National Geographic

Human jaws are surprisingly strong and efficient

human_jaws

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

More on skulls and super-bites:

//

There are 6 Comments. Add Yours.

  1. Nathan Myers
    June 22, 2010

    I’m used to reports of bite force of megalodons, tyrannosaurs, pliosaurs and mosasaurs measured in terms of what it takes to bite through a Yugo or a Hummer. I suppose for primates we need to use milli-Yugos.

  2. Ian Tindale
    June 23, 2010

    But look how weak our orbits around our eye sockets are, though.

    And what’s the point in a diagram that labels the items a) b) c) d) etc. on two rows, then the legend refers to it as “from left to right”. Come on, one or the other!

  3. southlakesmom
    June 23, 2010

    Interesting thoughts…my son just finished orthodontia to correct a bite. I wonder if the consistent misalignment that pays for many orthodontists to vacation in warm places is unique to humans. Do the other species mentioned “suffer” from similar problems? Of course, a misaligned bite will affect strength as well.

  4. jdmimic
    June 23, 2010

    While his data are certainly intriguing, I wouldn’t say that it necessarily creates a fatal flaw in all the earlier hypotheses. Two things to consider here; one, size does matter to an extent. When scaled to size, humans have the strongest bite force comparatively. But at actual sizes, we don’t. Absolute strength is still not as strong in the humans. When one is trying to break a rock, one doesn’t care that a smaller hammer may be proportionately stronger if it doesn’t have the absolute power to do the job.
    The second thing to consider is that, while the jaws may have evolutionarily gotten more efficient, they also appear to have gotten less capable of sustained activity, as they point out in the paper. This would seem to indicate that our dietary habits have shifted towards food that is more nutritious bite for bite, thereby reducing our need for prolonged chewing.
    I do find it interesting that it appears the efficiency developed by distributing the stress farther along the jaw, rather than concentrating it along the joint. The data does not create a “fatal flaw” in the hypotheses, but it does require the arguments to become more nuanced to address the new data.

  5. Steve
    July 12, 2010

    Anyone remember Charla Nash, whose face was bitten off by a pet chimp in Feb. 2009? She lost both eyes and her nose and portions of her jaws, which are now slowly being rebuilt by her surgeons. Is the human jaw even remotely capable of inflicting such damage?

  6. Stephen Wroe
    August 20, 2010

    The simple answer is yes – humans are entirely capable of inflicting truly terrible damage on other people by biting. There are hundreds of well-documented cases in the medical literature – noses, eyes, cheeks torn out – and even instances of human bites breaking through the bone of other humans. If you don’t have time to trawl through the literature then just ask any surgeon woring in emergency at a large metroplitan hospital.

    And to make it worse still – human bites are particularly likely to lead to serious infection.

Add Your Comments

All fields required.

Related Posts