Grizzly bears maintain circadian rhythm through hibernation, WSU study reveals

WASHINGTON — The internal clocks of grizzly bears keep ticking through hibernation, according to a genetic study led by researchers at Washington State University (WSU), highlighting the strong role of circadian rhythms in the metabolism of many organisms including humans.

The circadian rhythm is a 24-hour cycle that's part of the body's internal clock. It correlates to physical, mental, and behavioral changes. It responds primarily to sunlight and darkness and affects most living organisms.

The WSU study confirmed that bears' energy production still waxes and wanes in a daily pattern even though they hibernate for several months without eating. Researchers also found that during hibernation, the amplitude of the energy production was blunted—meaning that the range of highs and lows was reduced.

WSU researchers also added that the peak occurred later in the day under hibernation than during the active season, but the daily fluctuation was still going.

“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 Heiko Jansen, a professor in WSU’s integrative physiology and neuroscience department and senior author on the study in the Journal of Comparative Physiology B.

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According to the WSU article, other research has shown that circadian rhythms have ties to metabolic health. For example, in humans, major disruptions to these patterns have been linked to metabolic problems like weight gain and a higher prevalence of diabetes.

As the WSU research stated, bears are extreme shift workers since they take as much as six months off when they hibernate. Researchers attempted to figure out how they engage with "seemingly unhealthy" habits of gaining excessive weight and then going without food and not moving for several months, all without detrimental effects like bone mass loss or diseases like diabetes.

However, unlike hibernating rodents who are almost comatose, bears do move around occasionally during the dormant period, WSU researchers said. They added that through observation studies of the grizzly bears at the WSU Bear Center, researchers found out that the movements follow a circadian rhythm with more activity during the day than at night.

To see if the circadian rhythm was expressed on the cellular level, researchers took cell samples from six bears during active and hibernating seasons and cultured the cells to conduct an array of genetic analyses.

To see the circadian rhythm during hibernation, researchers examined the cells at the bears' typical lowered body temperature during hibernation at 34 degrees Celsius (93.2 F) and compared that to 37 C (98.6 F)during the active season.

Researchers found that thousands of genes were expressed rhythmically in hibernating bear cells. That means they maintained a circadian rhythm by producing adenosine triphosphate (ATP), the body's source of energy, later in the day in hibernation compared to active season conditions.

According to the release, researchers believe that altering the rhythm during hibernation helps the bears get some energetic benefit of the daily cycle without as much of the cost, helping them survive going without food for months.

“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,” Jansen said. “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.”