Confirming what observational studies have long thought, a genetic study has shown that a grizzly bear’s body clock ticks away routinely, even when the animal is deep in hibernation. What’s more, proof of their circadian rhythms mirrors our own, except these bears (Ursus arctos horribilis) can hibernate in the dark for four to seven months and emerge with few ill effects aside from having lost up to 30% of their body weight.
Washington State University (WSU) scientists believe the impressive physiology of these “extreme shift workers” may hold clues as to how humans can better cope with circadian rhythm disruptions. More prevalent in shift workers, circadian misalignment has been linked to heart disease, poor immune function, metabolic diseases such as diabetes and obesity, stroke, cancer and early death.
The researchers found that the hibernating bears' energy production fluctuates in a daily pattern, even though they haven’t eaten for months. During their hibernation, their energy peaks were subdued, with the highs occurring later in the day than when the animal is active, but they were still clear indications of the circadian rhythms being maintained rather than put on pause.
“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 senior author Heiko Jansen, professor in WSU’s integrative physiology and neuroscience department.
The team believes that knowing how bears can gain a massive amount of weight and then spend months with no food and little movement without ill-effects health-wise could open the door to human therapies. The bears may lose up to 100 pounds (45 kg), but they have little muscle wastage, no bone deterioration and no metabolic diseases triggered.
In the study, the researchers looked at expression of the animal’s internal body clock at the cellular level. Cell samples from six bears were taken during both their active and hibernating periods and cultured. They compared the cells taken from the hibernating animals, who have a typical lower body temperature of around 34 °C (93.2 °F), with cells taken during the active season, with a body temperature of around 37 °C (98.6 °F).
What they found confirmed the circadian activity, with thousands of genes being expressed rhythmically in the hibernating animal cells. This showed that adenosine triphosphate (ATP), or the body’s cellular energy, was being produced in a 24-hour cycle, albeit with lower peaks and valleys.
As mentioned earlier, the peak energy output also occurred later in the day than when the animal is active, suggesting that bears shift their circadian rhythm during hibernation to lower the cost of keeping their ‘battery’ running. Because of this, they can continue to burn enough fuel to power their bodies without eating for months – and with no detrimental health issues.
“It’s like setting a thermostat,” said Jansen. “If you want to conserve some energy, you turn down the thermostat, and this is essentially what the bears are doing.”
While observing hibernating animals at the WSU Bear Center, the researchers also noted that the animals were more active during certain periods of the day, even though the light conditions remained constant. The increased movements corresponded with when the cells from the hibernating animals had peaks of ATP production.
“They’re using the ability to suppress the circadian rhythm, but they don’t stop the clock from running,” he added. “It’s a really novel way of fine-tuning a metabolic process and energy expenditure in an animal.”
The researchers believe harnessing this ability might help humans cope with schedules that require them to work during the ‘biological’ night and sleep during the day.
It’s estimated 14% of the workforce endures night work, and there’s growing concern as to just how damaging keeping these hours is to human health, particularly regarding chronic disease, accidents, poor quality of life and healthcare costs.
If scientists could work out how to make our own internal clock smarter than the average bear, the potential for countering detrimental health impacts could be huge.
In another recent decade-long study, scientists found that bears also produce a unique anti-blood-clotting protein during hibernation, to safeguard them from risks of enduring immobility.
The study was published in the journal Journal of Comparative Physiology B.
Source: Washington State University
