Pedal and Manual Proximal Phalangeal Curvatures Among Pongo abelii and Pongo pygmaeus

Phalangeal curvature in hominoids correlates with locomotor behavior, with greater curvature associated with arboreality. Prior research using 2D geometric morphometrics (2DGM) demonstrated this relationship in the third manual proximal phalanges (mPP3) of orangutans. Specifically, the more terrestrial Bornean orangutans (Pongo pygmaeus) have less curved mPP3s than the less terrestrial Sumatran orangutans (Pongo abelii). This study extends the analysis to the third pedal proximal phalanges (pPP3) to assess if a similar pattern exists and further evaluate the link between curvature and locomotor diversity.

We analyzed pPP3 curvature in P. abelii (n = 16) and P. pygmaeus (n = 15) using a 2DGM approach. Comparisons were made with previously published mPP3 data. Generalized Procrustes analyses and principal components analysis were performed on 100 landmarks placed along the diaphysis's central longitudinal axis to quantify shape variation. ANOVA and post hoc tests determined significance between species and autopods.

PC1 accounted for 73.4% of shape variance, primarily reflecting dorsoplantar curvature. P. abelii exhibited significantly greater curvature, aligning with its more arboreal lifestyle. Additionally, curvature in pPP3s was greater than in mPP3s within species.

Results support the hypothesis that phalangeal curvature reflects habitual substrate use at lower taxonomic resolutions. The greater curvature in P. abelii corresponds to its predominantly arboreal locomotion, while the lesser curvature in P. pygmaeus aligns with increased terrestriality. Additionally, greater curvature in pedal versus manual phalanges may suggest differential adaptations related to substrate engagement. These findings enhance understanding of extant primate autopod functional morphology and can inform reconstructions of extinct species' positional behaviors.