While a few mammals can trot long distances, comparable to those that humans can run, they cannot run long distances while galloping in hot conditions without becoming hyper-thermic. This major constraint derives from two aspects of mammalian biology. First, the thermogenic effects of exercise increase in proportion to the number and rate of cross-bridges that are activated during muscular contractions. In humans for example, running can generate as much as tenfold more heat than walking (Cheuvront and Haymes, 2001), and a sprinting cheetah generates so much heat that it must stop after approximately 1 km (Taylor and Rowntree, 1973). Second, the major mechanism by which most mammals cool themselves, panting, is problematic during galloping. Panting occurs via shallow breaths, about ten times the normal rate of respiration, in the dead space of the upper pharynx without any gas exchange occurring in the lungs (Schmidt-Nielson, 1990). Panting mammals, however, cannot satisfy their respiratory demands for oxygen during galloping, and the 1:1 coupling of locomotion with respiration that occurs during galloping is biomechanically incompatible with panting (Bramble and Jenkins, 1993; Entin et al., 1999).
Humans, however, have evolved a number of specialized modifications for effectively dissipating copious quantities of heat while running in hot, arid conditions. For one, humans do not have to couple respiration with stride (Bramble and Carrier, 1983). In addition, humans are considerably derived in terms of the number of eccrine sweat glands and the loss of almost all fur. Sweating is an effective means of cooling (evapotranspiration of 1 ml H2O requires 580 cal of heat [Schmidt-Nielson, 1990]), but is ineffectual with fur, which traps air and moisture at the skin's surface, thereby considerably reducing convection (McArthur and Monteith, 1980). Therefore, other tropical cursorial mammals such as hyenas and hunting dogs that can run long distances are constrained to do so at night or during the dawn and dusk when the days are hot. Humans alone are capable of ER during midday heat. Human sweating, however, imposes high water demands, requiring as much as 1-2 l/h in well-conditioned athletes (Torii, 1995).
In short, humans are comparatively superb endurance athletes, particularly in hot, arid conditions that are conducive to heat-loss from sweating. In fact, humans appear to occupy a rare extreme in the general trade-off between aerobic and anaerobic capabilities (Wilson and James, 2004). Natural selection often favors speed over endurance because of the dynamics of predator-prey interactions: slower animals typically have lower fitness. Animals built for speed and power are rarely good at endurance and vice versa, in part because of muscle fiber composition. In most mammals, there is a predominance of Type lib (fast-glycolytic) and Type IIa (fast oxidative) relative to Type I (slow oxidative) muscle fibers. The former fast-twitch fibers can produce several times more force but are anaerobic and fatigue quickly. Slow-twitch fibers have higher aerobic capacity, but produce less force. Most human leg muscles have about 50% of each type (McArdle et al., 1996), but can increase slow-twitch fiber content to about
80% through aerobic endurance training. They can also increase fast-twitch fiber content to between 70-80% through power training (Thayer et al., 2000). Such training effects for fast twitch fibers are more common in humans with a novel form of the ACTN3 gene that predisposes individuals to have a high fast twitch muscle fiber content (Yang et al., 2003). In general, quadrupedal cursors have higher percentages of fast-twitch fibers in hind limb extensor muscles than humans, with cheetahs having the highest known-values (Armstrong et al., 1982; Acosta and Roy, 1987; Williams et al., 1997).
Human endurance capabilities raise two questions. First, when did they evolve? Second, why did they evolve? Accordingly, we first review a few points about the evidence for ER capabilities in the genus Homo and its relationship to walking. We then consider some alternative hypotheses about the sort of conditions that might have led to selection for ER capabilities.
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