Sexual Dimorphism In Human Male Faces May be the Result of Natural Selection

I’ve just spent the weekend mulling over the implications of a PNAS article: The feeding biomechanics and dietary ecology of Australopithecus africanus. Now this isn’t the usual sort of article that gets discussed on this blog, but I was intrigued by the following images:  

M. Fascicularis (left) and Australopithecus africanus (right).  

What intrigued me about these images was the connection between the areas of these skulls that are subjected to the highest stresses when biting hard objects (the warmer colors) and the areas of the human male face that are larger than predicted by ontogenetic scaling. What is immediately apparent is that the unusually large features in a human male skull are those that (likely) are subjected to the highest strains when biting hard objects. Additionally, human male – compared to female – skulls project farther forward, implying, perhaps, an adaptation to premolar loading.  

This extrapolation from Strait, et.al.’s analysis led me to speculate about the hypothetical circumstances in which these enlarged skull features could evolve in males only via natural selection. To begin, it seems likely that the adaptation would be for extreme circumstances rather than for a preferred food source. That is, there would have to be some compelling connection between the survival of human males and their ability to eat less desirable, harder to chew food. On the assumption that females gathered and males more often hunted, it seems likely that females would tend to stay in areas where foraging for optimal/desirable food would be successful. Assuming reasonable success with foraging: females would not need extremely powerful jaws. Males, on the other hand, might follow game – wherever the trail led. This would no doubt lead them outside of optimal foraging areas at times. If the hunt failed, or went-on for an extended period, males would be forced to eat what they could find along the way. If unsuccessful hunts occurred often enough, males with more versatile, powerful chewing capacity would perhaps survive when others died, simply by virtue of being able to chew through, for example the tough shells and husks of nuts.  

The implication of this speculation is that the larger facial structures of a human male are not clear signs of a successful hunter and powerful protector, but rather indicators that this one is merely more likely to make it back alive after the next failed hunt. The fact that these dimorphic traits exist suggest that hunting was, perhaps, at some point in our evolutionary past, a dangerous and often unsuccessful activity.

Wayne Hooke

David S. Strait, Gerhard W. Weber, Simon Neubauer, Janine Chalk, Brian G. Richmond, Peter W. Lucas, Mark A. Spencer, Caitlin Schrein, Paul C. Dechow, Callum F. Ross, Ian R. Grosse, Barth W. Wright, Paul Constantino, Bernard A. Wood, Brian Lawn, William L. Hylander, Qian Wang, Craig Byron, Dennis E. Slice, Amanda L. Smith (2009). The feeding biomechanics and dietary ecology of australopithecus africanus Proceedings of the National Academy of Sciences

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