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 For comparative purposes muscle weights were also gathered for Varecia, an arboreal quadruped, and previously unpublished dry muscle weights of several monkeys are included as well.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 characterized 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 therefore have less muscle force available per unit body weight to be propelled than their smaller-bodied counterparts.