© Shannon Curtis
8/28/2017 3:42:57 PM
Polar Bears and Astronauts: Muscle Atrophy
On March 1st, 2016, a returning space capsule touched down in the barren steppes of central Kazakhstan. Astronauts Scott Kelly and Mikhail Kornienko crawled out, immediately feeling the burden of Earth’s gravity after a year of weightlessness aboard the International Space Station. They soon began performing difficult physical tasks similar to those that future space explorers will face when arriving on new planets. However, the tasks were made extra challenging by their muscle atrophy: a gradual weakening and loss of mass that occurs when there is no need to work against gravity.
Meanwhile, a thousand miles north, polar bear mothers emerged from their dens, new cubs in tow. Although these bears had never been to space, they had been mostly motionless, curled up while denning for the past eight months. Decades ago, we discovered that a long-term reduction in activity in animals (and people) mimicked many of the effects of atrophy that were becoming more apparent as humans made longer and longer journeys to space. The fields of medicine, space exploration, and wildlife ecology have been linked by this unusual commonality ever since.
Insights from polar bears
In 2009, I won a prestigious research fellowship* from a partnership between the University of Wyoming and NASA, studying how the muscle physiology of polar bears changes throughout the year. Understanding mechanisms of muscle atrophy in polar bears could help us better understand atrophy in space-traveling astronauts. Our team completed multiple field seasons in the Southern Beaufort Sea (near Alaska and the Yukon), where we collected polar bear tissue samples. We then spent years in the lab analyzing those samples.
The first field season was on sea ice in the spring, when polar bears feast on seals after the long winter. We found evidence of muscle atrophy (e.g. reduced concentration of protein) but, surprisingly, no differences between bears that had hibernated recently (moms with new cubs) and those that had not hibernated (male bears and females with older cubs). Atrophy in non-hibernating bears could have been caused by reduced activity during the winter months, when food is harder to find due to dark and windy conditions. Just as lack of activity can cause atrophy, so can lack of food.
We returned to our study area in the summer and sampled bears on land. In the Southern Beaufort Sea, sea ice retreats north during the warm summer: about 70% of the polar bear population follows the ice while 30% swim to land. Most bears who choose land will swim to shore in July and August, having recently finished hunting seals. Accordingly, these bears showed no signs of atrophy: spring activity and feeding had allowed them to completely recover from winter.
That fall, we sampled bears on the land and on the sea ice. Since we had last seen them, the bears on land had been scavenging on the enormous bowhead whales carcasses left on the beach after subsistence whaling by the indigenous towns of Kaktovik, Nuiqsut, and Utqiaġvik (Barrow). These carcasses offer a calorie bonanza for the bears who spend summer on land. As expected, these bears showed few signs of atrophy; lack of summer activity was made up for by an abundance of food.
Meanwhile, the bears that had spent the summer on the sea ice displayed moderate atrophy from both food deprivation and less activity. Polar bears on the summer sea ice are mostly food deprived, likely because the ice now retreats far from the continental shelf where their seal prey is concentrated. And while the bears on ice were only slightly less active than those on shore, it was enough to make their muscles significantly different.
Polar bear muscle varied substantially throughout the year, which may reflect the extreme environmental variation between the Arctic summer and winter. Atrophy can be due to inactivity or food deprivation, and can have consequences for the 70% of bears who spend their summer on the sea ice. Weaker muscles can inhibit walking, swimming, and pouncing, though physical performance was not measured in this study.
As sea ice loss continues and the summer grows longer, polar bear activity could eventually be hindered because of muscle atrophy, yet another way the fate of sea ice and polar bears are intertwined. While humans seek to conquer muscle atrophy on our way into space, I hope that we can also protect sea ice on our home planet.
*Dr. Whiteman’s fellowship supplemented his role in a larger study, supported by the National Science Foundation, the Environmental Protection Agency, the US Geological Survey, and the US Fish and Wildlife Service.
This study was recently published by the peer-reviewed journal Conservation Physiology.