Biomechanics, muscle modeling, and the elevated bite force and tooth stress of piranhas.

The bite forces of vertebrate jaws and the application of those forces on a pointed or bladed tooth edge are crucial biomechanical parameters for any feeding event. Piranhas (fish family Serrasalmidae) exhibit powerful bites facilitated by large, multipennate, jaw muscles and high-efficiency lever mechanics, often accompanied by razor sharp teeth. Small animals like piranhas are often capable of impressive performance when viewed in a size-specific framework, yet the bite force capacity and tooth stress properties of piranhas have not often been measured or modeled. Here, we used morphometrics, computational biomechanical modeling, LiDAR teeth scanning technology, and comparative methods to explore the morphological underpinnings of the feeding mechanics in 11 species of piranhas representing disparate feeding guilds. Results show that piranhas produce highly variable tooth-specific bite forces (from < 1 N to almost 140 N) that yield elevated tooth tip stresses (from ~ 10 to > 440 MPa) for rapid removal of tissue from their prey. Our central conclusions are that some piranhas have mass-specific bite forces that exceed 1200 N/kg which greatly surpass large crocodylian, dinosaurian, and mammalian bite capacities, and that the greatest size-standardized bite forces and tooth stresses are found in an ectoparasitic species and omnivorous piranhas. Given the early-branching phylogenetic position of some of these omnivorous species, we conclude that diversification of the piranha feeding mechanism arose early in the evolutionary history of the clade.