The VCL hypothesis revisited: Patterns of femoral morphology among quadrupedal and saltatorial prosimian primates

Abstract

The descriptive and functional morphology of the postcranium of the vertical clinging and leaping prosimians is of great interest in both adaptational and phylogenetic studies of extant and extinct primates. An analysis of patterns of femoral morphology among quadrupedal and saltatory living prosimians indicates the presence of at least two, and possibly three, distinct femoral adaptations to the demands of an arboreal, saltatory existence. Osteological measurements were taken on 277 postcranial skeletons representing eight prosimian families, with skeletal trunk length (Biegert and Maurer, Folia Primatol. 17:142-156, 1972) used as an estimator of body size in both bivariate and multivariate (discriminant function) statistical analyses. Whereas all extant vertical clingers and leapers share certain femoral traits (i.e., long femur, proximally restricted trochanters, ventrally raised patellar articular surface), Galagidae and Tarsiidae share features of the proximal femur (i.e., cylindrical head, large posterior expansion of articular surface onto the neck) that clearly distinguish them from the specialized leapers of the Malagasy Republic (Indriidae and Lepilemur). These latter animals closely resemble active quadrupedal Lemuriformes in possessing spherical femoral heads, with little posterior expansion of articular surface and a distinctive greater trochanter shape. Some traits suggested as being characteristic of vertical clingers and leapers (i.e., deep femoral condyles facing posteriorly, narrow patellar groove with prominent lateral margin, greater trochanter overhanging anterior aspect of shaft) are shown to be common to all prosimians except the slow-climbing lorisids. The functional significance of aspects of femoral morphology among saltators is discussed, along with the relevance of these results to an assessment of vertical clinging and leaping among extant and extinct primates.