Center of mass mechanics during locomotion in the arboreal squirrel monkey (Saimiri sciureus) as a function of speed and substrate.

It is thought that the magnitude of center of mass (COM) oscillations can affect stability and locomotor costs in arboreal animals. Previous studies have suggested that minimizing collisional losses and maximizing pendular energy exchange are effective mechanisms to reduce muscular input and energy expenditure during terrestrial locomotion. However, few studies have explored whether these mechanisms are used in an arboreal context, where stability and efficiency often act as tradeoffs. This study explores three-dimensional center of mass mechanics in an arboreal primate-the squirrel monkey (Saimiri sciureus)-moving quadrupedally at various speeds on instrumented arboreal and terrestrial supports. Using kinetic data, values of energy recovery, center of mass mechanical work and power, potential and kinetic energy congruity, and collision angle and fraction were calculated for each stride. Saimiri differed from many other mammals by having lower energy recovery. Although few differences were observed in center of mass mechanics between substrates at low or moderate speeds, as speed increased, center of mass work was done at a much greater range of rates on the pole. Collision angles were higher, while collision fractions and energy recovery values were lower on the pole, indicating less moderation of collisional losses during arboreal versus terrestrial locomotion. These data support the idea that the energetic demands of arboreal and terrestrial locomotion differ, suggesting that arboreal primates likely employ different locomotor strategies compared to their terrestrial counterparts-an important factor in the evolution of arboreal locomotion.