Thermoregulation and endurance running in extinct hominins: Wheeler’s models revisited
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.
Section snippets
A revised Wheeler model of hominin thermoregulation
In order to facilitate comparison with Wheeler’s works and other papers that refer to his ideas, we have endeavored to retain as much of the structure of his model as we can. We have also kept as much of the same nomenclature and use of symbols as possible. Where we deviate from his model, we make the difference clear and we explain our motivation for the change.
Model summary
The sections above fully specify our modification of Wheeler’s model to consider a traveling individual. To evaluate the model for a specific type of hominin we must specify its mass M kg, and leg length L m (morphometric measurements for key hominins of interest are given in Table 1). With substitution of parameter values, the model will predict the net amount of metabolic heat that must be shed across the skin per unit time (in Watts) to avoid core temperature rise. This can be predicted for
Results
We first present the model’s predictions when parameterized for modern humans (Fig. 1): both males and females can maintain heat balance when running, even in the heat of the day, but this is only possible if body fluid levels are sufficient to allow sweating at the maximal rate. Predicted values for most energetically efficient running speeds are 3.0 ms−1 for females and 3.2 ms−1 for males. The estimated increases in metabolism above resting rates are 614 W for males and 455 W for females.
Discussion
Our model predicts that (in terms of thermoregulation) endurance running is only just possible in extremely hot conditions for modern humans. The predicted running speeds for modern humans generated by the model (3.2 ms−1 for males and 3.0 ms−1 for females) fall within the range of human ER speeds quoted by Lieberman et al. (2009): 2.3 ms−1 to 6.5 ms−1. The higher range of these values reflect elite athletes in marathon competitions, where individuals might be expected to run faster than is
Acknowledgments
We thank Dan Lieberman, Hannah O’Regan, Sally Reynolds, and Pete Wheeler, along with the editors and several anonymous referees, for discussion and comments on earlier drafts of this paper.
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