title: Adaptation to hard-object feeding in sea otters and hominins.
abstract: The large, bunodont postcanine teeth in living sea otters (Enhydra lutris) have been likened to those of certain fossil hominins, particularly the 'robust' australopiths (genus Paranthropus). We examine this evolutionary convergence by conducting fracture experiments on extracted molar teeth of sea otters and modern humans (Homo sapiens) to determine how load-bearing capacity relates to tooth morphology and enamel material properties. In situ optical microscopy and x-ray imaging during simulated occlusal loading reveal the nature of the fracture patterns. Explicit fracture relations are used to analyze the data and to extrapolate the results from humans to earlier hominins. It is shown that the molar teeth of sea otters have considerably thinner enamel than those of humans, making sea otter molars more susceptible to certain kinds of fractures. At the same time, the base diameter of sea otter first molars is larger, diminishing the fracture susceptibility in a compensatory manner. We also conduct nanoindentation tests to map out elastic modulus and hardness of sea otter and human molars through a section thickness, and microindentation tests to measure toughness. We find that while sea otter enamel is just as stiff elastically as human enamel, it is a little softer and tougher. The role of these material factors in the capacity of dentition to resist fracture and deformation is considered. From such comparisons, we argue that early hominin species like Paranthropus most likely consumed hard food objects with substantially higher biting forces than those exerted by modern humans.
authors.affiliation: Department of Biology, Marshall University, 1 John Marshall Drive, Huntington, WV 25755, United States. paulconstantino@gmail.com
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Constantino
authors.forename: James J-W
authors.initials: JJ
authors.lastname: Lee
authors.forename: Dylan
authors.initials: D
authors.lastname: Morris
authors.forename: Peter W
authors.initials: PW
authors.lastname: Lucas
authors.forename: Adam
authors.initials: A
authors.lastname: Hartstone-Rose
authors.forename: Wah-Keat
authors.initials: WK
authors.lastname: Lee
authors.forename: Nathaniel J
authors.initials: NJ
authors.lastname: Dominy
authors.forename: Andrew
authors.initials: A
authors.lastname: Cunningham
authors.forename: Mark
authors.initials: M
authors.lastname: Wagner
authors.forename: Brian R
authors.initials: BR
authors.lastname: Lawn
mesh_terms: D000220:Adaptation, Biological / Q000502:physiology*; D000818:Animals; D005075:Biological Evolution; D001696:Biomechanical Phenomena / Q000502:physiology; D003743:Dental Enamel / Q000502:physiology; D004032:Diet; D055119:Elastic Modulus / Q000502:physiology; D006244:Hardness / Q000502:physiology; D015186:Hominidae; D006801:Humans; D008334:Mandible; D008963:Molar / Q000033:anatomy & histology / Q000737:chemistry / Q000502:physiology*; D010041:Otters; D012107:Research Design; D014056:Tomography, X-Ray; D016474:Weight-Bearing