Relative leg-to-arm skeletal strength proportions in orangutans by species and sex

Among extant great apes, orangutans climb most frequently. However, Bornean orangutans (Pongo pygmaeus) exhibit higher frequencies of terrestrial locomotion than do Sumatran orangutans (Pongo abelii). Variation in long bone cross-sectional geometry is known to reflect differential loading of the limbs. Thus, Bornean orangutans should show greater relative leg-to-arm strength than their Sumatran counterparts. Using skeletal specimens from museum collections, we measured two cross-sectional geometric measures of bone strength: the polar section modulus (Zpol) and the ratio of maximum to minimum area moments of inertia (Imax/Imin), at the midshaft of long bones in Bornean (n = 19) and Sumatran adult orangutans (n = 12) using medical CT and peripheral quantitative CT scans, and compared results to published data of other great apes. Relative leg-to-arm strength was quantified using ratios of femur and tibia over humerus, radius, and ulna, respectively. Differences between orangutan species and between sexes in median ratios were assessed using Wilcoxon rank sum tests. The tibia of Bornean orangutans was stronger relative to the humerus and the ulna than in Sumatran orangutans (p = 0.008 and 0.025, respectively), consistent with behavioral studies that indicate higher frequencies of terrestrial locomotion in the former. In three Zpol ratios, adult female orangutans showed greater leg-to-arm bone strength compared to flanged males, which may relate to females using their legs more during arboreal locomotion than in adult flanged males. A greater amount of habitat discontinuity on Borneo compared to Sumatra has been posited as a possible explanation for observed interspecific differences in locomotor behaviors, but recent camera trap studies has called this into question. Alternatively, greater frequencies of terrestriality in Pongo pygmaeus may be due to the absence of tigers on Borneo. The results of this study are consistent with the latter explanation given that habitat continuity was greater a century ago when our study sample was collected.