The combination of locomotor features of the primate shoulder girdle by canonical analysis
Authors: Ashton E. H. , Healy M. J. R. , Oxnard C. E. , Spence T. F. 
Published: August 2009

Certain anatomical features of the pectoral girdle can be demonstrated to be functionally significant in the locomotor employment of the forelimb in different primate forms. In an earlier study, nine such shoulder features, examined individually, revealed the presence, within the Anthropoidea, of a graded contrast between brachiators, semibrachiators (progressing sometimes quadrupedally, sometimes by arm swinging) and quadrupeds. Within the Prosimii, corresponding contrasts obtain between hangers (resembling brachiators and semibrachiators in the frequent subjection of the arm to tensile forces) and quadrupeds (wherein, during locomotion, the arm functions as a strut).

The compounding by canonical analysis of these nine features into mathematical functions best separating the means obtained for individual genera, reveals more detailed correlations between anatomical structure and locomotor pattern.

Thus the first function disposes the Anthropoidea in a spectrum from the extreme brachiator Hylobates through the lesser brachiators Pan and Gorilla to such obtrusive semibrachiators as Ateles and Brachyteles, thence through intermediate members of the semibrachiator group to forms such as the infrequently arm‐swinging Presbytis, thence to more agile, arboreal quadrupeds like Pithecia and Cacajao, and finally through intermediate members of this last group (e.g. Macaca) to such wholly terrestrial forms as Papio and Erythrocebus.

The second function provides further information in that the genera are arranged in a sequence which extends from largely or wholly terrestrial forms at one extreme (e.g. Gorilla from among brachiators, low canopy species of Presbytis from among semibrachiators, Papio and Erythrocebus from among quadrupeds), to almost exclusively arboreal forms at the other extreme (e.g. Pongo among the brachiators, Brachyteles and Rhinopithecus among semibrachiators, Pithecia and Cacajao among quadrupeds).

For the Prosimii, neither of these functions yields information additional to their separation into hangers and quadrupeds (coincident respectively with the semibrachiators and quadrupeds of the Anthropoidea) established by study of individual dimensions.

All the subhuman primate genera lie close to the plane of the diagram formed by a combination of functions one and two, i.e. the third (and subsequent) functions are ineffective in their further separation. But, by the third function, man, whose individual dimensions agree sometimes with one group, sometimes with another, is clearly distinguished from all other primate forms—a distinction correlating with the unique functional activity of the human shoulder.

Although this analysis separates various primate taxonomic groups below familial level, the particular locomotor features themselves do not (either singly or in combination) differentiate between Prosimii and Anthropoidea, nor between prosimian infraorders, or superfamilies of the Anthropoidea.

However, other features of the shoulder girdle may make such a differentiation, and if so, a fossil form, studied in relation thereto and in relation to the analysis of locomotor features set out in this present study, could be interpreted both taxonomically and in terms of probable shoulder activity.