Dental Functional Morphology: How Teeth Work

  • Peter W. Lucas
Cambridge University Press: 2004. 372 pp. $130, £75 0521562368 | ISBN: 0-521-56236-8
Grabbing a bite: the main role of teeth is to break down large items of food into smaller ones. Credit: M. PETERSON/CORBIS

Science has made substantial progress since Aristotle wrote (apparently without doing much research) that women have fewer teeth than men. The sheer volume of published research on teeth since may lead some to conclude that we have over-compensated for Aristotle's ignorance. Yet teeth merit all this attention because of their tremendous biological importance — not to mention the dreadful pain they can cause. Dental development and function are the focus of much clinical attention. And for evolutionary biologists, teeth are invaluable sources of information about taxonomy, phylogeny and many other aspects of animal biology.

There are already many excellent texts on dental function and development within the context of craniofacial development and clinical dentistry, as well as several good reviews of dental variation and evolution among vertebrates. But Dental Functional Morphology provides a fresh perspective on dental function. Peter Lucas's basic argument is that because the primary function of teeth is to reduce the size of food particles, dental morphology must be analysed in the context of how teeth fracture food, and how foods resist this. So the book reviews in detail many of the key mechanical properties of food, such as toughness and elasticity, which influence how teeth initially deform food items and generate cracks in them to break down large particles. Lucas then considers how variations in tooth size and shape influence this. He also includes more general reviews of dental and oral anatomy, and provides an excellent summary of the processes of chewing and oral transport, viewed from the perspective of the mechanical properties of food, such as particle size and stickiness.

This food's-eye view leads to numerous insights and interesting ideas, such as Lucas's theory of fracture scaling. Bigger animals have bigger teeth, whose surface areas might be predicted to scale with body mass to the power of 0.67 (because tooth area increases to the power of two, and body mass increases to the power of three, yielding a scaling ratio of 2/3). Yet tooth surface area in mammals typically scales to the power of 0.61. Why this is so remains elusive, but Lucas argues that fracture mechanics plays a role.

The argument is complex, but boils down to the observation that once a crack is initiated in an object, little additional energy is needed to finish the job, regardless of its size. Bigger foods fracture at relatively low stress, which has several implications. One is that bigger animals (assuming that they chew bigger food) need relatively less muscle force (as quantified by muscle cross-sectional area), so this should only increase to the power of 0.5 relative to body mass, although this has yet to be tested. If tooth surface area does not increase relative to bite force, then tooth surface area should also scale to 0.5 relative to body mass. But teeth scale to the power of 0.61, so other factors must also influence tooth size, including other complex aspects of food mechanics also reviewed by Lucas. We can look forward to efforts to test this hypothesis and explore its implications.

Lucas does not consider in detail how dental function relates to tooth development and microstructure, or to the neuromuscular control of chewing. But readers interested in such topics as evolution, diet and ecology will enjoy his many other ideas about how vertebrate teeth work. The final chapter focuses mainly on mammals, creatively integrating biomechanics, anatomy, ecology and taxonomy in order to reconsider the evolution of dental adaptations for generating fractures in food for animals that eat insects, grasses, leaves, fruit and other animals. Primates, especially humans, get special attention. In general, we chew our food like other mammals, but the invention of cooking and other forms of food processing have drastically decreased the particle size and toughness of the food we eat. Palaeoanthropologists are still arguing about when cooking first evolved, but Lucas provides new reasons to suggest that the first species of the genus Homo, which had small teeth, was the first true chef of the animal world. Lucas calculates that your molars can be between 56% and 82% smaller to eat a cooked potato rather than a raw one, depending on whether you eat the skin and whether you roast or boil it.

Teeth often appear messy, confusing and dull to non-specialists, but Lucas succeeds in conveying his enthusiasm for the challenges of learning about the biology and ecology of organisms from such a small and humble organ. Although the book contains plenty of mechanics, the equations are presented clearly and well explained.

Lucas has filled the text with fascinating observations, humorous asides and wonderfully detailed footnotes. One particularly fun bonus is a flick-art animation running on the bottom corner of the book's pages that depicts the evolutionary transformation of a primitive single-cusped tooth into a human molar. Flicking through this cartoon will give your copy a thumb-worn look that Aristotle might have envied.