Cutting or crushing? Why mammalian teeth had to make a trade-off
Could you slice a steak and crack a nut with the very same tool? A thin blade can easily penetrate soft material, but it does not withstand heavy loads very well. A crushing tool, by contrast, needs to be broad and robust, but is much less effective at cutting. Yet over the course of their evolution, the vast majority of living mammals have combined these two functions within a single structure: a specialized molar. But how far can a single tooth reconcile two opposing functions?
To find out, our team compared these molars in living and extinct mammals and tested the crushing and cutting performance of 3D-printed replicas. By combining high-resolution scanning, 3D printing and mechanical testing, our new study, recently published in Science, sheds light on the constraints that have shaped mammalian dentition.
A tooth with two functions
Teeth form a crucial interface between an animal and its food. Their shape partly determines how food can be processed before swallowing, as well as the physical forces the teeth themselves are able to withstand.
Because teeth fossilize particularly well, they also provide valuable information about the diet and biomechanics of extinct species. They can therefore help us reconstruct some of the major stages in mammalian evolution, including the emergence of the “tribosphenic” molar.
This tooth has a distinctive morphology that combines a high, cutting region called the trigonid with a lower crushing region called the talonid. When the jaw closes, these regions interact with the upper teeth to shear and crush food within the same chewing movement.
By allowing food to be both sliced and crushed during the same bite, this architecture expanded the range of food resources available to mammals and may therefore have contributed to their ecological diversification. Yet these two functions favour largely contradictory tooth morphologies: efficient cutting requires tall, sharp cusps and well-aligned crests, whereas crushing requires a broad contact surface and a mechanically resistant structure. The question, then, is how this geometric incompatibility has shaped the evolution of mammalian molars.
Carnassial teeth in all their forms
Not all mammalian molars are alike. Over the past 66 million years, carnassial teeth (molars specialized for slicing flesh) have evolved independently in several mammalian groups.
Today, members of the order Carnivora (which includes cats, dogs and bears, among others) are the only living mammals with true carnassial teeth. Despite their name, carnivorans include not only specialized predators, but also omnivorous species and even strict herbivores such as the giant panda. This dietary diversity is accompanied by a remarkable variety of tooth shapes. In cats, the carnassial functions almost entirely as a blade, with a reduced or absent talonid. Dogs, bears and raccoons, by contrast, retain crushing surfaces that are useful for a more varied diet, extending all the way to the bamboo-based diet of the giant panda.
Our team therefore created 3D models of the lower carnassial teeth of 250 living and extinct species. Two major recurring morphologies emerged: tall, two-cusped teeth with a reduced or absent talonid, typical of specialized predators such as cats; and teeth with a well-developed talonid, associated with greater dietary diversity.
Ready, set… bite!
A selection of teeth was reproduced using 3D printing and then tested in two experimental setups using materials designed to mimic the properties of biological tissues.
To assess cutting performance, the models were pressed into layers of gelatin designed to mimic soft tissues such as skin and muscle. We measured both the force required to penetrate the gel and the size of the resulting lacerations according to the morphology of each tooth.
To assess crushing performance, the same tooth shapes were tested against a specially designed 3D-printed material with mechanical properties and resistance similar to those of bone.
Our experiments show that only a small number of tooth morphologies are highly effective at cutting soft tissues, and all of them share the same basic design: two tall cusps connected by sharp crests. This double-bladed configuration concentrates pressure over a very small area, allowing the tooth to penetrate deeply into the gelatin with relatively little force.
In hypercarnivores (animals whose diet consists almost entirely of meat) the carnassial tooth even approaches the theoretical optimum. This suggests that there is very little room for morphological variation: to cut efficiently, cusp height and orientation, together with crest alignment, must be combined with great precision.
By contrast, many different tooth morphologies can crush effectively. A well-developed talonid generally helps the tooth withstand high loads, but it is not the only possible solution. In some blade-like teeth, a small notch between the two cusps helps distribute mechanical stress and reduces the risk of fracture, much like the openings found in a circular saw blade. However, tooth shapes that perform particularly well at both functions remain exceptional: fewer than 1% of the morphologies we studied came close to the best possible compromise between cutting and crushing.
The trade-off is therefore not a balanced opposition between two equally demanding functions. Instead, it pits a highly restrictive function—cutting, for which only a limited number of shapes perform optimally—against a much more flexible function, crushing, which can be achieved through several different architectures. In other words, there are many more ways to build a good crusher than a good slicer.
When specialization closes certain doors
Over millions of years, this imbalance may have profound evolutionary consequences. Developing a larger talonid increases the number of ways in which a tooth can crush food and may allow animals to exploit a broader range of resources. By contrast, the evolution of a blade-like carnassial is associated with a much narrower degree of specialization.
This specialization may therefore produce an “evolutionary ratchet” effect: once the talonid has been strongly reduced and the tooth has become highly specialized for slicing flesh, returning to a more versatile tooth morphology becomes difficult.
Hyaenodonts and oxyaenodonts, close relatives of carnivorans that possessed highly specialized teeth, ultimately became extinct. While our study does not suggest that dental specialization caused their extinction, it does indicate that extreme specialization may restrict access to new food resources.
Evolution does not search for the perfect tool
So, can you slice a steak and crack a nut with the same tool? In mammals, the answer is yes—but rarely with equal efficiency. The tribosphenic molar made both functions possible within a single tooth, while also creating opportunities for specialization towards one function or the other.
This result reminds us that, in evolution, an innovation is never synonymous with perfection. It opens up new possibilities while simultaneously imposing new constraints, and the trade-offs that emerge from this process help shape the extraordinary diversity of life.
A French-language article presenting this work was published in The Conversation: “Trancher ou broyer ? Un compromis qui a orienté l’évolution des dents chez les mammifères.”