Functional aspects of metatarsal head shape in humans, apes, and Old World monkeys

Abstract

Modern human metatarsal heads are typically described as "dorsally domed," mediolaterally wide, and dorsally flat. Despite the apparent functional importance of these features in forefoot stability during bipedalism, the distinctiveness of this morphology has not been quantitatively evaluated within a broad comparative framework. In order to use these features to reconstruct fossil hominin locomotor behaviors with any confidence, their connection to human bipedalism should be validated through a comparative analysis of other primates with different locomotor behaviors and foot postures, including species with biomechanical demands potentially similar to those of bipedalism (e.g., terrestrial digitigrady). This study explores shape variation in the distal metatarsus among humans and other extant catarrhines using three-dimensional geometric morphometrics (3 DGM). Shape differences among species in metatarsal head morphology are well captured by the first two principal components of Procrustes shape coordinates, and these two components summarize most of the variance related to "dorsal doming" and "dorsal expansion." Multivariate statistical tests reveal significant differences among clades in overall shape, and humans are reliably distinguishable from other species by aspects of shape related to a greater degree of dorsal doming. Within quadrupeds, terrestrial species also trend toward more domed metatarsal heads, but not to the extent seen in humans. Certain aspects of distal metatarsus shape are likely related to habitual dorsiflexion of the metatarsophalangeal joints, but the total morphological pattern seen in humans is distinct. These comparative results indicate that this geometric morphometric approach is useful to characterize the complexity of metatarsal head morphology and will help clarify its relationship with function in fossil primates, including early hominins.

Metacarpal trabecular bone varies with distinct hand‐positions used in hominid locomotion

“…) and a relatively recent study captured their shape variation using a patch of 3D landmarks (Fernández et al. ). A recent study of Mc3 head shape used most of the same landmarks that bordered on this metatarsal patch, at the homologous metacarpal locations (Rein, ).…”

Trabecular variation in the first metacarpal and manipulation in hominids

“…Three random Mc1 specimens from each species were landmarked on their head and base, five times respectively, over several days with Checkpoint (Stratovan Corporation, Davis, CA), following Fernández et al (). The Morpho package in Rv3.3.0 (R Development Core Team, ; Schlager, ) was then used to generate Procrustes coordinates for the five repeats of three individuals per species and articular surface.…”

First metatarsal trabecular bone structure in extant hominoids and Swartkrans hominins

“…The superior aspect of the articular surface of the head expands to the dorsum of the bone, resulting in a raised appearance in relation to its shaft, which is thought to increase the range of dorsiflexion at the MTP joint (Stokes et al, 1979;Susman and Brain, 1988;Susman and de Ruiter, 2004;Griffin and Richmond, 2005;Griffin et al, 2010a). It is also medio-laterally wider on the dorsal aspect of the head than the plantar aspect, which has been argued to enhance joint stability during push-off and facilitate close-packing of the MTP joint (Susman and Brain, 1988;Hetherington et al, 1989;Susman and de Ruiter, 2004;Pontzer et al, 2010, Fernández et al, 2015.…”

“…If these plantar pressure patterns can be broadly applied to non-human apes, they would suggest that the first MTP joint incurs maximum loading when it is used for grasping. This is reflected within the shape of the MT1 head, which in all non-human apes is mediolaterally expanded on the plantar aspect (Susman, 1983;Latimer and Lovejoy, 1990;Marchi, 2005Marchi, , 2010Griffin and Richmond, 2010;Fernández et al, 2015). The same mechanism that allows for close-packing of the MTP joint during dorsiflexion in humans allows for close-packing during plantarflexion in non-human apes, increasing stability during pedal grasping (Susman, 1983;Griffin et al, 2010a).…”