Classification: Thermoregulation
Mnemonic: Sweat cooling
Specific Model: Thermorgulatory Hypothesis
Original Proponent(s): Wheeler 1984, 1991    
Basic Summary: Keeping cool in hot equatorial climates drove bipedalism by reducing the surface area of body exposed to UV radiation at noon and by placing the upper body further away from the ground into cool breezes.    
Assessment: 5th out of 9 broad categories of bipedalism models but equal 1st of individual models cited. 47% of texts surveyed referred to this model explicitly.
Simple: #35 / 42 (46%)
Detail: #36 /42 (45%)
Discussion: “The Evolution of Bipedality and Loss of Functional Body Hair in Hominoids” Wheeler 1984
Wheeler's first paper demonstrated that an upright posture provides distinct thermoregulatory advantages for a biped over a quadruped in open, equatorial habitats. Specifically, he argued, they would be subject to significantly less direct solar radiation whilst standing in an upright posture at around noon (where the amount of body surface exposed to the sun may be as low as 7%) and also that their upper bodies, being placed higher up away from the ground, would be subject to stronger convective air currents helping them to keep cool though evaporative sweat cooling. Most of his conclusions were drawn from measurements of body profiles in the frontal and vertical planes and estimating similar body profiles in Australopithecines. Most savannah-adapted mammals, Wheeler reminds us, tend to have an advanced set of features, including the ‘carotid rete’, to prevent their brain from overheating. As humans lack these features, some other mechanism for cooling the brain would be required if our ancestors had also evolved there. Wheeler’s hypothesis is that the upright posture offered by bipedalism may have been a significant part of another such mechanism. Using two small scale models of a putative hominid ancestors, one positioned in a quadrupedal posture and the other bipedal, Wheeler photographed the models from a range of angles corresponding to position of the sun. His results showed that the bipedal posture significantly reduced the amount of body surface exposed to the sun’s rays, particularly around noon.

“The thermoregulatory advantages of hominid bipedalism in open equatorial environments: the contribution of increased convective heat loss and cutaneous evaporative cooling” Wheeler 1991.
The arguments were developed mathematically in a later paper where Wheeler also tested his hypotheses using models placed in various grassy situations. He showed (Wheeler 1991:111) that a hominid which stood 1.25m tall when bipedal would gain an advantage, in terms of convective cooling, over the quadrupedal form, when standing in vegetation up to 1m tall. The same findings showed, however, that if the vegetation was taller than 1.25m then there would be no advantage at all. Basically, Wheeler suggested that bipedalism therefore conferred distinct thermoregulatory advantages over quadrupedalism for hominids living in open, grassy environments typical in the African savannah. This hypothesis, however, takes little account of the fact that the paleohabitats of the earliest bipeds appeared, even then in 1984, to have been predominantly wooded and not open grasslands. Since his paper was published almost all the accumulating fossil evidence of the early bipeds has reinforced this view (see, for example, WoldeGabriel et al 2001).
Wheeler’s thermoregulatory hypothesis for bipedal origins, based on movement across patches of open grassland between clumps of forest, is simply not workable because these habitats would offer shade from the sun and shield the hominids from breezes – both assumed in his model. Another criticism of the model is the implausibility of leaving the shade in the hottest part of the day in equatorial zones. To postulate that hominids would have done so in order to gain an advantage in foraging time against competitors, seems fanciful and generates more questions than it answers. For example: What benefit does an upright posture give a hominid, in terms of thermoregulation, when the sun is lower in the sky? And: If thermoregulation was achieved by sweat cooling, from where did these hominids replenish the water that would have been lost in this way? Assuming, as this model seems to, that early hominids always lived within close proximity to permanent fresh water supplies appears to defeat the premise on which the model is based: water-side vegetation, even grasses, are usually much taller than 1.25m.
 Strengths: Wheeler's model attempts to explain the reduction of functional body hair and adoption of sweat cooling mechanisms in addition to bipedalism and, assuming a switch to a more open habitat, at least provides a plausible means for human divergence from the other apes.    
Weaknesses: The model has a number of weaknesses. Firstly, positing early hominins moving around in the mid day sun in open savannah habitats is not a plausible way of increasing survival value. Secondly it has no analogues in extant mammals let alone primates. Perhaps most significantly, it is at odds with the growing consensus about a wooded, and not savannah-based, paleoecology for early hominins.    
1.1 Survival Value 3 (Poor) If hominids foraged around noon in equatorial habitats in waist high grasses, Wheeler’s model could work. At such times potential food sources might be left alone by other predators and scavengers alike, preferring to keep cool in the shade. However, the selective benefit of having this two-three hour window of opportunity open to them would appear to be limited at best and, perhaps, be more than negated by an increased risk of overheating    
1.2 Sexual Selection 5 (Fair) This model was judged neutral for this criterion.    
1.3 Not Teleological 4 (Fair) It is an undoubted consequence of our bipedalism that we have a reduced profile whilst standing in the mid day sun and that placing the upper body higher will subject it to cooling breezes. It is less clear that such factors could have caused the evolution of bipedalism in the first place. In it's favour one could argue that at least such benefits as Wheeler suggests would have been a factor all the way along at intermediate stages in the evolution too.    
2.1 Improved Food Acquisition 6 (Fair) The model was judges slightly better than neutral on the basis that mid day foraging could improve food acquisition.    
2.2 Accounts for Predation 3 (Poor) Diurnal foraging is less risky, in terms of predation, than foraging at night because most large predators do their hunting after dark. By the same logic it may be argued that the best time to avoid predators would be precisely when they are most likely to be sedentary themselves: during the hottest part of the day. However, this kind of behaviour would clearly encounter risks too. Any hominid straying too near a group of sleeping predators might still risk attack, no matter what time of day it was.    
2.3 Why Apes are not Bipedal 7 (Good) Assuming, Wheeler’s premise is correct that the hominid lineage lived in more arid and open areas than the ancestors of Pan/Gorilla. This model would indeed explain human-ape divergence. However much of the evidence emerging since Wheeler’s papers indicate that early hominid environments were relatively wet and wooded and not arid and open.    
2.4 Extant Analogues 1 (Poor) It is very rare for mammals to be observed foraging around noon, let alone any ape.    
2.5 Applies to Both Sexes 9 (Good) The model works equally well for both sexes.    
3.1 Hominid Anomalies 3 (Poor) Wheeler does not attempt to explain the anatomical anomalies seen in australopithecines.    
3.2 Fits Paleoecological Record 1 (Poor) Wheeler’s model appears to be contradicted by growing evidence that the habitats of early hominids was relatively wet and wooded.    
3.3 Precursor to Strider and knuckle Walker 5 (Fair) The assumption in this model is that the precursive form of locomotion was essentially quadrupedal, with little overlap to human-like bipedalism however, as intermediate forms are also favoured by the model the model was rated fair here.    
4.1 Extended Explanatory Power 8 (Good) Wheeler’s model, unusually, also explains the loss of functional body hair in humans and our efficient sweat cooling mechanisms.    
4.2 Complimentary 5 (Fair) The thermoregulatory hypothesis is generally complimentary to those proposing open habitats and contradictory to those based on wooded habitats.    
4.3 Falsifiable or Testable 5 (Fair) Wheeler’s approach was scientific and evidence-based but doesn't mae any falsifiable predictions.    
References Wheeler, P E (1984). The Evolution of Bipedality and Loss of Functional Body Hair in Hominoids. Journal of Human Evolution Vol:13 Pages:91-98.
Wheeler, P E (1991). The thermoregulatory advantages of hominid bipedalism in open equatorial environments: the contribution of increased convective heat loss and cutaneous evapourative cooling.. Journal of Human Evolution Vol:21 Pages:107-115