During hibernation, brown bears spend up to six months lying almost completely still, without eating, drinking or exercising. When spring arrives, they leave their dens with their muscles largely intact.
Author
- John Noone
Assistant Professor & Course Director BSc Sport and Exercise Sciences, University of Limerick
For humans, the same period of inactivity would usually mean severe muscle loss, weakness and long-term health problems. Even a few weeks in bed after surgery can reduce strength and mobility. For older adults, hospital patients and people with chronic illness, long-term immobility can permanently change quality of life. How do bears manage what humans cannot?
To explore this question, my colleagues and I studied how bears protect their muscles during hibernation. Our findings , published in Acta Physiologica, suggest that the answer lies deep inside their muscle cells, in the way they manage energy over long periods of inactivity.
Muscle cells rely on structures called mitochondria to supply the energy needed for movement and basic function. These structures convert nutrients into fuel, allowing muscles to contract, repair themselves and adapt to stress. In people who stop moving for long periods, mitochondria usually decline in both number and performance. Energy production drops. Muscles weaken. Recovery becomes harder.
Bears take a different approach. During hibernation, their muscles contain fewer mitochondria, but the ones that remain work more efficiently. Rather than allowing their energy systems to deteriorate, bears streamline them. They reduce what is unnecessary and preserve what is essential. It is similar to shutting down some power stations during low demand, while upgrading the remaining ones to run more smoothly.
Inside mitochondria, energy is generated through a series of linked chemical reactions. These reactions normally rely on several entry points and fuel sources. During hibernation, bears reorganise this system.
Our research shows that they shift towards alternative energy pathways that function well at low body temperatures. Some parts of the usual machinery are reduced, while others become more important.
At the same time, bears maintain the ability to use both fat and carbohydrates. Fat provides most of the energy during winter, but flexibility remains essential. If conditions change, their muscles can adapt quickly. This reorganisation allows bears to produce enough energy to preserve muscle tissue, even while their overall metabolism slows dramatically.
Temperature plays a central role. As bears cool during hibernation, chemical reactions slow. Instead of fighting this, their muscles adjust. The structure and function of mitochondria change in ways that suit colder conditions.
This temperature-driven control acts like a natural protective system. It limits damage, reduces waste and helps prevent the breakdown that usually follows long periods of inactivity.
To understand these changes, we studied wild brown bears in Sweden during both winter and summer.
In winter, locating hibernating bears meant tracking them to hidden dens beneath deep snow. Teams worked with wildlife experts and veterinarians to safely monitor the animals and collect small muscle samples.
In summer, the challenge was different. Active bears roam huge areas and are difficult to approach. Helicopters were used to locate them, and professionals carefully sedated the animals so that biopsies could be taken safely.
Once collected, the samples were swiftly analysed. The main technique we used measures how well mitochondria produce energy in real time and only works on fresh tissue.
Comparing winter and summer samples revealed a consistent pattern. Bears reduce the number of mitochondria during hibernation but preserve their function. Energy pathways are reorganised, fuel use remains flexible and cellular damage is limited. Together, these changes allow bears to remain strong despite months of immobility.
Connecting wildlife biology with human health
The findings help explain one of nature's most impressive examples of physical resilience. But their importance goes far beyond wildlife biology.
In humans, muscle loss is a major problem in ageing, long hospital stays, injury recovery and chronic disease. Once muscle is lost, it is difficult to rebuild. Weakness increases the risk of falls, disability and loss of independence.
Treatments rely mainly on exercise and nutrition, which are often hard to apply when people are very ill or immobile. Bears show that another approach is possible.
By making mitochondria more efficient, reorganising energy systems, and responding to temperature, their muscles remain protected even in extreme conditions. Understanding how this happens could guide future treatments that help preserve muscle in vulnerable people.
This research also has relevance beyond medicine. Astronauts lose muscle rapidly in space, and long space missions require better ways to protect physical health in low-gravity environments.
From frozen dens in Scandinavian forests to high-tech laboratories, this work connects wildlife biology with human health and space science.
Bears do not simply sleep through winter. Their muscles follow a carefully controlled programme of energy management, conservation and protection. In doing so, they leave a clear pawprint for human biology: resilience is not about maintaining everything, but about protecting the systems that matter most.
The long-term funding of Scandinavian Brown Bear Research Project (SBBRP) has come primarily from the Swedish Environmental Protection Agency and the Norwegian Environment Agency. This research was funded by the French National Space Agency (CNES, BEAR2MAN project), the French National Research Agency (ANR; B-STRONG project), the University of Strasbourg (H2E project) and the French National Center for Scientific Research (CNRS) MITI.
