Introduction

Studies of positional behavior have helped our understanding of postcranial adaptation in primates and this in turn has contributed to discussions concerning how and why particular directions in primate evolution occurred. At first, anatomists observed positional behavior, whether in captivity or in the wild, in order to describe what primates actually do. These observations led to simple categorization of primate species into, for example, brachiators or arboreal quadrupeds, as well as evolutionary scenarios reconstructing adaptive pathways in primate and human locomotor evolution (e.g. Keith, 1923; Clark, 1959; Napier and Walker, 1967). With the proliferation of field studies, many of the early categories proved less than useful and the association between particular anatomical features and specific behaviors came under closer scrutiny (e.g. Stern and Oxnard, 1973; Mittermeier and Fleagle, 1976; Morbeck et al., 1979). These works, which used more sophisticated methods, specifically the quantification of primate positional behavior, helped to direct studies of positional behavior towards ecology as well as morphology. Despite these efforts made in the 1970s, surprisingly few species have been adequately sampled quantitatively in the wild, and, perhaps more importantly, very few studies have focused upon a particular research problem (for example, changes in body size and its effect on arboreal locomotion; Napier, 1967; Cartmill, 1974; Fleagle and Mittermeier, 1980; Fleagle, 1985; Jungers, 1985).