Neanderthal barrel chests and aerobic capacity

           A reading from my Theory and Methods course this week bears indirectly on the question of running (or at least aerobic activity) in Homo.  I thought I'd share my summary and response, though I should note that I am not well-versed in the paleoanthropology of Neanderthals!

Churchill, S. E. (2006). Bioenergetic perspectives on Neanderthal thermoregulatory and activity budgets. In Neanderthals revisited: New approaches and perspectives (pp. 113-133). Springer Netherlands.

           The large chest volume of Neanderthals has been assumed to be adaptive, and several hypotheses have been proposed in explanation.  Bergmann’s rule predicts that body shape should evolve in cold climates so as to minimize heat loss; perhaps this explains the chest shape of Neanderthals.  Alternatively, the large chest could indicate selection for greater lung volume, which the author claims may have functioned to increase aerobic capacity.  Of course, Neanderthals don’t look like the sort of hominin that would need a big aerobic engine, but the byproduct of high aerobic activity is endogenous heat production.  Thus, Churchill hypothesizes that bigger chests are an adaptation to the cold climate.  (Though he does suggest that the resulting change in body shape, conforming to Bergmann’s rule, reinforced this selection.)

            Churchill’s argument rests on estimates of Neanderthal BMR (basal metabolic rate) and TEE (total energy expenditure).  Previous estimates of these values are based on primates and modern humans, and vary wildly. Cherry-picking these values could support or refute any hypothesized explanation of Neanderthal chest size. Clearly there is a need for more accurate estimations.  Towards this end, Churchill constructed a half-sized replica of (what is considered to be) an average-sized Neanderthal, and from this calculated skin surface area, weight, BMR, and TEE.  The TEE values were huge: upwards of 4,000 kcal/day.  This makes a certain amount of sense, as other calculations indicate that, to stay warm in the cold climate of glacial-period Europe, Neanderthals would indeed have needed tremendous amounts of energy.

            Several problems arise at this point: How did Neanderthals maintain such a high caloric intake? Estimates indicate that they would have needed greater meat intake, and therefore greater hunting success, than modern wolves, which are highly skilled hunting animals.  And through what mechanism is this high-calorie diet being converted into heat?  Heat is the byproduct of cellular metabolism, the breakdown of organic molecules.  Neanderthals may have generated extra heat by increasing what’s called “nonshivering thermogenesis”, where cells, in response to hormone signals, increase the rate of active transport and therefore increase the rate of metabolism.  This has been shown to increase metabolic rate (and therefore heat production) in humans by up to 40%. Churchill’s math indicates that such a mechanism, which does not require increased physical activity, could produce 133-165 Watts of heat, which he estimates would be enough to keep Neanderthals warm in “mild” winter weather.  The rest of the heat must come from another mechanism, the one that produces the most massive amounts of heat: muscular activity.  Here, presumably, is where the big chests come in: increased lung volume means more oxygen available for muscle metabolism.  (It’s not that we use all of our lung volume while exercising; rather, we use about 50%, and going beyond that is uncomfortable.  Increasing total lung volume would increase the usable lung volume, and therefore provide more oxygen.)  Trained athletes (he must mean endurance athletes?) can produce over 1000 Watts of heat for several hours consecutively.  Clearly this heat-generating mechanism, coupled with the requisite high-calorie diet, would keep Neanderthals warm.  And explain the barrel chests.

           Churchill’s argument necessitates a huge amount of work being performed by Neanderthals—nonshivering thermogenesis is not enough, so the extra heat must be generated through muscular activity.  How was this work being performed against the environment?  Such a huge aerobic output is hard to imagine in a species that was morphologically maladapted to covering long distances, at least via endurance running. Walking cannot produce the heat necessitated by this hypothesis, nor would it require increased lung volume, which is only a limiting factor in aerobic output at the upper limits of oxygen consumption (think fast running or sustained lifting, pushing, etc). Given what we know about the body structure of Neanderthals, it's hard to imagine them engaging in such rigorous mobility, the kind that would produce huge amounts of endogenous heat. 

          Moreover, this huge work production would have to be relatively constant in order to provide the benefit conferred by heat production.  And, irregular/large work outputs would lead to sweating, which easily leads to hypothermia in cold environments.

          Given all of this, why should we assume that the barrel-chest is adaptive (either genetically, or physiologically, as in altitude responses) rather than being a byproduct of other adaptations?  Or a product of genetic drift? (Neanderthals as an isolated and increasingly inbred population?)  Isn't this the more parsimonious (though less glossy) explanation?