Why did we evolve to experience "runner's high"?

We recently learned that women can have orgasms from exercising, and now it seems the pleasures of exercise are even more deeply woven into our evolution. The "runner's high" experienced after strenuous exertion was actually key to our species' success.

The runners high is something most athletes experience at one point or another. It's a rush of pleasurable endorphins released by the reward center of the brain. This response is triggered by a part of the brain known as the endocannabinoid system. The runners high allows people to surpass their normal physical limits by suppressing feelings of pain and sometimes causing feelings of happiness and euphoria. This potentially can be dangerous - after all, it allows a person to overexert beyond their actual capabilities - but it's not hard to see the benefits of such short-term super-performance.

Exactly where the runners high comes from is uncertain - one less than charitable theory suggests it's actually the brain's information processing centers becoming overtaxed and going haywire - but the consensus is that it is linked to the survival of our ancient hominid ancestors. The runners high, with its ability to suppress the pain of overexertion, is one of a few key adaptations - that allowed early humans to run for tremendous distances without needing to stop.

This is known as the endurance running hypothesis, and the adaptations needed to make such strenuous activity possible can potentially explain such diverse human traits as large gluteal muscles, hairlessness, short toes, and even our bodies' inability to deal with obesity and sedentary lifestyles without health complications. Such running might well have been our first real evolutionary advantage that allowed us to hunt bigger, more powerful animals. Before the development of stone tools, hominids could have used persistence hunting, in which they would chase their prey for miles until the animal collapsed due to exhaustion.

That last part relies on another big adaptation: our ability to sweat, which allows us to keep our body heat manageable even during peak performance. Galloping animals, on the other hand, must slow back down to a canter so that they can pant and lower their body temperature. Between sweating and the runners high, hominids could run fast enough and long enough that the animal could never stop galloping until it finally collapsed, making it an easy kill. Persistence hunting is still practiced by a few hunter-gather tribes, which you can see in the awesome clip below.

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.