The relation between hand morphology and quadrupedalism in primates

Primate hands can be classified into two broad categories on the basis of ray proportions and other features. Ectaxonic hands are characterized by a longer fourth ray and are found in most strepsirhines. Most haplorhines possess mesaxonic hands which are characterized by a longer third ray. Preuschoft et al. ([1993] in H. Preuschoft and D.J. Chivers (eds.): Hands of Primates. Berlin: Springer-Verlag, pp. 21-30) proposed a biomechanical model which predicts that, during quadrupedalism, a mesaxonic hand should be held in a more neutral position with respect to the forearm, whereas an ectaxonic hand should be more ulnarly deviated. The relation between hand positioning and the mesaxony/ectaxony categorization is investigated for 27 primate taxa. Videotapes were recorded for each species walking quadrupedally on arboreal supports. Several species were also videotaped during ground quadrupedalism. The degree of deviation of the hand relative to the substrate and the grips utilized were quantified for 18 species from the videotapes. Primates with mesaxonic hands use deviated hand positions and grips, especially when walking quadrupedally on small poles. Several species with ectaxonic hands use neutral hand positions and grips when walking quadrupedally on similar supports. Also, several primates, with either ectaxonic or mesaxonic hands, display a combination of deviated hand positions and grips when on arboreal substrates and neutral hand positioning when on the ground. The statistical results indicate that hand positioning during quadrupedal walking is more variable than expected based on the mesaxony/ectaxony classification. Furthermore, radiographic data suggest that primates evolved at least two different mechanisms of hand ulnar deviation.