Shape variation in the talus and medial cuneiform of chimpanzees and bonobos

Abstract

ObjectivesBonobos and chimpanzees do not differ from one another in overall frequencies of arboreality versus terrestriality as much as once thought. Thus, at a broad level, one would predict that there is little difference in foot morphology among Pan taxa. However, behavioral data suggest that bonobos more often use smaller diameter substrates (<10 cm) when climbing whereas western chimpanzees frequently climb larger diameter (>15 cm) substrates. This study tests the hypothesis that if Pan medial cuneiform and talus morphology reflects these substrate preferences, then the morphology of these bones should favor hallucial grasping in bonobos and an inverted foot set in western chimpanzees.Materials and MethodsThree‐dimensional geometric morphometric (3DGM) methods were used to explore shape variation in 126 talus and 127 medial cuneiform 3D surface models acquired from 108 chimpanzees (24 western, four Nigeria‐Cameroon, 33 central, 32 eastern, and 15 captive unknowns) and 22 bonobosResultsThe shapes of the talus and medial cuneiform in Pan covary as a functional unit emphasizing hallucial grasping with a less inverted foot set in bonobos and a more inverted foot set with a less abducted hallucial set in western chimpanzees. Other chimpanzee subspecies fall between these two extremes.DiscussionBonobo and western chimpanzee medial cuneiform and talus shapes are consistent with their differing preferences for using smaller and larger diameter substrates, respectively, when vertically climbing. These results suggest that even among closely related taxa, foot, hand, and other postcranial anatomy may be fine‐tuned for specific locomotor behaviors or preferences.

Morphological variation of the Pan talus relative to that of Gorilla

“…The two groups separate based on their ecological and locomotor affinities in some morphological traits (i.e., highland G. b. graueri exhibited a more similar talar morphology to G. b. beringei , while lowland G. b. graueri was more similar to G. g. gorilla ), however, such as relative curvature of the talar trochlea (Dunn et al, 2014; Knigge et al, 2015). Similar morphological trends have been recently documented among Pan taxa (Friesen et al, in press). P. troglodytes verus (from the Taï forest; most arboreal Pan taxon; Doran & Hunt, 1994; Doran, 1996; see below for more details) also exhibits more asymmetrical trochlear rims than the other Pan taxa, while P. paniscus (most terrestrial; Ramos, 2014) has comparatively even medial and lateral trochlear rims (Friesen et al, in press).…”

“…Similar morphological trends have been recently documented among Pan taxa (Friesen et al, in press). P. troglodytes verus (from the Taï forest; most arboreal Pan taxon; Doran & Hunt, 1994; Doran, 1996; see below for more details) also exhibits more asymmetrical trochlear rims than the other Pan taxa, while P. paniscus (most terrestrial; Ramos, 2014) has comparatively even medial and lateral trochlear rims (Friesen et al, in press).…”

“…We predict that this will manifest among and within both Pan and Gorilla in the following ways:P1 P. t. verus , the most arboreal of the Pan taxa, will exhibit a significantly different overall talar morphology than the more terrestrial Pan taxa driven by morphologies consistent with greater arboreality, such as more asymmetric trochlear rims which indicate a more inverted foot set for climbing (consistent with findings in gorillas (Dunn et al, 2014; Knigge et al, 2015) and in Pan (Friesen et al, in press)). P2 P. paniscus will exhibit significantly different talar morphology from the P. troglodytes (sub)species, consistent with the results of previous studies investigating morphological variation throughout the skeleton (e.g., Coolidge, 1933; Friesen et al, in press; Hare et al, 2012; Taylor & Groves, 2003; Zihlman et al, 1978; Zihlman & Cramer, 1978). For example, P. paniscus will have more even trochlear rims than the other Pan taxa consistent with findings from Friesen et al (in press). P3 All three gorilla taxa will exhibit significantly different talar morphologies, consistent with previous studies (Dunn et al, 2014; Knigge et al, 2015).…”

“…Aspects of talar variation, with a particular emphasis on articular facet morphology, have recently been analyzed on a smaller taxonomic scale within both Gorilla and Pan (Dunn et al, 2014; Friesen et al, in press; Knigge et al, 2015). Among gorillas, G. g. gorilla , generally considered the most arboreal gorilla taxon (e.g., Doran & McNeilage, 1998; Remis, 1998, Masi, 2004; Dunn et al, 2014; Knigge et al, 2015; see below for more detailed discussion on gorilla locomotor behavior), exhibits a more curved talar trochlea, which has been suggested to be an adaptation for increased joint mobility (Dunn et al, 2014; Knigge et al, 2015).…”

“…Here we build on the previous work of other authors (i.e., Dunn et al, 2014; Friesen et al, in press; Knigge et al, 2015) to quantify whole‐bone talar external shape within both Pan and Gorilla taxa (separately) using a weighted spherical harmonic (SPHARM) analysis and interpret morphological variation in light of degree of arboreality, phylogeny, and body size. We additionally analyze Pan talar variation relative to that of Gorilla within a single analysis to determine if there are consistent morphological trends within the two genera, which may shed light on broader evolutionary trends in the hominid talus.…”