Restructuring of Femoral Cortical Bone During Growth and Locomotor Development of Wild Chimpanzees (Pan troglodytes verus)

Objective: Chimpanzees are altricial in terms of their locomotor development and transition from being carried to engaging in suspensory and arboreal locomotor behaviors to eventually relying on terrestrial quadrupedalism as their main form of locomotion. Here, we consider the mechanical implications of femoral cortical bone restructuring during growth and locomotor development in wild chimpanzees.

Materials and methods: Cortical bone structure was examined in an ontogenetic sample of wild chimpanzees from a single subspecies (P. t. verus) spanning in age from 2 weeks to 12.6 years. Diaphyseal cross-sections were extracted from micro-CT scans of the femur at 35%, 50%, and 65% of total intermetaphyseal length and variation in cortical bone structure was assessed based on bending rigidity (Imax/Imin, Ix/Iy), relative medullary area, and cortical bone porosity.

Results: Diaphyseal shape is relatively circular with a high amount of cortical bone porosity and a large relative medullary area during early infancy. Distinct shifts in cortical bone structure occurred for each studied parameter with the biggest changes occurring within the first 5 years. Values appear to stabilize as quadrupedal walking increases in frequency and is established as the main form of locomotion.

Discussion: Collectively, the results suggest a degree of integration in which cortical bone restructures in response to rapid changes in locomotion in addition to nonmechanical influences such as hormonal, and growth factors, without compromising function and structural integrity. The extent of influence of each factor varies throughout growth and highlights the need for caution in functional interpretations of cortical bone geometry.