Gorilla calcaneal morphological variation and ecological divergence

Abstract

Objectives: The primate foot has been extensively investigated because of its role in weight-bearing; however, the calcaneus has been relatively understudied. Here we examine entire gorilla calcaneal external shape to understand its relationship with locomotor behavior. Materials and methods: Calcanei of Gorilla gorilla gorilla (n = 43), Gorilla beringei graueri (n = 20), and Gorilla beringei beringei (n = 15) were surface or micro-CT scanned. External shape was analyzed through a three-dimensional geometric morphometric sliding semilandmark analysis. Semilandmarks were slid relative to an updated Procrustes average in order to minimize the bending energy of the thin plate spline interpolation function. Shape variation was summarized using principal components analysis of shape coordinates. Procrustes distances between taxa averages were calculated and resampling statistics run to test pairwise differences. Linear measures were collected and regressed against estimated body mass. Results: All three taxa exhibit statistically different morphologies (p < .001 for pairwise comparisons). G. g. gorilla demonstrates an anteroposteriorly elongated calcaneus with a deeper cuboid pivot region and mediolaterally flatter posterior talar facet. G. b. beringei possesses the flattest cuboid and most medially-angled posterior talar facets. G. b. graueri demonstrates intermediate articular facet morphology, a medially-angled tuberosity, and an elongated peroneal trochlea. Discussion: Articular facet differences separate gorillas along a locomotor gradient. G. g. gorilla is adapted for arboreality with greater joint mobility, while G. b. beringei is adapted for more stereotypical loads associated with terrestriality. G. b. graueri's unique posterolateral morphology may be due to a secondary transition to greater arboreality from a more terrestrial ancestor.

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

“…Similar positively allometric scaling of the power arm relative to the load arm in the triceps surae muscle complex has been observed in mammals more generally (Biewener, 1989). Among gorilla taxa, G. gorilla has a relatively anteroposteriorly longer calcaneus for estimated body mass compared with G. beringei, which has been suggested to increase the length of the foot as a whole, providing a greater surface contact area for climbing in these more arboreal gorillas (Harper et al, 2021).…”

“…The increased pivot region depth has been suggested to increase mobility at the calcaneocuboid joint (Gebo, 1992;Lewis, 1980a;Rose, 1988). A recent analysis of external variation among gorilla taxa found this trend to hold true on a smaller taxonomic scale, with Gorilla gorilla gorilla, the most arboreal gorilla species (Doran-Sheehy, Andrainady, & Lodwich, 2009;Masi, 2004) exhibiting the relatively deepest pivot region, while G. beringei beringei, the most terrestrial (Doran, 1996;Doran & McNeilage, 1998) has the relatively shallowest pivot region (Harper, Ruff, & Sylvester, 2021). G. b. graueri, which is intermediate in degree of arboreality (Doran & McNeilage, 1998), is also intermediate in cuboid facet pivot region depth (Harper et al, 2021).…”

“…While previous studies have found some significant relationships between calcaneus morphology and body size broadly across primates, little work has been done to investigate these relationships among and within humans and nonhuman great apes. Studies have found significant differences in calcaneus morphology among these groups that have been suggested to be related to varying locomotor strategies (e.g., Gebo, 1992;Harper et al, 2021;Kidd, O'Higgins, & Oxnard, 1996;Prang, 2015;Prang, 2016a). Postcranial morphologies can be influenced by many factors other than locomotion, however, including body size and phylogeny.…”

Morphological differences in the calcaneus among extant great apes investigated by three-dimensional geometric morphometrics

“…The extracted shape variations along PC1 include the tendency of the medial and lateral pivot regions of the cuboid facet to be more deeply concave and convex, respectively, with decreasing PC1 (in orangutans) but flatter with increasing PC1 (in gorillas). The morphology of the cuboid facet pivot region is linked to greater and lesser midfoot joint mobility, which is considered to reflect an adaptation for arboreal and terrestrial locomotion, respectively 1 , 28 .…”

“…Harcourt-Smith 27 analyzed the patterns of morphological variations of human and great ape calcanei based on 20 landmarks, but only crude comparisons were possible because of the small number of landmarks. Harper et al 28 has recently clarified the calcaneal morphological variations among the three subspecies of gorilla taxa with differences in the degree of arboreality based on geometric morphometrics of approximately a thousand landmarks and demonstrated that the calcaneus is anteroposteriorly more elongated and possesses more concave cuboid and flatter posterior talar articular surfaces. However, only these studies have explored morphological variations among great apes calcanei using geometric morphometrics.…”