Long distance running isn't just good for your physique. If you're a man, and you're fast over long distances, you might just have a healthier sex drive and better reproductive fitness, a new study suggests.
Biologists at the University of Cambridge in the UK were interested in how endurance running might have been beneficial to male hunters in early human societies, and whether this trait still held any value to reproductive success.
In a study looking at half-marathon runners, the team determined that the best performers were likely exposed to higher levels of testosterone in their mother's blood before birth. In men, this hormone exposure has previously been linked to stronger sex drives, higher sperm counts, and better cardiovascular fitness, the researchers say.
Following the 2013 Robin Hood half-marathon in Nottingham, England, the team photocopied the hands of 542 race finishers, including 439 men and 103 women. They were looking for differences in the length of the runners' index and ring fingers, something known as the 2D:4D ratio.
Previous studies have shown that people who are exposed to greater levels of testosterone in the womb have a longer ring finger - or fourth digit - compared to their index finger - the second digit.
The researchers found a positive correlation between the 2D:4D ratio and race timing, meaning the fastest runners tended to have longer ring fingers, a marker of more 'masculine' hands. This correlation doesn't necessarily imply that higher prenatal testosterone exposure leads to improved running abilities, but it's an interesting takeaway.
And while this correlation was true for both male and female athletes, the results for men were more pronounced. On average, the 10 percent of men with a lower 2D:4D ratio - that is, a longer ring finger - completed the race 24 minutes and 33 seconds faster than the 10 percent of men with the highest 2D:4D ratio.
For the female participants, the difference was just under 12 minutes.
The results, which were published in the journal PLOS ONE, suggest that the skills associated with endurance running could be the result of prenatal testosterone exposure, and might act as a reliable signal for reproductive potential.
"The observation that endurance running ability is connected to reproductive potential in men suggests that women in our hunter-gatherer past were able to observe running as a signal for a good breeding partner," said lead author and evolutionary biologist, Danny Longman, in a press release.
As the authors point out, "the hunting of mid-sized animals using hunting technologies such as spears favours a method of disadvantaging or weakening the animal before approaching for the kill."
As such, many successful hunters would have needed to rely on a technique known as persistence hunting, where they chased prey to the point of exhaustion, often during the hottest part of the day.
"This technique has been observed in the Kalahari in Africa, and the Tarahumara tribe of Northern Mexico, who have been reported chasing deer until they collapse before strangling them by hand," the researchers write.
Naturally, endurance running would have been a desirable trait, necessary to secure vital food resources. And apparently, the best runners are still being rewarded.