Energetic costs of hindlimb‐dominated locomotion in sifakas (Propithecus verreauxi)

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Human bipedal locomotion is unique, requiring a suite of musculoskeletal adaptations that were acquired gradually throughout hominin evolution. For example, lordotic lumbar spines move the center of mass over the hip joint and resists axial compression, short, laterally‐oriented iliac blades increase the leverage of the hip extensors and abductors, and the adducted hallux facilitates weight transfer during a propulsive toe‐off. Sifakas and other primates occasionally move bipedally in the wild, especially in terrestrial settings, but because they lack specialized anatomy, bipedal locomotion should be less efficient. In this study we test the hypothesis that energy costs differ among sifaka locomotor modes and predict that terrestrial bipedal locomotion is associated with greater energy expenditure than their more habitual mode of arboreal locomotion, vertical clinging and leaping. Bipedal (n=77) and vertical clinging and leaping (n=74) gait cycles were filmed in nine sifakas at the Duke Lemur Center. Locomotor data (stride length, peak body height of the cycle, and velocity) were measured from these videos using ImageJ and used to calculate energy expenditure using published equations. Linear mixed models were used to statistically test the differences in energy expenditure between locomotor modes while accounting for the effects of velocity and repeatedly measuring the same individuals. Results demonstrate that the cost of bipedalism is significantly greater than that of vertical clinging and leaping (p<0.0001). This result supports the idea that the locomotor adaptations in the hominin musculoskeletal system reduce the cost of locomoting bipedally. Furthermore, because sifakas rely on bipedal locomotion in terrestrial settings, these results have conservation implications for understanding the energetic demands of sifaka locomotion in deforested habitats.