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Following the trail of the bear's secrets

How can bears go into hibernation for up to seven months and come out just as healthy as they started? Scientists bring bears out of their dens to find answers to this riddle. We are now getting closer to answers that may help solve some of our major public health problems.

A bear lying in a den, peeking at the photographer.

Published Apr. 16, 2023 8:14 PM - Last modified Oct. 24, 2025 10:58 AM

Four cars in a row pictured from the back driving on a snowy road with a forest to their left. — Dalarna in Sweden, February 2023. Early morning. A group of international researchers is heading out on today's mission.

A forest photo taken from the snowy forest floor upward towards the sky. — Somewhere in this forest lies the bear they have to find. A three-year-old female without cubs. The researchers will anaesthetise her and take her out of the den.

Jon Arnemo on skis on a snow-covered road. — Professor Jon Arnemo has been involved in the research project since its inception. He has helped retrieve more than 70 young bears during the winter.

It's always exciting. Sometimes the bear wakes up and gets away when we arrive. And the bear can be a dangerous animal when you crawl into its den.

Jon Arnemo, professor of applied ecology, INN University

hand holding phone showing map of where the bear might be located. — The location of the radio-tagged bear is marked on the map.

Researcher holding measuring equipment in the air in front of one of the cars with an open trunk. — They search for the bear with radio direction-finding antennas.

Ski tracks in the snow. — Researchers from institutions in Germany, France, Denmark, Sweden and Norway are part of the group heading in the direction of the den.

When they arrive at the den, the bear is anaesthetized.

A closeup photo of a bear's head with a yellow blindfold over its eyes. — It is a large three-year-old weighing 74 kilos that they have taken out of the den.

The hibernation of the bear is fascinating:

It is not in deep sleep and its body temperature is only a few degrees lower than in summer.

But its heart only beats ten times a minute. The bear hardly moves, eating and drinking nothing. The bear does not urinate nor has bowel movements.

For up to seven months, it eats away at the fat reserves it has acquired during the autumn. It saves the body's resources and uses very little energy.

The bear’s muscle and bone mass are almost unchanged when it leaves the den in the spring

Jon Arnemo standing over an anaesthetised bear. — It is almost a miracle. The bear has quite an unhealthy lifestyle. It eats way too much sugar in the autumn and gets fat. Then it goes to bed for half a year. Still, it doesn't get any lifestyle diseases. Jon Arnemo

The largest animal in the Scandinavian forests has many secrets. Secrets that no researcher has ever gone to such great lengths to uncover.

A researcher taking samples from a bear. — The researchers have little time: the anaesthesia lasts about an hour.

Now the whole bunch starts taking samples and measurements.

Of the thigh muscles.

Of the bear's size.

A closeup photo of a test tube filled with bear blood. — And not least: of the blood

This research into bear hibernation began in 2010, as part of the Scandinavian Bear Project. They have taken samples of the same radio-tagged individuals in both winter and summer.

Right from the start, the aim was to use the brown bear as a model species for human medical studies, so-called "translational medicine".

The dream: that the bear can provide us with a “miracle cure” for some of our major public health problems.

A clipping from the paper in the journal Science.

After 13 years, the researchers have published 60 scientific papers with new knowledge about the bear's physiology.

And now they have had a real breakthrough.

The findings of the international research group are now published in the prestigious journal Science, featuring Norwegians Jonas Kindberg (NINA) and Alina Evans (INN University) as co-authors.

Unlike humans, bears almost never get blood clots.

In the blood samples from the bear, the researchers saw that the platelets became less sticky in winter, but they did not understand how this happened. After three years of collaboration with researchers at Ludwig Maximilian University of Munich the researchers now believe they have found the mechanism: a so-called heat shock protein, HSP47, causes the platelets to change, triggered by the bear lying motionless for a long time. This prevents the formation of blood clots.

Based on the bear research, they have now demonstrated the same mechanism in mice, pigs, and indeed humans as well.

The researchers believe the findings are promising. Perhaps in the future we will have a new type of pill against blood clot formation, developed with knowledge from the bear project?

It was the Danish cardiologist Ole Frøbert, a professor at Örebro University and Aarhus University, who took the initiative to investigate the bear's hibernation.

I am extremely happy that after all these years, with all the challenges we have faced, we have found answers that can actually be used in human medicine

Ole Frøbert

There are still many questions and few answers about how the bear can biochemically and physiologically adapt to such a long hibernation period. And not least how this can be transferred to another mammal – the human.

Researchers studying the samples they have taken from the bear, still out in the field. — Is it knowledge of the bear's hibernation that could one day enable the human body to make the long journey to Mars?

Can the bear's physiology provide us with solutions to one of the biggest public health problems of our time – obesity and overweight?

Researchers carry the anaesthetised bear. — The researchers have finished collecting their samples, the bear's anaesthetic is about to wear off and they carry it back to the den.

The bear is carefully placed back where it lay under a large rock somewhere in the forest in Dalarna.

A key consideration of the project is investigating how the bears are affected by researchers disturbing their winter hibernation, and continuous ethical assessments are made of the methods used. The studies have been approved by animal ethics authorities in Sweden.

The Scandinavian Bear Project, which is led by the Norwegian Institute for Nature Research (NINA), is a collaboration between NINA, the Swedish University of Agriculture (SLU), Inland Norway University of Applied Sciences (INN University), and a number of other research institutions both in Scandinavia and the rest of Europe.