Integrative Primate Gait Dynamics

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Until recently, mechanistic studies of animal locomotion have routinely recorded steady-state, linear progression, often on highly artificial substrates (e.g., treadmills). In their natural habitats, animals stop and go, move up and down, use a variety of substrates, and change direction. Great variability certainly characterizes primate locomotor repertoires. Being an arboreal radiation, all primates spend at least some time in trees. Here they are faced with discontinuous and unsteady substrates that require constant adjustments and frequent turns. Gait mechanics in the real world is therefore almost certainly different from that typically studied in a gait lab. Demes will address two aspects of gait variability in this project: (1) the variability of limb bone loading, and (2) the modulation of gait mechanics in response to arboreal substrates. (1) Limb bone loading is often described as stereotypic, but this characterization is based on studies of animals moving linearly at steady state speed. The primates in this study will perform a variety of motor tasks (linear progression and directional changes, overground and on simulated branches) while crossing force transducers. Demes will evaluate the reaction forces acting on their limbs in conjunction with limb position data (obtained via video), thus indirectly testing the variation in bending regimes that the limb bones are exposed to. These, in turn, are important to understand shape variation in long bone cross sections. Aside from testing a range of locomotor modalities, both an arboreal and a terrestrial species will be enrolled in this research. This will put to test notions of variable loading regimes and circular bone cross sections being associated with arboreal locomotion, and stereotypic loading and elliptical bone cross sections with terrestrial locomotion.(2) It has been demonstrated that primates, and in particular arboreal primates, use compliant (as opposed to stiff) gaits that make travel on thin and unstable substrates safer. Such gaits reduce the vertical fluctuations of the center of mass of the body. However, these fluctuations are requisite for energy-saving pendulum and spring mechanisms. Demes will analyze whether there is a tradeoff between the advantages of compliant gaits and energy efficiency by tracking the fluctuations of the center of mass of both arboreal and terrestrial primates, while they are moving overground as well as on simulated branches. The Primate Locomotion Laboratory at Stony Brook is a unique facility that has produced major contributions to the field of primate functional morphology over the last 30 years. Numerous students and postdoctoral fellows have trained here that are now leaders in experimental research. This project offers further training opportunities for graduate students. The two aspects being addressed in this research are fundamental to primate functional morphology: One tests a paradigm in functional morphology upon which interpretations of fossil long bone diaphyseal shapes are based; the other addresses potential adaptive tradeoffs of primate gait specializations.