The extent to which the mandibular corpus exhibits developmental plasticity has important implications for interpreting variation in adult and juvenile mandibular morphology in the archaeological and paleontological record. Here, we examine ontogenetic changes in mandibular corpus breadth, rigidity, and strength in two population samples with contrasting diets: late prehistoric Tigara from Point Hope, Alaska, characterized by a very demanding masticatory regime, and proto-historic Arikara from the Sully Site in South Dakota, with a less demanding regime. A total of 52 juvenile and 11 adult Tigara, and 32 juvenile and 10 adult Arikara were included in the study. Juveniles ranged in age from 1 to 17 years, with good representation of younger (1–6-year-old) juveniles (20 Arikara, 18 Tigara). Superoinferior and buccolingual external and cortical bone breadths of mandibles were measured at the Pm4-M1 and M1-M2 junctions using calipers and biplanar radiographs, respectively. An asymmetrical hollow beam model was employed to reconstruct cross sections and calculate bending rigidities and strengths in the sagittal and transverse planes. Among adults, Tigara have greater transverse corpus width, bending rigidity, and strength, and ratios of transverse to sagittal dimensions than Arikara. This shape difference develops gradually during growth, with only weak trends among young juveniles, increasing to near-adult contrasts among adolescents. These results support a role for functional mechanical loading of the mandible during growth in producing adult differences in mandibular corpus morphology. Am J Phys Anthropol 2011. © 2011 Wiley-Liss, Inc.