Our teeth are a mystery. The set we grow during late childhood stays with us throughout our lives, biting and chewing thousands of times a day. They can withstand forces of up to 1,000 newtons and yet, the material that coats them – enamel – is little tougher than glass. How does this extraordinarily brittle substance not shatter into pieces every time we crunch a nut or chomp on an apple?
Herzl Chai from Tel Aviv University found the answer, and it’s a surprising one. At a microscopic level, our teeth defend against fractures by developing with cracks already built in. These pre-made defects are known as “tufts” because of their wavy appearance. They are scattered throughout the enamel and share any physical burdens placed on a tooth, so that no one part has to take the full brunt.
By pressing down on individual teeth using a metal rod, Chai found that it’s relatively easy to create a crack in a tooth, but much harder to actually make it grow bigger to the point where the tooth properly breaks. The tufts, together with structures that prevent cracks from growing, are responsible – they allow us to chew without catastrophe. Our teeth aren’t built to avoid damage, but they’re incredibly good at containing it.
Humans aren’t alone in this – Chai compared out teeth to those of sea otters, and found the same adaptive features under a microscope. It may seem like an odd pairing, but we share a fondness for hard-shelled foods with sea otters – we like nuts and seeds, while they can’t get enough of shellfish. These similarities are reflected in our teeth.