The Changing Belly Bugs of Brown Bears: A New Study Of The Role of Gut Microbiota in Hibernation of Ursus arctos

Brown bears are well known for their hibernation period, or period of hypometabolic fasting, which can last for up to 6 months during the winter. Prior to this they go through an intense period of eating and weight gain, known also as hyperphagia, to help stock enough fat to survive the winter.

Sommer et al. Visual Abstract

Sommer et al. Visual Abstract 2016


A new study (available through Cell Press, link to full paper in reference section) has for the first time demonstrated that changes in Brown Bear microbiota during different times of the year play a very important role in the efficiency of both their weight gain during summer and their use of fat stocks during the winter.

The study analyzed the microbiota of free-ranging Brown Bears during the active phase and the hibernation phase. Their aim was to investigate the role of microbiota in Brown Bear hibernation as well as if a seasonally altered microbiota contributed to “the healthy obesity phenotype” of weight gaining individuals during summer.

The study’s secondary aim was to study possible links between mechanisms of bear weight gain and loss with that of human obesity and resistance to insulin in the hope of a new approach to developing therapies for obesity.

Copyright. The Telegraph. Alamy.

Copyright. The Telegraph. Alamy. 2014.

The team analyzed the fecal microbiota of 16 bears captured during hibernation in February and during the active weight-gaining period in June of the same year.

They found that hibernation or ‘winter’ microbiota had reduced diversity but that it contained more microbiota phylums specialised in using lipid and protein stocks in the absence of active food intake (ie. during fasting). This change in gut flora composition allows hibernating bears a more efficient use of their fat stocks during hibernation thus leading to loss of body weight and body fat.

They also noted that bears produced more red blood cells and hemoglobin during hibernation and less bilirubin (used during hemoglobin recycling processes); this last element correlating with the lessened production of bile acid.

Summer microbiota was much more diverse, specialised in processing the Brown Bear’s omnivore diet and additionally might play a role in optimised weight gain.


To test whether the observed seasonal changes in bear gut microbiota was indeed responsible for changes in host physiology germ-free mice (mice bred in a sterile environment) were colonised with either ‘summer’ or ‘winter’ Brown Bear microbiota. It was then observed that mice colonised with the ‘summer’ microbiota showed greater fat gain that those with ‘winter’ microbiota. Further more those with ‘winter’ microbiota had higher observable levels of cholesteryl esters and triglycerides, demonstrating higher mobility of fats. Thus transferring Brown Bear microbiota to the germ-free mice led to the host acquiring some of the donors metabolic tendencies: It appears that the seasonal differences in Brown Bear microbiota actively contributes to observed seasonal metabolic changes. This allows them to more efficiently adapt to the physiological demands of either hyperphagia during summer or hibernation with prolonged fasting in winter.


Copyright. Telegraph. 2015.

Copyright. Telegraph. 2015.



Sommer et al. (2016) “The Gut Microbiota Modulates Energy Metabolism in the hibernating Brown Bear Ursus arctos” Cell Reports (14): 1-7

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Lillian Stanton

Lillian is a student currently studying biology and media communications. She is hoping to follow a post graduate degree in scientific journalism.

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