Natural selection

You've probably had the feeling. Your running shoes are pounding the pavement — then suddenly your pain fades away, and you're feeling euphoric. The runner's high. But that biological perk may be limited to mammals that evolved for endurance 1 — like us. So says a study in the Journal of Experimental Biology. Researchers had humans and dogs — both natural-born runners — jog a half hour on a treadmill. Then they sampled their blood for endocannabinoids, some of the 2 thought to trigger the runner's high. As expected, humans and dogs had much higher levels after the run. But when ferrets — a 3 species — took the same 30-minute trot, they had no spike in those 4 molecules. The authors say that's because long-distance running could have helped our hunter-gatherer ancestors find more food — thus increasing their reproductive success. And they speculate that natural selection may have linked up a feel-good reward to that beneficial 5. These days of course, this ancient trait won't help us find extra calories — but it may encourage us to run 'em off.