Phenotypic Plasticity: The Impact of Habitat and Behavior (Substrate Use) on Adult Talo-Crural Appositional Articular Joint Shape Both Between and Within Closely Related Hominoid Species

Key words: phenotypic plasticity, effect of use on Hominoid ankle shape, effect of habitat on behavior. The hominin fossil record is punctuated by variation and rapid change in talo-crural articular joint shape. Prior studies in a diverse Catarrhine sample using Singular Warp analysis has revealed that similar shape is observed across superfamilies due to substrate use, but genera can differ in shape in response to the same behavioral stimulus. Ontogenetic data show these changes arise in the adult. Phenotypic plasticity, the response of the joint to an epigenetic behavioral factor, substrate use, is explored in three comparably sized closely related extant homi-noid species. Matched talo-crural articular surfaces were laser surface scanned, landmarked and analyzed in 114 adult specimens of Pan paniscus, Pan troglodytes (three subspecies) and Homo sapiens (5 global locations). Appositional articular shape was compared to previously described flexible (arboreal), and stable (terrestrial) morphologies, and the effect of habitat evaluated. Pan paniscus had a flexible talo-crural joint shape profile consistent with its highly arboreal substrate use. Differences within the Pan troglodytes and Homo sapiens subsets reflected differing substrate use due to habitat, and, in the case of the latter, technology (footwear). The three Pan troglodytes sub-species clustered with a more stable joint surface profile, with the exception of a series of Pan troglodytes troglodytes from southern Cameroon. A more flexible profile was manifest in this group associated with a human ground presence (Baka) for an extended period. Homo sapiens, a single species with a diverse global habitat, showed a continuum of grades from stable, in heavily shod populations in hard surface habitats, to flexible, in unshod populations in soft surface habitats. Overall, Pan paniscus showed uniformity of talo-crural shape, Pan troglodytes an environmental effect, and Homo sapiens provided insights into the mechanism of rapid shape change, phenotypic plasticity, due to the behavioral factor, substrate use, observed in the fossil record.