Thermoregulation and endurance running in extinct hominins: Wheeler’s models revisited

Abstract

Thermoregulation is often cited as a potentially important influence on the evolution of hominins, thanks to a highly influential series of papers in the Journal of Human Evolution in the 1980s and 1990s by Peter Wheeler. These papers developed quantitative modeling of heat balance between different potential hominins and their environment. Here, we return to these models, update them in line with new developments and measurements in animal thermal biology, and modify them to represent a running hominin rather than the stationary form considered previously. In particular, we use our modified Wheeler model to investigate thermoregulatory aspects of the evolution of endurance running ability. Our model suggests that for endurance running to be possible, a hominin would need locomotive efficiency, sweating rates, and areas of hairless skin similar to modern humans. We argue that these restrictions suggest that endurance running may have been possible (from a thermoregulatory viewpoint) for Homo erectus, but is unlikely for any earlier hominins.

Introduction

In an influential paper, Bramble and Lieberman (2004) argued that modern humans are conspicuously unusual in comparison to other mammals in their ability to run continuously for long periods of time. They further argued that the evolution of this unusual ability requires explanation, because it seems unlikely to be a byproduct of selection for walking long distances. It has been suggested that such endurance running may have played an important role in hominin evolution (Carrier, 1984, Bramble and Lieberman, 2004, Lieberman et al., 2009). Specifically, if the ancestors of humans had similar abilities, then these could have been used for hunting, giving them access to otherwise difficult-to-catch but highly-rewarding large mammalian prey. Such persistence hunting would have involved chasing prey continually, and not allowing it time to rest until the prey was overcome by hyperthermia (Lieberman et al., 2009).

The viewpoint that endurance hunting was important in hominin evolution remains contentious (see Pickering and Bunn, 2007, Lieberman et al., 2007, Liebenberg, 2008). Previous works (e.g., Bramble and Lieberman, 2004) considered the structural basis and fossil evidence for endurance running in extinct hominins. However, one of the keys to endurance running in modern humans is our ability to shed heat. Specifically, humans are considerably derived in terms of our number and function of eccrine sweat glands and near-bare skin, and these adaptations play a vital role in heat dissipation. Since running is a highly energy-expensive activity and persistence hunting is likely to be most effective in hot conditions and in open habitats with little available shade (Liebenberg, 2006, Lieberman et al., 2009), heat balance is a central aspect of the evolutionary importance of endurance running in hominins. Further, persistence hunting is predicted only to be effective against otherwise healthy prey if the prey is driven to move at high speeds. At moderate speeds, many mammalian quadrupeds (e.g., dogs, horses, and deer) can trot for long distances at speeds that exceed human walking. Only if the human pursuers run such that the prey must often break into a gallop, will the chase ultimately be successful. This occurs because of both the high rate of heat generated during galloping and the inability to lose heat by panting whilst galloping (Lieberman et al., 2009). Thus, the key to success in persistence hunting is to achieve hyperthermia in the prey (rather than simple physical exhaustion) by chasing sufficiently quickly enough that the prey must regularly move faster than a trot. This will often require sustained running from the hominin pursuers (but see Pickering and Bunn [2007] for an alternative viewpoint), and thus very effective dissipation of the considerable heat produced.

Here, we use a mathematical modeling approach to explore heat balance in putative persistence hunting hominins. Specifically, we modify the most widely-cited model of thermoregulation in hominins (Wheeler, 1984, Wheeler, 1985, Wheeler, 1991a, Wheeler, 1991b, Wheeler, 1992a, Wheeler, 1992b, Wheeler, 1993, Wheeler, 1996). Wheeler’s model dealt with a hominin standing still, so we have developed a model to allow for both the heat generated by a running individual and the increase in convective heat loss due to an individual moving quickly through the surrounding air. We also modify the model in a number of small ways to take into account improved estimates of some parameter values as a result of research done since the original models were published. Our aim is to explore whether or not endurance running would have been thermally possible for extinct hominins: that is whether or not extinct hominins could have run in the heat of the day for sustained periods of time without overheating. We also seek to identify what thermal adaptations would have been required to make such activity possible. We stress that such a modeling approach cannot demonstrate that persistence hunting actually happened at any point during human evolution but could potentially rule it out as a possibility on thermoregulatory grounds.