Hindlimb articular surface allometry in hominoidea and Macaca, with comparisons to diaphyseal scaling

Abstract

The scaling of hindlimb articular surface dimensions with body mass is investigated in Pan, Gorilla, Pongo, Macaca fascicularis, and, for the femoral head, two population samples of recent Homo sapiens sapiens. Articular dimensions scale very strongly with body mass within pongids at close to isometry. Body mass can be estimated relatively precisely given the appropriate reference group, with femoral head dimensions giving the best estimates. Several deviations from general scaling trends are also present and have functional implications. Positive allometry of the medial femoral condyle, and thus condyle asymmetry, is shown to be related to degree of varus (bowleggedness) of the knee, most marked in gorillas and declining in the smaller pongids and macaques. Macaques show somewhat smaller hindlimb articulations for their body mass, while modern humans have large femoral heads relative to body mass. Relative to diaphyseal cross-sectional dimensions (Ruff, 1987), orangutans and modern humans have large articulations and macaques small articulations. These proportional differences are explained in terms of differences between species in joint excursion, mode of locomotion, and activity level. Orangutans load their hind limbs less than other pongids due to more pronounced forelimb suspensory behavior and thus have less robust diaphyses, but maintain relatively large articulations to allow greater joint excursion, particularly hip abduction. Relatively smaller hindlimb articulations in macaques may be linked to reduced joint mobility relative to pongids. Modern humans, due to their bipedality, load the lower limb more than quadrupedal primates and thus have relatively large joints, but shaft dimensions remain smaller than expected, possibly at least partly because of lower activity levels. Within recent humans, femoral head dimensions are highly positively allometric. Because of this, femoral head size in two relatively complete early hominids (AL 288-1 and KNM-WT 15000) is approximately at or above what would be predicted for hominids of their respective body masses.

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