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Why humans are wired to run - and ferrets are not


Ferrets don't get "runner's high," but dogs do.

That curious observation, published in the latest issue of The Journal of Experimental Biology, offers surprising new insights into the evolutionary forces that guide the behaviour of ferrets and dogs – and humans. Unlike ferrets, we're wired to run in pursuit of "neurobiological rewards," so figuring out how to trigger that response more effectively could help encourage people to exercise more.

The ability to run long distances played a key role in our evolution, according to Harvard anthropologist Daniel Lieberman. Back in 2004, he made a list of 26 distinct features of the modern human skeleton that appear to be specifically designed for running, from the specialized neck tendon that keeps our head from flopping when we run to the unusually short toes that improve our stability and leverage.

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But, as everyone who has struggled to maintain an exercise program knows, the ability to run means nothing without the desire to get out there and do it. Since that desire would have been crucial to the survival of our earliest ancestors, University of Arizona anthropologist David Raichlen wondered whether it, too, might have been shaped by evolution.

To test that idea, he and his colleagues decided to look for the presence of runner's high after a 30-minute run in three groups of subjects: humans, dogs and ferrets. The "high" they were looking for wasn't necessarily the powerful euphoria often associated with the term, Dr. Raichlen notes: "That seems to be rarely experienced by runners in general, making it much more difficult to study."

Instead, the researchers measured pre- and post-exercise blood levels of anandamide, which is an endocannabinoid – a chemical produced in the body that, like marijuana, reduces pain and anxiety and creates a sense of well-being. These mood changes are much milder, to the point that many runners don't even notice them happening – but they still provide what the researchers call a "neurobiological reward," which makes you less likely to skip your next workout.

The animals were chosen because dogs, like humans, are "cursorial" mammals, which means they're adapted to run long distances in search of food. Ferrets, on the other hand, are non-cursorial. In fact, they typically sleep 18 hours per day and rarely engage in sustained endurance activity in the wild.

"The nice thing about ferrets is that we were able to adopt them out to the community after the study was completed," Dr. Raichlen adds. "This fit within our ethical guidelines of working with animal models."

The results were exactly what the scientists predicted: After running, endocannabinoid levels in the humans jumped to 2.6 times their pre-run values, and levels in the dogs increased by a factor of 3.3. The humans who reported the greatest change in mood also had the biggest jump in endocannabinoid levels. (The dogs had no comment, but it seems reasonable to assume a similar relationship.)

In the ferrets, on the other hand, there was no significant change – their survival never depended on running long distances, so evolution didn't bother to make it feel good.

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As a final test, the researchers repeated the experiment with a 30-minute walk instead of a run – and this time, saw no increase at all in endocannabinoid levels. So this evolutionary cattle-prod really seems to encourage humans, dogs, and (hypothetically) other cursorial mammals to exercise vigorously, presumably because that's what offered the greatest advantage in our hunting and gathering days.

The endocannabinoid system may actually be just one part of a more complex system to encourage endurance exercise, since researchers have also found links between runner's high and endorphins, the body's internal equivalent of morphine. In fact, one study found that rats who are accustomed to running suffer from typical signs of morphine withdrawal – teeth chattering, "wet dog shakes," and so on – if they're injected with a drug that blocks the action of opioid drugs.

All of this leaves an important question, though: If exercise feels so great and is so powerfully addictive, why do we have so much trouble doing it?

"Inactive people may not be fit enough to hit the exercise intensity that leads to this sort of rewarding sensation," Dr. Raichlen says. To test that hypothesis, he and his colleagues plan to study the endocannabinoid response in more detail, to determine whether it gets stronger or weaker as you go from sedentary to active.

It's a tempting thought – that if you can just stick with your exercise plan long enough, you'll eventually get hooked on it by powerful chemicals. That's what millions of years of evolution want you to do – unless, of course, you're a ferret.

Alex Hutchinson blogs about research on exercise at His latest book is Which Comes First, Cardio or Weights?

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