NATIONAL GEOGRAPHIC - Some of this variability in motivation or ability to do hard exercise is related to genetics. For example, Theodore Garland, Jr., an evolution biologist at the University of California Riverside, wanted to understand how complex traits—like marathon running—evolve at multiple levels of organization, ranging from behavior to DNA. He has shown in an ongoing experiment launched in 1993 that a strain of super-runner mice—bred over more than one hundred generations—evolved specific changes in their DNA and ran over three times longer than average. These mice also have different microbiomes than their less active counterparts.
To test whether eliminating the gut microbiome would affect the motivation to exercise Garland gave the athletic mice antibiotics. It drastically and irreversibly reduced the voluntary exercise behavior of super runners. The mice with depleted gut bacteria ran about 21 percent less every day, even though they continued to eat well and were otherwise unaffected.