The ontogeny of knuckle-walking and dorsal metacarpal ridge prominence in chimpanzees

Objectives This study examines the sensitivity of the development of the dorsal metacarpal ridge (DMR) to the frequency of knuckle-walking during ontogeny in chimpanzees, and compares the prevalence and prominence of the DMR in hominoids and baboons. Materials and Methods We characterized the type and frequency of quadrupedalism of wild chimpanzees of different ages from the Ngogo community, Uganda. Using museum specimens, we quantified third metacarpal DMR angle and height in Pan individuals of different developmental stages, and in adult Gorilla, Pongo, Hylobates, and Papio. Results When terrestrial, all quadrupedalism consisted of knuckle-walking, although older individuals knuckle-walked more than younger. The total amount of quadrupedalism engaged in by different age groups while arboreal was consistent; however, knuckle usage while arboreally quadrupedal increased with age. The DMR ridge did not emerge in Pan until older infancy, after the onset of knuckle-walking, and reached prominence by the juvenile stage. The DMR angle increased significantly after juvenility but relative height did not. Adult Pan and Gorilla have more prevalent and prominent DMRs than Papio; the DMR was rare in Pongo and absent in Hylobates. Discussion The timing of development of the DMR in chimpanzees supports it as a knuckle walking character, as does its occurrence in gorillas, and virtual absence in suspensory orangutans and gibbons. The DMR in baboons differs in appearance and frequency from DMRs in African apes, and so differentiates knuckle-walking from digitigrady in catarrhines. The relationship between the DMR and loading of a vertical hand while terrestrial deserves further study.