Paradox explained: Why hibernating bears don't get blood clots uncovered

Researchers figure out how bears in hibernation, as well as paraplegic people, can be motionless without getting dangerous blood clots
Paul Ratner
Brown bear
A brown bear in the woods of Sweden in the summer.

Tobias Petzold / Ludwig-Maximilians University Munich.  

  • Researchers studied bears to understand why they don't get thrombosis while remaining motionless for long periods.
  • The scientists found a special mechanism in the blood of the bears that explains this ability.
  • This mechanism is shared among other mammals.

Hibernation is one of the most extraordinary traits of bears and yet, not all bears possess that feature. However, bears with the ability to hibernate spend months without moving, with few active bodily functions other than breathing.

They don’t eat or drink and their heart rate and other metabolic functions slow down. Although it can be risky, it helps the bears to get through long periods including winters when food is usually scarce.

What’s more, despite the bears laying motionless for extended periods, they don’t seem to get blood clots.

Usually, prolonged stillness causes health to deteriorate in individuals, for instance, those experiencing lengthy flights or immobility due to being bedridden for long periods can attest to it.

This unique ability in brown bears, Ursus actos (also shared by other hibernating animals) has been recently studied by an international research team, who discovered a protein that prevents blood clots from forming during the bears' hibernation. 

The study was led by Matthias Mann, Director at the Max Planck Institute of Biochemistry, and Dr Tobias Petzold, a cardiologist at the LMU Hospital Munich.

The scientists conducted their research on brown bears residing in central Sweden. The bears were tracked for over a decade by Danish cardiologist and professor, Dr Ole Fröbert from the University Hospital in Örebro, Sweden, who also collaborated on this study.

Discovering the bears' secrets

During the research process, the bears were first sedated for blood sampling. The mammals were then equipped with GPS transmitters so they could be tracked in the summer.

Blood samples were obtained from 13 hibernating bears in their dens during the winter and summer from the same bears.

After initiating a variety of blood tests, researchers found that the quantities of more than 150 proteins varied greatly between the blood of hibernating and active bears.

They also found that the process of interaction between the platelets and immune cells of the immune system was slowed down. This accounted for the absence of venous thrombosis. The same explanation held true for healthy volunteers who participated in a month-long spaceflight simulation study carried out by NASA and the German Aerospace Center.

The mechanism behind the slowdown observed in the blood was further probed using mass spectrometry-based proteomics — an analysis of the proteins involved.

The researchers found that 71 proteins were being upregulated and 80 downregulated during hibernation periods. In particular, they focused on the proteins in platelets—the components of blood that cause clotting.

The study established that one of the components – the heat shock platelet protein 47 (HSP47, which uses an enzyme called thrombin to help platelets stick together and form clots) was being downregulated by a 55-fold in bears that were hibernating in contrast to when they were active. The scientists found that this phenomenon was key to preventing thrombosis.

They also discovered that this downregulation of HSP47 was likely evolutionary and occurred in a whole range of mammals when they are immobilized for long time, including humans, pigs, and brown bears. 

Insight into the study

Interesting Engineering (IE) reached out to Dr Johannes Bruno Müller-Reif of the Max Planck Institute of Biochemistry for more insight into the research.  

The following exchange has been lightly edited for clarity and flow.

IE: Why did you and your colleagues look at bears for this study?  

Dr Müller-Reif: Free-ranging bears have evolutionarily adapted to several biological challenges during their hibernation period. The observation that they do not develop venous thrombosis in this period is just one interesting study purpose (paper from one of our co-authors on this).

We (professor Mann and I at the Max Planck Institute of Biochemistry MPIB) got contacted by experts in the field of venous thrombosis from the Ludwig Maximilians University (LMU), Munich (Dr Petzold, Dr Thienel, and professor Massberg) because they collected samples from free-ranging brown bears (plasma and blood platelets) together with the organizers of the bear captures (professor Frobert and the Scandinavian brown bear project).

They were able to measure the lower rate of thrombogenicity and thrombo inflammatory potential in the hibernating bears as opposed to the bears during their activity period in summer, but finding the underlying mechanisms in a non-model organism is difficult with traditional biochemical techniques. This is where the power of our method (mass spectrometry (MS)-based proteomics) lies and we previously showed that we can quantify proteins from any organism across the tree of life, as long as there is a draft genome available.

With our method, we were able to find several interesting proteins changed in the plasma and platelet proteome of bears between hibernation and activity phase and the one protein which was sticking out was HSP47 (or SERPINH1) with – 55-fold lower expression levels in platelets during hibernation.

In short, here we applied an evolutionary medicine approach. We studied an organism that adapted to a specific biological niche and found the underlying mechanism of hibernation.

At this stage, it was not clear if we were able to translate our findings to other species, especially humans. In a follow-up study, we found that this mechanism also exists in human individuals who remain motionless for prolonged periods, therefore, the mechanism could also be of therapeutic use.

One common question is, why we did not study other less dangerous or more accessible hibernating animals, like ground squirrels or hamsters (and other studies have done this of course). The reason is that rodents have a different mechanism of hibernation in which a substantial decrease in body temperature below 10°C  leads to profound sequestering of platelets in the liver, neutrophil margination to the vessel wall, and lymphocyte storage in secondary lymphoid organs.

In bears, those remain in circulation at a higher body temperature of – 30°C, thus, specific antithrombotic mechanisms are needed which we studied here.

Paradox explained: Why hibernating bears don't get blood clots uncovered
Seasonal differences in bear platelets

IE: How does the interaction between platelets and immune cells of the immune system in hibernating bears prevent venous thrombosis?

Briefly, platelet-neutrophil interaction is a part of the physiological mechanism of venous thrombosis, whereupon the neutrophils undergo a process called Neutrophil Extracellular Trap (NET) formation. They externalize nuclear DNA-histone strands that foster intravascular thrombosis.

We found that reduced HSP47 expression on the platelet surface (e.g. during hibernation) or pharmacological HSP47 inhibition reduced the interaction of platelets with neutrophils and therefore lead to a reduction of NET formation and venous thrombosis.

IE: Is it possible to block the HSP47 protein in humans to prevent thrombosis or what suitable substances are you looking to find that would achieve the same effect? 

We pharmacologically inhibited human platelets in vitro in co-culture with neutrophils and observed reduced NET formation. This supports the hypothesis that HSP47 inhibition under physiological conditions would also reduce this interaction, leading to a reduced venous thrombosis potential in patients.

Unfortunately, the small molecule inhibitors we used in this study are not fit for in vivo use, and the development of specific drugs is time-consuming and cost-draining. Nevertheless, we are working on this topic as a follow-up to our published work as we are convinced to have discovered a novel targetable mechanism of venous thrombosis.

IE: Are there other secrets of bear hibernation that may be helpful to humans? 

As described above, bears have adapted to several challenges during hibernation and our team will continue to study those in the future. For example, bears during hibernation do not develop severe sarcopenia, the decrease of muscle material such as humans do during bed rest post surgery. 

Bears do not get bed sores during hibernation. Bears during their motionless state develop plaques in the brain unlike how humans develop neurodegenerative diseases.

There are several interesting and partly untouched topics waiting to be investigated around hibernating bears in the future.

Check out the paper “Immobility-associated thromboprotection is conserved across mammalian species from bear to human” in the journal Science


Venous thromboembolism (VTE) comprising deep venous thrombosis and pulmonary embolism is a major cause of morbidity and mortality. Short-term immobility-related conditions are a major risk factor for the development of VTE. Paradoxically, long-term immobilized free-ranging hibernating brown bears and paralyzed spinal cord injury (SCI) patients are protected from VTE. We aimed to identify mechanisms of immobility-associated VTE protection in a cross-species approach. Mass spectrometry–based proteomics revealed an antithrombotic signature in platelets of hibernating brown bears with heat shock protein 47 (HSP47) as the most substantially reduced protein. HSP47 down-regulation or ablation attenuated immune cell activation and neutrophil extracellular trap formation, contributing to thromboprotection in bears, SCI patients, and mice. This cross-species conserved platelet signature may give rise to antithrombotic therapeutics and prognostic markers beyond immobility-associated VTE.

Add Interesting Engineering to your Google News feed.
Add Interesting Engineering to your Google News feed.
message circleSHOW COMMENT (1)chevron
Job Board