Scaling and relative size of the human, nonhuman ape, and baboon calcaneus

Abstract

Among human and nonhuman apes, calcaneal morphology exhibits significant variation that has been related to locomotor behavior. Due to its role in weight-bearing, however, both body size and locomotion may impact calcaneal morphology. Determining how calcaneal morphologies vary as a function of body size is thus vital to understanding calcaneal functional adaptation. Here, we study calcaneus allometry and relative size in humans (n = 120) and nonhuman primates (n = 278), analyzing these relationships in light of known locomotor behaviors. Twelve linear measures and three articular facet surface areas were collected on calcaneus surface models. Body mass was estimated using femoral head superoinferior breadth. Relationships between calcaneal dimensions and estimated body mass were analyzed across the sample using phylogenetic least squares regression analyses (PGLS). Differences between humans and pooled nonhuman primates were tested using RMA ANCOVAs.Among (and within) genera residual differences from both PGLS regressions and isometry were analyzed using ANOVAs with post hoc multiple comparison tests. The relationships between all but two calcaneus dimensions and estimated body mass exhibit phylogenetic signal at the smallest taxonomic scale. This signal disappears when reanalyzed at the genus level. Calcaneal morphology varies relative to both body size and locomotor behavior. Humans have larger calcanei for estimated body mass relative to nonhuman primates as a potential adaptation for bipedalism. More terrestrial taxa exhibit longer calcaneal tubers for body mass, increasing the triceps surae lever arm. Among nonhuman great apes, more arboreal taxa have larger cuboid facet surface areas for body mass, increasing calcaneocuboid mobility.

Changes in human calcaneal morphology throughout the Pleistocene-Holocene Levant

“…Nevertheless, sexual dimorphism of the calcaneus size increased with the transition to a sedentary lifestyle. This might be related to the positive association between body mass and calcaneus size (Harper et al, 2022a). The overlap in variance between males and females in calcaneal indices with only a few significant differences (LAW/MaxL, LAL/MaxL, and CFB/CFH) corresponded with previous findings demonstrating no significant differences in calcaneal shape between the sexes (Harper, 2023).…”

“…Furthermore, the human calcaneus had to adapt to higher ground reaction force applied to it at heel strike (Latimer and Lovejoy, 1989). These adaptations included an asymmetric and convex cuboid facet, a broader and shorter calcaneal tuberosity, and a larger lateral plantar process, which was more plantarly positioned compared to apes (Bojsen-Møller, 1979;Latimer and Lovejoy, 1989;Boyle et al, 2018;Harper et al, 2021;Harper et al, 2022a). Furthermore, it has been suggested that human calcaneal morphology of shorter calcaneal tuber (body) improves running economy (Raichlen et al, 2011).…”

A new early hominin calcaneus from Kromdraai (South Africa)

“…as calcaneal tuber breadth scales to estimated body mass with positive allometry in both modern humans and nonhuman primates (Harper et al, 2022). The KW 6302 calcaneal robusticity index is close to what is preserved in A. africanus, another smallbodied hominin (Prang, 2015;Ruff et al, 2018), indicating that the two specimens may have experienced similar loading regimes.…”

“…Gorillas have been recently shown to use a heel‐only heel strike with an impact spike, however, suggesting that KW 6302 may have utilized a modern human‐like heel strike (Zeininger et al, 2019; Wunderlich et al, 2019). The relatively narrow tuber may also be the product of KW 6302′s small body size (reconstructed to be 33.3–35.9 kg) as calcaneal tuber breadth scales to estimated body mass with positive allometry in both modern humans and nonhuman primates (Harper et al, 2022). The KW 6302 calcaneal robusticity index is close to what is preserved in A. africanus, another small‐bodied hominin (Prang, 2015; Ruff et al, 2018) , indicating that the two specimens may have experienced similar loading regimes.…”

“…Linear measures are compared to those previously collected on all known Plio‐Pleistocene hominin calcaneus fossils (DeSilva et al, 2018; Latimer et al, 1982). Total calcaneal length was estimated based on the relationship between calcaneal tuber length and total calcaneal length in modern humans, chimpanzees/bonobos, and gorillas using least‐squares regression analyses (extant relationship in Figure S1; linear measures from Harper et al, 2022).…”