Endurance running and digit ratio (2DAD): Implications for fetal testosterone effects on running speed and vascular health

There is anatomical and physiological evidence that endurance running (ER), i.e., running one or more kilometers using aerobic metabolism, originated early in the evolution of Homo, and the consequences of early selection for ER may be important in modern Homo. Here we examine ER performance in competitive ER. ER is sex dependent such that men tend to run faster than women, and the influence of sex on ER suggests that it may be modified by testosterone M. It is shown that a putative proxy for prenatal T, the ratio of the length of the 2nd and 4th digits (2DAD), is correlated with ER. Thus performance in training for ER was associated with high prenatal T, as measured by low 2DAD, in both men and women. In cross-country races from 1 to 4 miles, 2DAD explained about 25% of the variance in both male and female ER. Therefore, speed in ER was dependent on a proxy for prenatal T. 2DAD correlates 'with performance in sport and exercises, which test a mix of strength and fitness, but the associations are in general quite weak with 2DAD accounting for less than 10% of the variance in performance. Our finding that 2DAD explains about 25% of the variance in ER suggests that prenatal T is important in determining efficiency in aerobic exercise. Early populations of Homo may have been strongly selected for ER and high prenatal T. The implications of this for patterns of predisposition to cardiovascular disease in modern Homo are discussed.