Normal mice run three to four miles each night.
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Garland took a group of average mice and separated them into two subgroups: those that chose to run less than average each night, and those that chose to run more than average. Garland then bred “high runners” with other high runners, and “low runners” with other low runners. After just one generation of breeding, the progeny of the high runners were, of their own accord, running even farther on average than their parents. By the sixteenth generation of breeding, the high runners were voluntarily cranking out seven miles each night.
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When mice are bred for endurance capacity — not voluntary running, but when they are forced to run as long as they physically can — successive generations have more symmetrical bones, lower body fat, and larger hearts.
In his voluntary-runner breeding program, Garland saw body changes, “but at the same time,” he says, “clearly the brains are very different.” Like their hearts, the brains of the high runners were larger than those of average mice. “Presumably,” Garland says, “the centers of the brain that deal with motivation and reward have gotten larger.”
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Whatever Ritalin does in the brains of normal mice is already occurring in the brains of the high-running mice.