In 2014, NASA sent young and older mice into space; within 9 to 11 days, the younger ones began running group laps in zero gravity

Back in 2014, NASA sent 20 mice to the International Space Station, dividing them into young and old age groups. The young mice began to run around the habitat walls in circles within 9 to 11 days of launch. They ran as a collective and appeared to be well-coordinated with one another. In contrast, the older mice were floating in the same environment but did not seem to care about the exercise at all. Researchers have proposed several possible explanations for this behavior, but none has been confirmed.

The project was called NASA's Rodent Research-1 (RR-1), which saw 20 female mice, the "young" group of which aged 16 weeks and the "old" group aged 32 weeks, being aboard the ISS for 37 days. According to a 2019 peer-reviewed study titled “Behavior of mice aboard the International Space Station,” published in Scientific Reports, the lead researcher of which is Dr. April Ronca from NASA Ames, the race-tracking behavior of the young mice began when they started pushing against the habitat walls individually.

A mouse mosh pit, by the numbers


The younger “Validation” mice circled for 24% of the time during their activities; that is, 3,267 times around the tank. Meanwhile, the older “Experimental” group of mice circled only around 21 times, which accounted for less than 1% of their recorded activities despite the fact that researchers recorded almost ten times more video of them. In just three days, individual circling turned into group circling with some mice circling in unison around the tank. Apparently, it mimicked the same pattern that mice exhibit when provided with a wheel to run on Earth.

Why didn't the older mice bother?

No one knows the answer, and it is precisely this fact that makes the story worth considering. Three hypotheses were proposed: this may have been either a stress or stereotyped motor reaction, or the well-known rewarding effects of physical activity, or the vestibular apparatus of the mice, a sensory system located in their inner ear that detects motion and maintains balance, in response to the unusual experience of moving by itself in zero gravity.

Old age could simply mean a more inflexible or less motivated nervous system in space. According to NASA’s own records of the results, all 20 mice were fine during the entire experiment, with comparable weight and good fur, so this was not distress but rather looked like play.

Why should this matter

The researchers themselves noted a loose parallel with human spaceflight: astronauts aboard the International Space Station also move around by pushing off surfaces, and the study's authors suggested the mice's behavior could offer a useful analogue for understanding how physical movement affects the body in microgravity. That said, the comparison only goes so far; the mice's circling looked like spontaneous, unprompted play, whereas astronaut workouts are a strict medical requirement. Astronauts have to work out for about two hours each day precisely because microgravity can cause deterioration in the physical condition of their muscles, bones, and hearts very quickly.

As stated on NASA’s webpage dedicated to astronaut fitness, current practices are viewed by NASA as vital measures against this issue, not just something that is done for fun; it is similar to your gym time, except not doing it wouldn't just be not uncool but rather medically dangerous. This is one mission, one rodent habitat, 20 mice, and three conflicting theories that are yet to be confirmed. This is why there are ongoing studies, and there is even a planned Rodent Research-26 mission, all thanks to the questions that RR-1 brought to light. Neither mice nor humans completely understand microgravity, and we’re just all doing laps in hopes of discovering something from it.