Energetics of Brachiation versus Walking: A Comparison of a Suspended and an Inverted Pendulum Mechanism

Abstract

It has been argued that brachiation is an energetically cheap way to move since, like a swinging pendulum, it involves an alternate storage and recovery of kinetic energy in gravitational potential energy during each swing. Walking also involves an alternate storage and recovery of kinetic energy in gravitational potential energy during a stride, and it has been likened to an inverted pendulum. There seems no a priori reason to suppose that the suspended pendulum of the "brachiator" is an energetically cheaper mechanism for locomotion than the inverted pendulum of the "walker." We therefore measured the energetic cost of locomotion as a function of speed as spider monkeys brachiated on a rope mill and on a horizontal ladder mill and as they walked on a treadmill. We compared the energetic cost of brachiation, the most specialized form of suspended locomotion, with that of the simplest-upside-down walking in the loris. We found that spider monkeys used significantly more energy to move at any speed when they brachiated than when they walked. The lorises, on the other hand, used the same amount of energy whether they moved upright or suspended beneath a rope. Although brachiation requires more energy to move a given distance, it may allow animals to drastically reduce the distance per trip in an arboreal habitat by moving from branch to branch in the periphery of trees.

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