Why are our lower jawbones fused? It’s all about strength, study finds

Author: Deanna Csomo McCool

Using testing methods more common in engineering research, Professor Matthew Ravosa, in the Department of Biological Sciences, determined that the lower jawbones of certain primates – including humans – are stronger than those of their “cousins” because the bones are fused.

Matthew Ravosa

Ravosa, who is also a concurrent professor in the Department of Aerospace and Mechanical Engineering and the Department of Anthropology, collaborated with Christopher Vinyard, formerly one of Ravosa’s graduate students who is now a professor at Northeast Ohio Medical University, on the article that was published last month in Nature Research Scientific Reports.

Anthropoidea is a suborder of primates that includes humans, apes and monkeys. These mammals have a fused or bony mandibular symphysis, which is the place where the bones are closely joined. Species in the Strepsirrhini suborder – lemurs, for instance – have an unfused mandible. Ravosa and others in the field have long wondered if the complete fusion of the lower jaw joint came about for increased strength or stiffness. Those who advocated for the “stiffness” model have said there isn’t a correlation between fusion and the strength of the joint – and that an unfused mandible is just as strong. Others hypothesized that fusion makes the mandible stronger.

Ravosa and Vinyard looked at two types of forces on mandibles that occur during molar biting and chewing. One is lateral transverse bending, also called wishboning, in which the outer part of the chin is compressed when one side of the jaw is pulled outward while chewing. The other is dorsoventral (DV) shear in which the jaw halves vertically slides by one another in opposite directions.

They obtained different skulls from a wide variety of species from both suborders and used an Instron universal testing machine, which is designed to evaluate the mechanical properties of many types of materials, to break the jaws at the symphysis. They recording the force needed during each test. After extensive testing, it became clear that the fused mandible is stronger.

“This is a perfect example of using engineering approaches to address long-standing biological questions,” Ravosa said.

The question of which type of mandible is stronger has been one Ravosa pondered since his time as a postdoctoral fellow at Duke University Medical Center, and his work at the Duke Lemur Center, which is world leader in the study, care, and conservation of lemurs.

But more importantly, learning about why Anthropoidea, and more specifically humans, have fused lower mandibles points to clues about our past, especially becayse a bony jaw joint is rare among mammals.

“It gives us input into what a fused mandible means functionally, and why we look the way we do,” Ravosa said. “It has implications for fossils of hominids, and also tells how bones behave, and about skeletal biomechanics and design.”