Choice excerpt:

It turned out that, as expected, the humans had shown significantly increased levels of endocannabinoids after running. So had the dogs, suggesting, for the first time, that they, too, experience a runner’s high.

But neither species had developed increased endocannabinoid levels after walking.

And the ferrets didn’t show higher endocannabinoid levels after either session. They gained, it seems, no neurobiological pleasure from running.

What these findings suggest, besides that ferrets will not make ideal training partners for marathon runners, is, Dr. Raichlen says, that “a reward response” to aerobic activity “appears to be part of our evolutionary history.”

Liking to run, it seems, may have helped to make humans what they are.