Stephen Wroe has built a career out of analysing some of the planet’s most formidable skulls. His group at the University of New South Wales have studied the strength, sturdiness and biting power of the sabre-toothed cat, the great white shark, and the Komodo dragon. Now, he has turned his attention to a predator whose skull is far less impressive but yields surprises all the same – us.
Humans, it is said, have relatively weak jaws that can’t inflict or withstand high bite forces. Some have suggested that we are adapted to eat foods that aren’t very tough, or that our use of tools and cooking has lessened the evolutionary pressure on maintaining sturdy jaws. Some have even suggested that our weedy jaw muscles made way for our large brains and thus facilitated their evolution. But according to Wroe, all of these explanations have a fatal flaw – our jaws aren’t weak at all. They’re actually remarkably efficient for a primate.
The notion of weak human chops was based on very unrefined models that treated our jaws as two-dimensional levers. Of course, in real life, we chew in three glorious dimensions. To really understand how strong our mandibles are, we need to add that third dimension to the models.
That’s exactly what Wroe did. He used his signature technique, called finite element analysis, to create a virtual model of a human skull (belonging to a San hunter-gatherer). The technique is commonly used by engineers to test the properties of machines and vehicles, but Wroe uses it to put animal skulls through a ‘digital crash-test’.
For good measure, Wroe also digitised the skulls of six other primates – the gorilla, chimpanzee, orang-utan and white-handed gibbon, and two extinct species, Australopithecus africanus and Paranthropus boisei. All of the skulls came from adult females. The images below show an example of these virtual models, displaying the forces that act upon the skulls as they chomp down on the second molar. The blue regions are those under the least amount of stress, while the red, pink and white regions are enduring the highest stresses.
The results revealed that human skulls, far from being weak, are quite tough and unusually efficient for their size. Our second molars can exert a bite force between 1,100 and 1,300 Newtons, beating the orang-utan, gibbon and Australopithecus but lagging behind the gorilla, chimp and Paranthropus. These forces are roughly what you’d expect for a primate of our size. We’re never going to bite with the sheer power of a Megalodon, or the predators that Wroe usually studies, but we’re no slouches when compared to closely related species.
And if you scale all the skulls to the same size, we suddenly become the leader of the pack. If all the jaw muscles clenched with the same force, our teeth would exert a bite force that’s at least 40% greater than any of the other primates, save the gibbon. So not only is our bite very respectable, our jaw muscles need to exert considerably less force from to produce it.
This explains some peculiar characteristics of our skulls. Our teeth are as tough as those of other primates because they still need to withstand the relatively high forces exerted by our bite. But the rest of our skull can afford to be comparatively flimsier. The jaw muscles attach to the skull and inflict stress upon it when they work. But our jaw muscles can produce a strong bite through less effort than those of other primates. As such, they inflict fewer stresses upon the skull, which can afford to abandon some of its sturdiness.
Reference: Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2010.0509
