Functional optimality underpins the repeated evolution of the extreme "saber-tooth" morphology

“Sabre teeth” – elongate blade-like canines – are a classic example of convergence, having evolved repeatedly throughout mammalian history. Within canine teeth there is a trade-off between the aspects of shape that improve food fracture and those that increase tooth strength. Optimal morphologies strike a balance between these antagonistic functional criteria. The extreme sabre-tooth morphology is thought to confer functional advantage for more specialised predatory adaptations and optimisation; however, the adaptive basis underpinning their evolution remains unclear. To determine whether sabre-tooth shape reflects selection for functionally optimal morphologies we generated a morphospace of the 3D shape of 70 non-sabre and 25 sabre-tooth species, a subset of which were used to quantify functional metrics of puncture performance and breakage resistance. These data were combined using a Pareto rank-ratio algorithm to evaluate optimality. We demonstrate that extreme sabre-tooth morphologies are functionally optimal, occupying a peak in our optimality landscape. Unlike other optimal canine morphologies, extreme sabre teeth optimise puncture performance at the expense of breakage resistance. This identifies functional optimality as a key driver underpinning the repeated evolution of this iconic tooth.