Meeting Abstract

43.1  Saturday, Jan. 5  Ontogeny of joint mechanics in squirrel monkeys (Saimiri boliviensis) YOUNG, J. W.; Univ. of Texas, Austin jesse.young@mail.utexas.edu

Ontogenetic studies of a small, yet diverse, mammalian sample have shown negative allometry of bony lever arms, suggesting developmental declines in effective mechanical advantage (EMA: i.e., muscle moment arm/substrate reaction moment arm). Increased EMA among young mammals has been interpreted as an adaptation to facilitate adult-like levels of performance despite ontogenetic limits on muscle force. This model implicitly assumes that substrate reaction (SR) moments are isometric or decrease over development. If, however, SR moments increase with body size, young mammals may not require longer muscle levers to achieve adult-like performance. I filmed developing squirrel monkeys (Saimiri boliviensis; age: 87-302 days, mass: 240-540g) weekly as they traversed a 3m instrumented trackway or elevated pole. 2-D Kinematic and kinetic data were used to calculate elbow moments at peak vertical force for a total of 360 strides. Data were grouped by substrate and gait (i.e., symmetrical [walk/run] or asymmetrical [bound/gallop]). Elbow flexion increased with body mass in all conditions except for symmetrical strides on the ground. Because SR angles did not change with size, increasing elbow flexion led to positive allometry of SR moment arms and moments. SR moment arms and moments were isometric for symmetrical strides on the ground. In sum, even if elbow extensor muscle moments scale isometrically, young squirrel monkeys are never at a mechanical disadvantage relative to older ages. These data challenge the hypothesis that negative allometry of bony lever arms is necessarily a mammalian adaptation to counteract ontogenetic limits on locomotor performance. More data on the ontogenetic scaling of muscle force would be required to fully evaluate this hypothesis. Supported by L.S.B. Leakey Foundation Grant 38648.