B. Demes & J. G. Fleagle Department of Anatomical Sciences, State University of New York, Stony Brook, N.Y. 11794-8081, U.S.A. E-mail: bdemes@mail.som.sunysb.edu jfleagle@mail.som.sunysb.edu P. Lemelin Department of Anatomy, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272-0095, U.S.A. E-mail: pl1@riker.neoucom.edu Received 15 July 1997 Revision received 7 November 1997 and accepted 10 November 1997 Keywords: locomotion, vertical clingers and leapers, relative muscle masses, strepsirhines. Myological correlates of prosimian leaping Although the skeletal correlates of vertical clinging and leaping behavior in primates have been studied in great detail, myological information on this locomotor group is not readily available. We here provide relative muscle mass data for the hindlimb of four prosimian leapers, representing indriids as well as the small-bodied tarsiers and galagos. Wet weights of all hindlimb muscles, with the exception of the intrinsic muscles of the foot, were determined. For comparative purposes muscle weights were also gathered for Varecia, an arboreal quadruped, and previously unpublished dry muscle weights of several monkeys are included as well. The specialized leapers are character- ized by a predominance of muscles for hindlimb joint extensions. Indriids have larger hip extensors than ankle plantarflexors, whereas the galago and tarsier display the reverse condition. This dichotomy in relative muscle mass corresponds to a dichotomy in leaping kinematics, with indriids going through a greater range of movement at the hip joint and galagos and tarsiers at the ankle joint. However, the most striking result is the overwhelming dominance of the quadriceps femoris muscle in both groups. This suggests that power may be transferred from the knee and thigh to adjacent joints and segments. In contrast, quadrupedal primates have more extensor musculature at the hip, suggesting that the need for a short swing phase pendulum constrains muscle mass distribution within the limb of quadrupeds. Total muscle mass of the hindlimb as well as the mass of the propulsive muscles scale with body mass at exponents below the functional equivalence expectation. Larger-bodied leapers there- fore have less muscle force available per unit body weight to be propelled than their smaller-bodied counterparts.  1998 Academic Press Limited Journal of Human Evolution (1998) 34, 385–399 Introduction The locomotor category of vertical clingers and leapers comprises at least two sub- groups of primates that differ in locomotor performance and body design: galagos and tarsiers on the one hand, and indriids on the other. This dichotomy in body proportions and skeletal features has been described in a series of papers, following soon after the locomotor category was introduced by Napier & Walker in 1967 (Cartmill, 1972; Martin, 1972; Stern & Oxnard, 1973; Jouffroy & Lessertisseur, 1979; Gebo & Dagosto, 1988; Godfrey, 1988; Anemone, 1990). It seems to be, at least in part, size-driven. Tarsiers and specialized galagid leapers are all well below 1 kg, whereas all indriids fall above that threshold. Features that characterize tarsiers and small-bodied galagos, and distinguish them from indriid leapers include an elongated tarsus, and transverse tarsal joints promoting flexion and extension. They have been described as tarsifulcrumators (Morton, 1924), long-tarsal jumpers (Jouffroy & Gasc, 1974; Jouffroy & Lessertisseur, 1979) or foot-powered jumpers (Gebo & Dagosto, 1988). In con- trast, indriid leapers have a grasping foot with long digits and no elongation of tarsal Send correspondence to: Brigitte Demes, Dept. of Anatomical Sciences, State University of New York, Stony Brook, N.Y. 11794-8081, U.S.A. e-mail: bdemes@mail.som.sunysb.edu 0047–2484/98/040385+15 $25.00/0/hu970203  1998 Academic Press Limited