Skeletal Muscle Atrophy in Polar Bears

Figure 1: Example of polar bear with Atrophy

Seasonal declines in food availability causes many animals to reduce their physical activity and use their tissue storage for energy and function. This decline in activity brings a decrease in skeletal muscle loading and neural activation. These factors potentially cause skeletal muscle atrophy that is characterized by loss of muscle mass and strength. Catabolism of tissue caused by fasting is also a cause of muscle atrophy in animals.

Across the Arctic tundra, polar bears (Ursus maritimus) experience food deprivation and changes in physical activity over seasonal changes. Pregnant female bears hibernate, give birth, and nurse in maternal dens from November until March.  Other bears such as males and non-pregnant females are still active during these months, however, their hunting ability is decreased due to seals not being active on the ice during winter months. Once offspring are grown, mothers and their cubs emerge onto the ice and become highly active from April through July.

Figure 2: Mother bear surfacing from hibernation with cubs

Due to annual ice melts that peak during August through October, polar bears are forced to fast once more. In one of the 19 sub-populations in the Southern Beaufort Sea (SBS) area, nearly 70 percent of individuals follow the retreating sea ice as it recedes north beyond the continental shelf waters, while 30 percent  of the animals come to the shore. The bears that follow the sea ice that retreats beyond the continental shelf waters become food deprived. Seal food sources follow the ice retreat while terrestrial prey that is rich in nutrients becomes scare.

This study examined the nutritional status and physical activity of polar bears in the SBS after winter food deprivation and during summer fasting. Predictions tested were that bears on the sea ice over the continental shelf during the months of April through May are recovering from atrophy induced by reduction in activity and food deprivation. The second prediction was that bears that are on-shore in August demonstrate no atrophy because of high activity and excessive eating. Third, they predicted that polar bears on-shore in October exhibit little to moderate atrophy due to competing influence of reduced activity and feeding from washed up whales. The last predication tested was that bears on sea ice over deep water in October show moderate to substantial atrophy from reduced activity and fasting over the summer months.

Table 1: Idealized timeline of polar bear cycles in SBS

Samples included bear's bicep femoris muscle that was collected during each season. Captures of bears were done between Barrow, Alaska and the USA-Canada border both on the shoreline and offshore on coastal sea ice. Figure 1 below shows the study area with (A) being the Beaufort Sea Region boxed, (B) being the Beaufort sea region of the Alaskan coasts and distribution of the sea ice (white) in May of 2009. The picture (C) shows the same area with sea ice as of August 2009. The circles represent locations of 31 polar bears over the previous week.

Figure 3: Mapping of polar bears in SBS region

Atrophy was most pronounced during the spring months of April through May as a result of food deprivation during the previous winter. The bear's muscles exhibited reduced protein concentration, increased water content, and lower kinase mRNA. These polar bears increased their feeding and activity in the spring when seals became more readily available to them. This increase in feeding initiated a period of muscle recovery.

During the following ice melt of late summer, nearly 30 percent of the SBS polar bears abanded retreating ice for the land dwelling animals. In August, the shore bears exhibited no muscle atrophy. This indicated that the bears had fully recovered from food deprivation in the winter. These individuals were found to eat whale carcasses during October, and they had retained good muscle condition because of this. In contrast, nearly 70 percent of SBS bears follow the ice north in the late summer. This takes bears to an area of deep water and less prey. These bears fast, and by October, they exhibited muscle protein loss and rapid changes of myosin because of their reduced activity.

In summary, these findings indicate that unlike other bears during winter hibernation, polar bears without food in summer cannot escape muscle atrophy. Prolonged summer fasting resulting from climate change-induced ice loss creates an increased risk of greater atrophy and reduced abilities to hunt and travel.

Whiteman, J.P., Harlow, H. J., Durner, G.M., Regehr, E. V., Rourke, B.C., Robles, M., ... Ben-David, M. (2017). Polar bears experience skeletal muscle atrophy in response to food deprivation and reduced activity in winter and summer. Conservation Physiology, 5(1), cox049. http://doi.org/10.1093/conphys/cox049   https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5550809/#cox049C64