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Z.J. Tsegai, M.M. Skinner, D. H. Pahr, J. Hublin, T.L. Kivell:
"Trabecular bone ontogeny in the forelimb and hindlimb of chimpanzees";
Poster: 8th Annual Meeting of the European Society for the study of Human Evolution (ESHE), Faro, Portugal; 2018-09-13 - 2018-09-15.

Internal bone structure remodels during life and, as such, has the potential to reflect the behaviour of an individual during its lifetime. Extant apes, including humans, undergo locomotor transitions during ontogeny, although the mode and timing of these transitions differs across taxa. In chimpanzees, locomotor transitions are characterised by increasing frequencies of hindlimb loaded locomotor modes (e.g. knuckle-walking) and reduced frequencies of forelimb loading (e.g. suspension) with increasing age [1-3]. As trabecular bone is likely to respond to changing loads during ontogeny [4, 5], its structure across an ontogenetic series may reflect the mechanical loading that characterises these locomotor stages.
We investigated changes in trabecular bone structure during ontogeny in the proximal humerus, proximal femur and distal tibia across a single population of chimpanzees (Pan troglodytes verus) from the Taï National Park, Côte d´Ivoire. Using high-resolution microtomographic scans, trabecular structure was quantified in volumes of interest VOIs) and across the entire metaphysis in a sample of 21 subadult individuals. Spherical VOIs were placed in similar locations across age categories, and bone volume frac-tion (BV/TV), trabecular hickness (Tb.Th), trabecular spacing (Tb.Sp), trabecular number (Tb.N), and degree of anisotropy (DA) were quantified. Ratios of each trabecular variable between the elements were used to interpret the differing patterns in each skeletal region. Morphometric maps were generated to visualise the distribution of BV/TV and the orientation of trabecular bone throughout the metaphysis.
Results from VOIs indicate that trabecular structure beneath the rowth plate differs across ontogenetic stages in chimpanzees. There is an increase in both BV/TV and Tb.Th with increasing age that is more pronounced in the hindlimb elements than that of the humerus. There is little change in Tb.N and Tb.Sp across ontogeny. The pattern of DA differs between elements. In the proximal humerus and distal tibia, DA is initially high but reduces until around five years of age, and then increases again. In contrast, in the proximal femur, DA is relatively similar across younger age groups and increases at adolescence. Morphometric maps of BV/TV distribution show that bone is homogeneously distributed at birth, but with increasing age,trabecular bone becomes more heterogeneous. In all elements, trabeculae are initially proximodistally oriented throughout the metaphysis, with the primary orientations becoming more variable after one or two years of age.
The pattern of trabecular ontogeny in the chimpanzee proximal humerus, proximal femur and distal tibia is characterised by an increased amount of bone and thicker trabecular struts with age, but limited changes in the spacing between trabeculae. Changes in DA within the humerus and tibia may reflect a locomotor transition from more variable, arboreal loading before five years of age, to more stereotypical loading as knuckle-walking becomes more frequent. Relative to the humerus and tibia, the proximal femur has a greater increase in robusticity during ontogeny. This first study of trabecular ontogeny in chimpanzee long bones reveals striking differences with published analyses of humans, including the presence of a high BV/TV at birth and rapid changes in trabecular structure around two years of age in humans [5], neither of which are documented in chimpanzees. Overall, these findings demonstrate that analysis of trabecular ontogeny in extant apes has the potential to reveal developmental changes in the locomotor repertoire and that the trabecular pattern varies between chimpanzees and humans, both of which have important implications for robust reconstruction of behaviour in fossil hominins.

This research was supported by the Max Planck Society (ZJT, TLK, MMS and JJH) and European Research Council Starting Grant #336301 (TLK and MMS). For access to specimens we thank Christophe Boesch and Roman Wittig (Max Planck Institute for Evolutionary Anthropology and Taï Chimpanzee Project).

References:
[1] Doran, D.M., 1992. The ontogeny of chimpanzee and pygmy chimpanzee locomotor behavior: A case study of paedomorphism and its behavioral correlates. Journal of Human Evolution
23, 139-157.
[2] Doran, D.M., 1997. Ontogeny of locomotion in mountain gorillas and chimpanzees. . Journal of Human Evolution 32, 323-44.
[3] Sarringhaus, L.A., MacLatchy, L.M., Mitani, J.C., 2014.
Locomotor and postural development of wild chimpanzees. Journal of Human Evolution 66, 29-38.
[4] Raichlen, D.A., Gordon, A.D., Foster, A.D., Webber, J.T., Sukhdeo, S.M., Scott, R.S., Gosman, J.H.,
Ryan, T.M., 2015. An ontogenetic framework linking locomotion and trabecular bone architecture with applications for reconstructing hominin life history. Journal of Human Evolution 81, 1-12.
[5] Ryan, T.M., Krovitz, G.E., 2006. Trabecular bone ontogeny in the human proximal femur. Journal of Human Evolution 51, 591-602.