The indriid ischium and the hominid hip

Abstract

In their now classic paper, Napier & Walker (1967) noted that primate vertical clingers and leapers are unusual among mammalian leapers in having a relatively short ischium. In this paper we propose that the short ischium in vertical clingers and leapers reflects the fact that this bone has been reoriented to increase its dorsal projection. The short, dorsally projecting ischium of vertical clingers and leapers is functionally analogous with that of bipedal hominids and related to regular use of an extended hip joint. Morphometric comparisons of ischial shape in a wide range of prosimians and platyrrhines, together with a review of the naturalistic locomotion and posture of these species, supports the association between dorsally projecting ischia and frequent leaping from vertical supports. A brief examination of the ischia of several Eocene prosimians indicates that these taxa regularly lept from vertical supports. On the basis of our studies of primate ischium and a consideration of our present understanding of prosimian phylogeny, we feel it is not possible to accurately reconstruct the phylogeny of vertical clinging and leaping adaptations in early primate evolution or to reject Napier and Walker's hypothesis that this is the initial locomotor adaptation among cuprimates.