Hibernating Bears and their Gene Expression

                The average black bear hibernates for 5-7 months, depending on their location. During this period, many physiological changes occur within the animal's body. Their body temperatures lower, they stop producing waste, metabolic rates severely decline and they even stop eating for these long periods. In order to prepare for this hibernation period, bears will eat upwards of 20,000 calories a day in the summer months, adding around 4-5 inches of fat for insulation under their coat. By the time the hibernation period is over, they will have lost almost 20% of their body fat, and again be ravenously hungry. While it is still being studied whether or not these changes come from environmental factors, like scarcity of food, or if they come from different hormones being released, it is evident that bears express different genes during winter hibernation, and the summer active periods.

              In a recent study of the America Black Bear, 245 genes in the heart, and 319 genes in the liver identified to be expressed differently during the summer and winter months. Of these a select 24 genes had significantly elevated expression. It was found that these genes mainly were involved in the lipid catabolism, and protein synthesis. During hibernation, these processes are linked to important things like keeping muscles from experiencing atrophy, and keeping the heart strong as the animals slows down its metabolic processes. A bear in hibernation will severely slow down it's heart rate. These different gene expressions can help keep the bear's heart strong, and at the same time allow the bear to walk when it is time to come out of the den. After all, wouldn't it be hard to stand up after taking a nap for 6 months? The expression of these genes help contribute to the muscle preservation of the bear. 

          As the bear sleeps through the winter, another phenomena occurs, thanks to genes being expressed differently. At the mRNA and protein level, a slow shift from glucose catabolism to glucose synthesis occurs in the liver. This allows the bear to have energy, which will be used to power the brain and other tissue that is being starved.  While the bears hibernate and slowly stop breaking down glucose for cellular respiration, they are able to reduce their metabolic rate by nearly 50%. 

          

        I have always love animals, but bears are truly incredible, and have always interested me. It is a wonder that they can accomplish the feat of going months and months on end without eating or defecating. I first started looking into this topic about a year ago, and have been interested ever since. There are a few researchers who say that bears never actually enter a true "hibernation" but rather a state of torpor. To myself, it doesn't matter how they classify the bear's winter nap. It is interesting to see what the expression of genes can do to keep a bear alive through the winter. When reading this article, I felt like I was reading science fiction. The bear had the ability to make it's heart slow down, keep itself from losing muscle, and even make itself stop producing waste.

https://www.sciencedirect.com/science/article/abs/pii/S1043276009001568
https://bear.org/do-black-bears-hibernate/
https://bmcgenomics.biomedcentral.com/articles/10.1186/1471-2164-12-171

Yellowstone Grizzlies May Soon Commingle With Northern Cousins

  Photo: David Grubs

The grizzly bears of Yellowstone National Park have been protected from hunting for years. The areas outside Yellowstone however were routinely hunted in order to protect cattle herds. Because of this, Yellowstone bears have been cut off from the rest of their kind. "Their closest kin prowl the mountains some 70 miles north, in and around Glacier National Park". Today, bears from the north are venturing further south which may have a positive impact on the Yellowstone bears. The population of Yellowstone bears was fewer than 150 in 1975 which resulted in a very small gene pool. If this lower genetic diversity were to continue, it could mean that in hundreds of years from now, the Yellowstone bears may be less able to adapt to changing conditions such as disease resistance, climate change, and changing food sources. Some scientists have even considered trucking bears from the north into Yellowstone if the bears were not able to reunite naturally. The state recently took the Yellowstone bears off the endangered species list because of the come back in population since 1975. Even though that is true, the state has also said that it will not allow any hunting in areas where the northern population may reconnect with the Yellowstone bears. I think that letting these two  populations of bears naturally reunite should help the diversity greatly and the state should put in many more restrictions that will better the odds in the areas where this can occur. 

https://www.nytimes.com/2017/11/03/science/grizzly-bears-yellowstone-genes.html
https://www.doi.gov/pressreleases/secretary-zinke-announces-recovery-and-delisting-yellowstone-grizzly-bear

Bear Breeds and Evolution

     Is it possible to analyze the evolutionary history of all bears at the genome level? Now it is! Scientists have sequenced the entire genomes of four bear species. It shows that gene exchange between species occurs through extensive hybridization. DNA samples of different species were taken from European zoos that are important for both conservation and research. Through these studies, it discounts previous assumptions of hybrid bears occurring due to climate change.

     I too was under the impression that climate change played a huge role in different hybrids of bear species. According to the article, brown bears invade northern regions and polar bears move onto the sea ice later than usual due to changes in climate. This new genomic data showed there must have been gene flow between the polar and sun bears, however the two live in completely different geographic areas and thus have never met. The researchers explanation of this suggested an "intermediate host" has passed the genes on in various directions.




Bear Hybrids
Types of Bears

Bear Hunting Altered Genetics More Than Ice Age Isolation

It was previously believed that the Ice Age, which effected most species twenty thousand years ago, also effected the distribution of bears.  However, recent studies of mitochondrial DNA led by Anders Gotherstam of Sweden, have shown that this is not the case.  In fact, the European Brown Bear was able to roam freely across Southern and Central Europe throughout the Ice Age.



However, these bears no longer live in these regions.  It is believe that this is due to historical hunting and human activity in what would have been the brown bears natural environment.  Today only a few populations of brown bears remain in Spain, Italy, the Balkans, and Scandinavia.

This study was conducted by researches from Sweden, Spain, the UK, Germany, and France.  It was published in the Molecular Ecology journal.  I was surprised that human activity effected the distribution of bears more than the Ice Age did!  It's depressing to know that even twenty thousand years ago humans were already having a negative effect on our environment.