Years of movies and television have cultivated the image of the dumb jock: athletically gifted, but not so gifted academically. A new study conducted by researchers at the University of California, Riverside has found that the stereotype may not be entirely truthful - kind of. Researchers found that mice that have been selectively bred for generations to love exercise have larger midbrains than mice that were not selectively bred.

The study examined the brains of mice that had been selectively bred to voluntarily enjoy wheel running. These mice have been bred for the past 20 years, which accounts for 65 generations of laboratory mice. The researchers took the brains of mice and dissected them into two different sections: the cerebellum, which is responsible for controlling movement, and the non-cerebellar regions.

They found that mice bred to be athletes had midbrains that were 13 percent larger than mice that were not. Midbrains are responsible for reward learning, motivation and the reinforcement of behavior. The rest of their non-cerebellar portions of the brains were also larger. However, researchers found that the athletic mice did not have larger cerebella or greater brain mass.

"The finding," the researchers write in the article, "supports the mosaic theory of brain evolution." That theory is breeding for a selected trait would cause the brain to evolve, but only in certain sections of the brain. The other theory suggested that selective breeding would cause the entire brain to evolve.

However, the findings for humans are not yet clear. "It is possible that individual differences in the propensity or ability for exercise in humans are associated with individual differences in the size of the midbrain, but no one has studied that," study author Theodore Garland said in a statement. "If it were possible to take MRIs of babies' midbrains before these babies started 'exercising' and then follow these babies through life, it may be that inherent, genetically-based differences in midbrain size detected soon after birth will influence how much they would be likely to exercise as adults."

The study was published in the Journal of Experimental Biology.