The remarkable diversity of mammalian carnivores may be built on a surprisingly limited set of dental solutions - teeth that are good at slicing meat cannot also excel at crushing hard foods, and vice versa. According to a new study, this inherent tradeoff between cutting and crushing performance has repeatedly steered the evolution of carnassial teeth toward two recurring forms, helping explain how predators evolved to exploit different diets. Teeth are among the most informative features in vertebrate evolution because they directly reflect how animals obtain and process their food. This, combined with their durability in the fossil record, makes them an indispensable tool to understand the ecology and evolutionary history of mammals. A defining innovation in mammals is heterodont dentition, or the presence of specialized tooth types that perform different tasks. One of these specializations is the tribosphenic molar, a tooth whose structure enables both cutting and crushing in a single bite – an adaptation that allows mammals to eat a wide range of foods. This innovation, however, came with a fundamental trade-off: teeth specialized for cutting sacrifice crushing efficiency, while those adapted for grinding reduce cutting performance. How mammals have balanced these competing functions to shape ecological adaptability and evolutionary diversification remains unknown.

To address this question, Narimane Chatar and colleagues analyzed the three-dimensional shape of the lower carnassial tooth in 250 living and extinct carnivorous mammals, revealing two recurring evolutionary designs. One is a highly specialized, blade-like tooth with a reduced grinding surface, characteristic of obligate meat-eaters such as cats. The other has a larger grinding region that supports a broader, more omnivorous diet, as seen in dogs and bears. These patterns suggest that relatively small developmental changes can produce substantial functional differences while remaining constrained by underlying genetic mechanisms. Experimental tests with 3D-printed teeth showed a clear tradeoff between slicing and crushing performance. Fewer than 1% of species approached optimal performance in both functions. In the vast majority of species, teeth optimized for slicing flesh were poor at crushing hard materials, whereas teeth adapted for crushing sacrificed slicing efficiency. For example, the blade-like carnassials of hypercarnivores closely matched the theoretically optimal shape for cutting, indicating that natural selection has repeatedly favored an efficient design despite diverse evolutionary histories.