The hibernation state of grizzly bears has long been a focus of scientific interest, particularly the physiological and metabolic changes that occur during this time.

Recent research led by Washington State University has shed new light on the role of biological clocks during hibernation, providing deeper insights into the importance of circadian rhythms in metabolism.

Grizzly bears' biological clocks do not stop working during hibernation, according to a genetic study led by Washington State University. The findings highlight the critical role of circadian rhythms in the metabolism of many organisms, including humans.

The study confirms previous observations that bears' energy production still rises and falls in a daily pattern, even when they go months without eating.

Researchers found that during hibernation, the fluctuations in energy production decreased, meaning the range of energy highs and lows narrowed. At the same time, the peak energy in hibernation occurred later than during the active season, but the daily fluctuations still existed.

"This emphasizes the importance of circadian rhythms themselves – they give organisms the flexibility to function in extreme states like hibernating bears," said Heiko Jansen, a professor in the Department of Integrative Physiology and Neuroscience at Washington State University and senior author of the study published in the Journal of Comparative Physiology B.

Circadian rhythms, the 24-hour physiological cycles shared by most animals on Earth, are closely related to metabolic health. In humans, major disruptions to these patterns, such as night shift work, are associated with metabolic problems like weight gain and higher rates of diabetes.

Grizzlies rest for up to six months while hibernating, similar to extreme shift workers. Researchers are trying to figure out how they stay healthy with this seemingly unhealthy habit.

Unlike hibernating rodents, which are almost comatose, grizzly bears occasionally move around during hibernation. Through observations at the Washington State University Bear Center, researchers found that these activities also follow a circadian rhythm, with the bears being more active during the day than at night.

In the latest study, the researchers further explored whether circadian rhythms are expressed at the cellular level. They took cell samples from six grizzly bears during the active and hibernation seasons and then cultured the cells for a series of genetic analyses.

To simulate hibernation, the researchers compared the cells at the low temperatures typical during hibernation (about 34 degrees Celsius) and the body temperature of the active season (about 37 degrees Celsius).

The results showed that thousands of genes are expressed rhythmically in the cells of hibernating grizzlies. The expression of these genes is converted into an energy rhythm through the rise and fall of adenosine triphosphate (ATP) production.

During hibernation, ATP is still produced in a daily pattern, but its fluctuations are reduced, with lower peaks and valleys. At the same time, the peak production point under hibernation conditions is also later than during active season conditions.

Maintaining the circadian rhythm itself requires energy. The researchers believe that by adjusting this rhythm during hibernation, grizzly bears can get some of the energetic benefits of the daily cycle without expending too much energy. This may help them survive months without food.

Professor Jansen explained: "It's like adjusting the thermostat. If you want to save some energy, you turn down the thermostat, and this is basically what the bears are doing.

They are using the ability to suppress the circadian rhythm, but they are not stopping the clock. This is a new way to fine-tune the animal's metabolic processes and energy consumption."

The study co-authors include Ellery Vincent, Blair Perry, and Charles Robbins of Washington State University, and Joanna Kelly of the University of California, Santa Cruz. The research was supported by the National Science Foundation and the Washington State University Bear Research and Conservation Fund.

The research conducted by Washington State University highlights the intricate and essential role of circadian rhythms in the survival strategies of hibernating grizzly bears.

By maintaining a subdued yet functional biological clock, these bears manage to balance energy conservation with the physiological demands of hibernation.

This delicate balance ensures their survival during long periods without food and offers valuable insights into the broader significance of circadian rhythms in metabolic health.

As we continue to explore these rhythms across different species, the lessons learned from grizzly bears may pave the way for advancements in both wildlife conservation and human health management.