Cortical bone distribution of the proximal phalanges in great apes: implications for reconstructing manual behaviours

Abstract

Primate fingers are typically in direct contact with the environment during both locomotion and manipulation, and aspects of external phalangeal morphology are known to reflect differences in hand use. Since bone is a living tissue that can adapt in response to loading through life, the internal bone architecture of the manual phalanges should also reflect differences in manual behaviours. Here, we use the R package Morphomap to analyse high‐resolution microCT scans of hominid proximal phalanges of digits 2–5 to determine whether cortical bone structure reflects variation in manual behaviours between bipedal (Homo), knuckle‐walking (Gorilla, Pan) and suspensory (Pongo) taxa. We test the hypothesis that relative cortical bone distribution patterns and cross‐sectional geometric properties will differ both among extant great apes and across the four digits due to locomotor and postural differences. Results indicate that cortical bone structure reflects the varied hand postures employed by each taxon. The phalangeal cortices of Pongo are significantly thinner and have weaker cross‐sectional properties relative to the African apes, yet thick cortical bone under their flexor sheath ridges corresponds with predicted loading during flexed finger grips. Knuckle‐walking African apes have even thicker cortical bone under the flexor sheath ridges, as well as in the region proximal to the trochlea, but Pan also has thicker diaphyseal cortices than Gorilla. Humans display a distinct pattern of distodorsal thickening, as well as relatively thin cortices, which may reflect the lack of phalangeal curvature combined with frequent use of flexed fingered hand grips during manipulation. Within each taxon, digits 2–5 have a similar cortical distribution in Pongo, Gorilla and, unexpectedly, Homo, which suggest similar loading of all fingers during habitual locomotion or hand use. In Pan, however, cortical thickness differs between the fingers, potentially reflecting differential loading during knuckle‐walking. Inter‐ and intra‐generic variation in phalangeal cortical bone structure reflects differences in manual behaviours, offering a comparative framework for reconstructing hand use in fossil hominins.

Cortical bone architecture of hominid intermediate phalanges reveals functional signals of locomotion and manipulation

“…We expect to observe similar relative patterns and interspecific differences in cortical morphology of the intermediate phalanges that we did in the proximal phalanges (PPs) (Syeda et al, 2023). Specifically, we expect Pongo and H. sapiens to show similar patterns between their respective IPs and PPs, while African apes will show greater differences between their phalangeal elements due to direct loading of the IPs during knuckle-walking.…”

“…We previously explored cortical bone distribution patterns and properties in the proximal phalanges of digits 2-5 (PP2-PP5) in extant great apes and showed that the pattern of cortical bone within the non-pollical proximal phalanges is capable of distinguishing varied hand postures employed by each taxon and corresponds with predicted loading during these hand postures (Syeda et al, 2023). Results also indicated that cortical bone patterns and properties reflect the variable digital loading within the hand of each taxon (Syeda et al, 2023). Here, we build upon this research and provide the first detailed, comparative study of the cortical morphology of extant hominid intermediate phalanges in digits 2-5.…”

“…The majority (74%) of our human sample did not have all four associated digits and therefore we assigned phalanges to a digit using morphological characteristics described in Susman (1979) and Case and Heilman (2006). For individuals in our sample that had associated PPs (see Syeda et al, 2023), we compared cortical distribution and properties with the IPs. voltages of 100-160 kV and 100-140 μA using a 0.2-0.5 mm copper or brass filter.…”

“…Functional inferences regarding manual behaviors of these fossil specimens have been made using elements of the carpus (Kivell et al, 2013;Marzke et al, 2010;Tocheri et al, 2007), the metacarpus Galletta et al, 2019;Skinner et al, 2015) and the phalanges (Almécija et al, 2010;Kivell et al, 2015Kivell et al, , 2018Kivell et al, , 2022Syeda et al, 2022), but the intermediate phalanges are relatively understudied. Here we build on our previous work (Syeda et al, 2023) and investigate variation in cortical bone structure of the intermediate phalanges of digits 2-5 (IP2-IP5) within the context of inferred hand use in humans and other extant hominids (Pongo, Gorilla, and Pan). We also conduct intra-digit comparisons of both the proximal and intermediate phalanges and discuss how the combined cortical structure of these two elements may reflect function of the fingers during manual behaviors.…”

“…Phalangeal external morphology, as well as the internal bone structure, have been shown to be functionally informative (Jungers et al, 1997;Karakostis et al, 2018;Matarazzo, 2008;Patel & Maiolino, 2016;Susman, 1979;Syeda et al, 2023). The structure of both cortical and trabecular bone can adapt in response to mechanical loading by removing bone in skeletal areas where stress is low and adding bone where stress is high (Barak et al, 2011;Currey, 2013;Pearson & Lieberman, 2004;Ruff et al, 2006) as well as by changing the orientation and alignment of the trabecular struts (Barak et al, 2011;Pontzer et al, 2006).…”