption and temperature. Five wild-caught black bears were implanted with additional sensors for monitoring core body temperature, electromyograms (EMG), and electrocardiograms (ECG). The bears were continuously monitored during and after hibernation (Tøien et al. 2011).

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Figure 2. A female black bear (Ursus americanus) and her cub forage along a road. (From Flickr/Lee Coursey)

As previously reported, the body temperature of hibernating black bears dropped to 33
oC on average (an 11% drop). However, metabolic rates dropped to 25% of BMR. Interestingly, when the bears emerged from hibernation in April they maintained a reduced metabolic rate (53% of BMR) for several weeks as they foraged and moved about the enclosure. The researchers suggest that a return to normal BMR may require food intake and a return to “full capacity of the digestive system.”

Heart rate slowed during hibernation to 8 to 12 beats per minute (46 to 64 beats per minute at rest are typical in summer months). However, the heartbeats were not evenly spaced over time. Rather, hibernating black bears experience periods of arrhythmia. A short burst of heartbeats occurs as they breathe in, which is followed by a period of no heartbeats (arrhythmia) lasting up to 20 seconds before the beginning of the next inspiration burst. Thus hibernating black bears are able to decouple body temperature and heart rate during, and for a few weeks after, hibernation.

But how do hibernating bears prevent muscle and bone loss that occurs during prolonged inactivity and periods of anorexia (inability to eat)? For example, reduced heart rates and muscle atrophy could result in potentially damaging heart failure as the heart chambers atrophy and blood pools in the chambers during hibernation? New research is helping answer some of these questions.

In hibernating grizzly bears (
Ursus arctos) the expression of important heart muscle proteins changes during hibernation (Barrows et al. 2011). The myosin heavy chain protein, which comes in two forms, controls heart contractions. The beta form of myosin heavy chain produces a slower but more forceful contraction than the alpha form. During hibernation, the left atrium produces more alpha protein, resulting in a weaker beat that reduces the forces and helps prevent atrial damage.

Skeletal muscle also appears to be protected during hibernation. A human patient bed-ridden for 3 months would be expected to lose over 50% of their muscle strength as the muscle atrophies. Hibernating black bear, essentially immobile and anorexic for over 3 months, lost only 29% of muscle strength (Lohuis et al. 2007).

In sum, advances in telemetry equipment and genomic studies are now shedding some light on hibernation in bears. We now know that 1) bears do hibernate, 2) core body temperature changes independent of metabolic rate, 3) reduced metabolic rates are maintain for several weeks after emergence from the den, 4) that bone and muscle loss is far less than would be expected for immobile mammals of similar size, and 5) that gene expression plays an important role in preventing muscle damage during hibernation. Of course, many questions remain.



References

Barrows, N.D., Nelson, O.L., Robbins, C.T., and B.C. Rourke. (2011) Increased cardiac alpha-myosin heavy chain in left atria and decreased myocardial insulin-like growth factor (IGF-I) expression accompany low heart rate in hibernating grizzly bears.
Physiological and Biochemical Zoology, 84:1-17.

Lohuis, T.D., Harlow, H.J., Beck, T.D.I. and P.A. Iaizzo, (2007) Hibernating bears conserve muscle strength and maintain fatigue resistance.
Physiological and Biochemical Zoology, 80: 257-69

Øivind Tøien, John Blake, Dale M. Edgar, Dennis A. Grahn, H. Craig Heller, & Brian M. Barnes (2011). Hibernation in Black Bears: Independence of Metabolic Suppression from Body Temperature Science, 906-909 : 10.1126/science.1199435