Mice on Earth may like running in wheels, but mice in space apparently take advantage of the microgravity to run in circles by bouncing off the walls.
A new study of mice living aboard the International Space Station suggests such projects could reveal key insights to support human voyages to Mars and beyond, scientists added.
A mission to Mars and back would take years, but scientists aren't sure how a crew would adapt or suffer over such a long time in outer space. Spaceflight involves exposure to microgravity, radiation, isolation and myriad other factors that may pose a variety of physical and psychological risks for astronauts.
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Studying lab rats and mice is an obvious way for scientists to learn what effects spaceflight might have without endangering astronauts. "Since rodents develop and age much faster than humans, studying rodent model organisms allows scientists to study diseases that may take years or decades to develop in humans," study lead author April Ronca, a space bioscientist at NASA Ames Research Center in Moffett Field, California, told Space.com.
However, previous work on rodents in space often flew the animals in apparatuses that were ill-equipped to reveal what the animals were actually doing in flight and how well they were adjusting to life in space, the researchers wrote in the paper. For example, in 2013, 45 male mice flew for 30 days on the uncrewed Russian Bion-M1 spacecraft, but a food dispenser malfunction and other equipment failures killed about two-thirds of the mice. The quality of video recordings of the mice also declined precipitously after the first week as debris obscured the camera lenses.
So researchers designed a new and improved rodent hotel to send to the space station, called the NASA Rodent Habitat. During the first two uses of this habitat, the NASA Rodent Research-1 mission, scientists recorded videos of 20 female mice for between 17 and 33 days. A group of similar mice were kept on the ground and monitored for comparison during each procedure.
The NASA Rodent Habitat was designed to house mice in groups, which reduces their stress levels and helps scientists monitor their social activity. The habitat also sports grates on its walls that mice can grab onto and bounce off of. The habitat also provides both a relatively large space for mice to move around in and a smaller area they can retreat to and engage in the huddling behavior typical of mice.
The design of the NASA Rodent Habitat also helped keep its lenses fairly clear throughout the mission. "The camera lenses are positioned in the habitat in a way that reduces exposure of the lens to floating debris and liquids," Ronca said. "More recently, a regular lens-cleaning step has been incorporated into the Rodent Research system, which has further improved image quality."
In these initial tests, the scientists found that mice in space were active and mobile throughout the experiment, exploring their habitat and engaging in the full range of behaviors typical of the species, such as feeding, grooming their fur and huddling together. The rodents quickly adapted to their new weightless circumstances — for example, by anchoring themselves to the habitat walls with their hind legs or tails and stretching out their bodies. This pose was similar to mice on Earth standing up on their back legs to explore their environment.
Throughout their time spent on board the space station, the mice explored the entire habitat. At the end of the mission, they weighed about the same as their counterparts on the ground and their coats were in excellent condition, the researchers reported, which are both signs of good health.
A unique behavior was seen within seven to 10 days after launch — the younger, but not older, mice in the experiment started running in circles around the habitat walls. This "race-tracking" quickly evolved into a coordinated group activity. The researchers noted this appeared similar to exercises that human crew participated in routinely.
But the scientists aren't sure why these mice started running laps. Mice given the chance to scurry on a running wheel in the wild will do so about as much as captive mice, suggesting that circling of all kinds is something mice enjoy doing. Or, such circling is the kind of abnormal repetitive behavior that prior research has shown animals perform under stress. However, the researchers think stress was not a likely cause of this circling in space, because the mice were in excellent health, showed no overt signs of stress and otherwise behaved normally.
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Of course, the circling could still have been prompted by the unique conditions imposed by spaceflight. Bones in the inner ears of mice and humans alike help the body keep balance, but in microgravity, the constant feeling of free fall experienced in orbit can feel uncomfortable. Circling may generate forces to stimulate the inner ear and help the rodents feel comfortable, the researchers said. In the future, the scientists plan to identify the molecular and cellular changes occurring in mice to help pinpoint the causes of the circling, Ronca said.
And the mice's shorter life spans may help scientists shed light on long-term human spaceflight. "The mice flew for up to 37 days, which is approximately 18 months for humans," Ronca said. "The results of long-duration rodent studies are important for learning how to protect the health of astronauts who will embark upon long duration exploration missions, and can help treat diseases on Earth such as suppressed immune systems, muscle atrophy and bone loss."
The scientists detailed their findings online April 11 in the journal Scientific Reports.
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