Circadian rhythms persist during bear hibernation

In a new study led by Washington State University, researchers have discovered that grizzly bears maintain their internal circadian rhythms even during their hibernation period. 

This finding provides significant insights into the adaptability of circadian rhythms across different species, including humans.

Circadian rhythms

“Hibernation is a highly seasonal physiological adaptation that allows brown bears (Ursus arctos) to survive extended periods of low food availability,” wrote the study authors. 

“Similarly, daily or circadian rhythms conserve energy by coordinating body processes to optimally match the environmental light/dark cycle.”

“Brown bears express circadian rhythms in vivo and their cells do in vitro throughout the year, suggesting that these rhythms may play important roles during periods of negative energy balance.”

Energy production 

Contrary to the common perception that hibernation is a state of complete inactivity, the researchers observed that energy production in bears continues to follow a daily pattern, even as they sleep for months. 

The experts also found that the amplitude of energy production was blunted during hibernation, with a reduced range of highs and lows. The peak occurred later in the day compared to the bear’s active season, but the daily fluctuation was still there, noted the researchers. 

“This underscores the importance of the circadian rhythms themselves – that they give organisms the flexibility to still function in a state as extreme as a hibernating bear,” said study senior author Professor Heiko Jansen.

How to study bear hibernation

This phenomenon was observed through genetic analyses on cell samples from six bears, taken during their active and hibernating periods. 

The cells were cultured and examined at two different temperatures, mimicking the bears’ body temperatures during hibernation and active seasons. 

The results showed a rhythmic expression of thousands of genes, leading to a consistent yet modulated production of adenosine triphosphate (ATP), the primary energy carrier in cells.

Study implications 

The study’s findings have broader implications for understanding metabolic health in humans. Disruptions in circadian rhythms, such as those experienced by night shift workers, have been linked to metabolic issues like weight gain and diabetes. 

By studying bears, who are akin to “extreme shift workers” during their hibernation, scientists can gain insights into how prolonged periods of inactivity and fasting do not result in health issues typically seen in humans under similar conditions.

Energy conservation 

The research also highlighted differences in hibernation behaviors between bears and other hibernating mammals like rodents. 

Unlike the near-comatose state of hibernating rodents, bears show occasional movements during hibernation, following a circadian rhythm with more daytime activity.

“It’s like setting a thermostat. If you want to conserve some energy, you turn down the thermostat, and this is essentially what the bears are doing,” said Jansen. 

“They’re using the ability to suppress the circadian rhythm, but they don’t stop the clock from running. It’s a really novel way of fine-tuning a metabolic process and energy expenditure in an animal.”

More about bear hibernation 

Bear hibernation is a complex and fascinating process that involves several unique biological adaptations. Here are some key aspects of bear hibernation:

Annual cycle and stages

Black bears, for example, go through a five-stage annual cycle, including hibernation, walking hibernation, normal activity, hyperphagia (excessive eating), and fall transition​​.

Metabolic changes during bear hibernation

Bears are considered true hibernators. This status was confirmed after biologists discovered the many metabolic changes that occur during their hibernation. This process was traditionally defined only in terms of temperature reduction, which is not significant in bears​​.

Temperature regulation

Unlike some hibernators, such as Arctic ground squirrels, that can lower their body temperature dramatically, bears maintain a comparatively high body temperature during hibernation. 

Their metabolism slows significantly, but they hardly lose any heat. This is a unique approach among hibernating animals​​.

Eating habits

In preparation for hibernation, bears enter a phase of hyperphagia, where they consume enormous amounts of food. For instance, grizzly bears in Yellowstone can consume up to 20,000 calories a day during this period, gaining up to three pounds daily​​.

Reproduction and bear hibernation

The reproductive cycle of bears is also linked to hibernation. Female bears can delay the implantation of fertilized eggs in their womb until they enter hibernation, which is believed to be a strategy for energy conservation and population control based on food availability​​.

Bear hibernation and den construction

Bears typically create heat-efficient dens, often on slopes at high elevations. The dens are constructed just large enough for the bear, with an entrance that quickly gets covered by insulating snow, maximizing heat retention​​.

Health preservation during hibernation

Bears maintain their health during hibernation in ways that are not fully understood but are of great interest to scientists. 

For instance, they are able to prevent osteoporosis, a notable exception to what happens in most other animals, including humans, when inactive for extended periods. 

A study showed that hibernating bears shut down genes involved with bone breakdown, suggesting a potential avenue for preventing osteoporosis in humans​​​​.

Muscle atrophy prevention

Grizzly bears can hibernate for months without experiencing muscle atrophy, a common problem in other animals and humans during periods of inactivity. Their metabolism and heart rate drop during hibernation, and their blood becomes rich in nitrogen. 

Researchers have identified a unique set of genes in grizzly bears that boost amino acid metabolism during hibernation, leading to increased concentrations of certain non-essential amino acids that fuel muscle cell growth​​.

Potential medical applications 

The study of bear hibernation has potential applications for human health. For example, replicating a hibernation-like state in humans could be beneficial for stroke victims, extending the window of opportunity for treatment. 

Such research is inspired by the way bears significantly reduce their metabolism while maintaining vital functions during hibernation​​.

Hibernation timing

Unlike some other hibernating animals that follow strict schedules based on day length, bears decide for themselves when to hibernate and when to recover. This timing is influenced by various factors, including food supply and temperature​​.

The study is published in the Journal of Comparative Physiology B.

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