The Runner's High - you are destined to enjoy exercise

I saw this report pass over my Google Reader today, based on a study just published in the Journal of Experimental Biology:

Wired to run: exercise-induced endocannabinoid signaling in humans and cursorial mammals with implications for the ‘runner’s high’

Humans report a wide range of neurobiological rewards following moderate and intense aerobic activity, popularly referred to as the ‘runner’s high’, which may function to encourage habitual aerobic exercise. Endocannabinoids (eCBs) are endogenous neurotransmitters that appear to play a major role in generating these rewards by activating cannabinoid receptors in brain reward regions during and after exercise. Other species also regularly engage in endurance exercise (cursorial mammals), and as humans share many morphological traits with these taxa, it is possible that exercise-induced eCB signaling motivates habitual high-intensity locomotor behaviors in cursorial mammals. If true, then neurobiological rewards may explain variation in habitual locomotor activity and performance across mammals. We measured circulating eCBs in humans, dogs (a cursorial mammal) and ferrets (a non-cursorial mammal) before and after treadmill exercise to test the hypothesis that neurobiological rewards are linked to high-intensity exercise in cursorial mammals. We show that humans and dogs share significantly increased exercise-induced eCB signaling following high-intensity endurance running. eCB signaling does not significantly increase following low-intensity walking in these taxa, and eCB signaling does not significantly increase in the non-cursorial ferrets following exercise at any intensity. This study provides the first evidence that inter-specific variation in neurotransmitter signaling may explain differences in locomotor behavior among mammals. Thus, a neurobiological reward for endurance exercise may explain why humans and other cursorial mammals habitually engage in aerobic exercise despite the higher associated energy costs and injury risks, and why non-cursorial mammals avoid such locomotor behaviors.

They basically found that that exercising mammals release pleasurable endocanabinoids in response to exercise.

Having suggested that natural selection used the endocanabinoid system to motivate endurance exercise in humans and other animals that walk and run over long distances, Raichlen adds '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'.

There is some more background at Eureka here.

The team publish their discovery that animals that evolved for endurance exercise benefit from endocanabinoids while animals that did not don't experience the pleasures, leading them to propose that natural selection used the endocanabinoid system to motivate endurance exercise in humans.

Given that we do experience the runners' high, the implication is that we evolved for endurance....or were designed for endurance. 

It is also interesting from the perspective of motivation and the ideas of Samuele Marcora that I mentioned here.