AQCI
Paleoanthropology Seminar 19th Oct 2000
The thermoregulatory advantages of hominid bipedalism: the contribution of convective heat loss and cutaneous evaporative cooling.
Peter E. Wheeler
Journal of Human Evolution 1991 21 107-115
General Quotation: “The thermoregulatory advantages conferred by bipedalism to a large brained primate on the African savannah could have been significant factors contributing to the adoption of this unusual form of locomotion.”
but “... there is a distinct thermoregulatory advantage in remaining a quadruped and moving beneath the canopy shaded from direct solar radiation.”
Argument: Wheeler makes the astonishing claim that bipedalism arose because of thermoregulatory advantages of convective heat loss, evaporative sweat cooling and physical posturing at noon specifically on the open equatorial grasslands of the African savannah. (The importance of this location is vital to his argument and cannot be fudged.)
His line of argument is as follows: Air moves faster further from the ground and is cooler. Therefore if an animal can place more of its body further from the ground it will stay cooler because of this faster moving air through convective heat loss. His theory is very closely tied up with, and is dependent on, the height of any vegetation present. If there is no vegetation height at all, he finds there is actually little bipedal advantage. His data shows that the thermoregulatory benefit peaks if the vegetation is 100cm tall and then tails off again, dropping back to 0 before 200cm.
He then, separately, considers cutaneous evaporative cooling, claiming that the convective cooling discussed earlier will have reduced the requirement for sweating - suggesting that bipedalism would actually reduce the amount of sweating required to keep cool.
More importantly than both of these factors, he argues, is the fact that an erect posture (versus standing on all fours) would most reduce the amount of direct radiation on the body at noon (but, he concedes, much less so at other times of the day.)
Questions: How could Wheeler, in 1991, publish a paper totally based on the assumption that hominids evolved on the Savannah (5 specific references) when for fifteen years, since Johanson's Lucy paper, it had already been clear that the savannah had nothing to do with the origins of bipedalism? Suggesting now, as some people do, that it was some kind of intermediate, mosaic environment totally invalidates his whole hypothesis. (see second quote above.)
It is very important in his argument that a distinction be made between convection cooling and evaporative sweat cooling. His model assumes that convection cooling reduces the requirement for sweat cooling. Is this a valid assumption? Why did he not perform any experiments on human subjects?
Experiential connection: “Mad dogs and Englishmen go out in the mid-day sun” but chimpanzees shade under trees.
Textual connection: In this paper Wheeler makes no attempt to evaluate the cost in energy and water budgets of bipedalism in a savannah-based hominid. To be fair, at least he does address the issue in a sister paper in the same journal - "The influence of bipedalism on the energy and water budgets of early hominids" pp 117-136 (same vol..)
However even using his figures, that assume convective heat loss does reduce sweat cooling, Wheeler still concludes that these savannah-based hominids would need to find water to drink at least once a day and “not normally be found more than half a day's travel from a river or water hole.” Half a day's travel? How far is that, remembering that they would, presumably, spend most of the twelve hour day looking for food? 10 km? 5 km? 1 km? The closer to water they went the more trees there would be and consequently the less valid his theory of a thermoregulatory cause for bipedalism would become. Isn’t it likely, in fact, that they would have stayed very close to water, inhabiting a gallery forest environment, where quadrupedalism would have actually had “a distinct thermoregulatory advantage.” It would appear that Wheeler has defeated his own argument.
Implications: Wheeler's hypothesis should have been dismissed long ago.
Algis Kuliukas
18th October 2000