The relation between hand morphology and quadrupedalism in primates

Area of Science:

• Primate Anatomy and Biomechanics
• Evolutionary Morphology
• Locomotor Adaptations

Background:

• Primate hands are broadly classified as ectaxonic (longer fourth ray, common in strepsirhines) or mesaxonic (longer third ray, common in haplorhines).
• A biomechanical model predicted distinct hand positions (mesaxonic neutral, ectaxonic ulnarly deviated) during quadrupedalism.

Purpose of the Study:

• To investigate the relationship between primate hand categorization (mesaxony/ectaxony) and actual hand positioning during quadrupedal locomotion.
• To test the predictions of a biomechanical model regarding hand posture in different primate taxa.

Main Methods:

• Analysis of quadrupedal locomotion (arboreal and terrestrial) via videotaping for 27 primate taxa.
• Quantification of hand deviation and grip types for 18 species from video recordings.
• Radiographic data were also considered to understand evolutionary mechanisms.

Main Results:

• Primates with mesaxonic hands utilized deviated positions and grips, particularly on small arboreal supports.
• Some ectaxonic-handed primates used neutral hand positions on similar supports.
• Hand positioning was more variable than predicted by the mesaxony/ectaxony classification, with some species showing mixed strategies across substrates.
• Radiographic evidence suggests multiple evolutionary pathways for hand ulnar deviation.

Conclusions:

• The mesaxony/ectaxony classification alone does not fully predict hand positioning during primate quadrupedalism.
• Primate hand use during locomotion is more complex and variable than previously modeled.
• Evolution has resulted in diverse mechanisms for achieving hand ulnar deviation in primates.