Are humans evolved specialists for running in the heat? Man vs. horse races provide empirical insights.

Abstract

What is the central question of this study? Do available comparative data provide empirical evidence that humans are adapted to endurance running at high ambient temperatures? What is the main finding and its importance? Comparing the results of races that pit man against horse, we find that ambient temperature on race day has less deleterious effects on running speed in humans than it does on their quadrupedal adversary. This is evidence that humans are adapted for endurance running at high ambient temperatures. We debate whether this supports the hypothesis that early man was evolutionarily adapted for persistence hunting. Many mammals run faster and for longer than humans and have superior cardiovascular physiologies. Yet humans are considered by some scholars to be excellent endurance runners at high ambient temperatures, and in our past to have been persistence hunters capable of running down fleeter quarry over extended periods during the heat of the day. This suggests that human endurance running is less affected by high ambient temperatures than is that of other cursorial ungulates. However, there are no investigations of this hypothesis. We took advantage of longitudinal race results available for three annual events that pit human athletes directly against a hyper-adapted ungulate racer, the thoroughbred horse. Regressing running speed against ambient temperature shows race speed deteriorating with hotter temperatures more slowly in humans than in horses. This is the first direct evidence that human running is less inhibited by high ambient temperatures than that of another endurance species, supporting the argument that we are indeed adapted for high temperature endurance running. Nonetheless, it is far from clear that this capacity is explained by an endurance hunting past because in absolute terms humans are slower than horses and indeed many other ungulate species. While some human populations have persistence hunted (and on occasion still do), the success of this unlikely foraging strategy may be best explained by the application of another adaption - high cognitive capacity. With dedication, experience and discipline, capitalising on their small endurance advantage in high temperatures, humans have a chance of running a more athletic prey to exhaustion.