A big question is exactly why our capacity for the runners high evolved - for instance, is it actually our brain's way of incentivizing us to run and exercise? To test this idea, researchers at the University of Arizona, Florida's Eckerd College, and the University of Texas San Antonio decided to see whether the runners high can also be found in other mammals. For this task, they recruited two very different mammals - dogs, which like us are naturally athletic and capable of strenuous exercise, and ferrets, which are not. (No offense to any of our readers out there who happen to be ferrets, you lazy bums.)


The researchers had human, dog, and ferret participants both run and walk on treadmills, and then they collected blood samples after the exercise was completed. Though the ferrets were difficult to work with, they were able to get enough samples to reach one inescapable conclusions - humans and dogs both experience a massive spike in an endocanabinoid known as anandamide after a brisk run. Anandamide is responsible for signalling the reward center of the brain, and it helps explain why the human participants all reported feeling much happier after the strenuous run. The ferrets, however, showed no increase at all in their anandamide levels.

This all strengthens the idea that the endorphin release is a mechanism that specifically evolved to motivate humans and other athletic mammals to run for great distances. Lead author David Raichlen explains:

"'Aerobic activity has played a role in the evolution of lots of different systems in the human body, which may explain why aerobic exercise seems to be so good for us...These results suggest that natural selection may have been motivating higher rather than low-intensity activities in groups of mammals that evolved to engage in these types of aerobic activities."


Unfortunately, while this reward system helped keep our hominid ancestors fast enough to survive in a time before stone tools, it can't be of as much help in motivating humans to start exercising today. After all, the runners high is something that can only be accessed through peak physical exertion, which means people who otherwise live a sedentary lifestyle aren't going to suddenly get that endorphin rush just by jumping on a treadmill. As Raichlen explains, "Inactive people may not be fit enough to hit the exercise intensity that leads to this sort of rewarding sensation." Of course, if you ask me, that sounds like a wager to me...

Via the Journal of Experimental Biology. Image by Dudarev Mikhail, via Shutterstock.