Trabecular data of the capitate and third metacarpal through ontogeny in chimpanzees and gorillas

Chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla) both knuckle-walk in adulthood but have been argued to develop their locomotor strategies differently in terms of timing and in positioning of the manus. Using dentally-defined age groups of both Pan and Gorilla, this study presents an internal trabecular bone approach to better understand the morphological ontogeny of knuckle-walking in these taxa. Capitate and third metacarpal bones were microCT scanned at 23–40 micron resolution with scaled volumes of interest placed centrally within the head of the capitate and base of the third metacarpal. Trabecular measures associated with activity frequency were expected to decrease through ontogeny, while degree of anisotropy (DA) was expected to increase through ontogeny in Pan alone. Bone surface area to volume ratio (BS/BV) showed expected patterns related to activity level in both bones. DA did not show statistical support for predicted patterns, but this may be due to sample size, as observed changes through ontogeny reflect expected trends in the capitate. Analyses of principal trabecular orientation corroborated known behavioral differences related to variation of hand use in these taxa, but only Pan showed biomechanical patterning associated with suggested wrist posture. Assessment of allometry showed that trabecular bone of larger animals is characterized by fewer and more spaced out trabeculae, supporting a constant trabecular geometry model of changes associated with body size changes through ontogeny. In combination, these findings lend support to behavioral and biomechanical arguments that knuckle-walking has evolved independently in Pan and Gorilla