Hominoid Hallucal Metatarsal Divergence and Tarsometatarsal Joint Morphology

The objective of this study is to test the hypothesis that variation in hallucal metatarsal divergence and tarsometatarsal joint morphology reflects mobility and stability across hominoids using a comparative morphological approach.

The comparative sample includes 896 3D models of extant hominoid first metatarsals, second metatarsals, intermediate cuneiforms, and medial cuneiforms representing 224 individuals across 10 taxa. The variables include the hallucal metatarsal divergence angle, articular surface areas, curvatures, and joint congruence indices. Linear modeling and information criteria were used to evaluate taxon, locomotor frequency, and ecology and locomotion effects.

Extant hominoids vary in a predictable manner based on their ecological and locomotor differences. Humans and eastern gorillas have morphologies suggestive of a less divergent, less mobile hallux, whereas orangutans and hylobatids fall on the opposite end of the morphological spectrum. The models using a graded ecology and locomotion predictor variable substantially outperform those using binary locomotor predictors based on frequencies of arboreality and terrestriality.

The morphology of the hominoid hallucal tarsometatarsal joint reflects variation in ecology and locomotion. A large-bodied, forest-living ecological niche may select for hallucal grasping features in taxa that spend a large proportion of their locomotor budget on the ground because foraging for preferred foods and avoiding predators relies on vertical climbing, which is a relatively infrequent but potentially selectively advantageous locomotor behavior of African apes.