brachiation_gibbon_desktopbBrachiation – the arm-swinging gait used by gibbons – is a useful place to start when thinking about how locomotion can be very largely ‘passive’ – achieving motions without requiring any energy inputs from the muscles. When moving fast, gibbons can swing like a pendulum, and fly through the air ‘ballistically’ (like a ball). Both of these can happen at any speed without (ignoring drag) requiring energy. So why do gibbons have long arms, and should we think of brachiation as an amazingly efficient form of locomotion?

My answer to these questions focusses on the transitions between ballistic and pendular paths. If the gibbon overshoots the ideal trajectory to make sure of making the next handhold (which it does appear to do), there is a wasteful ‘clunk’ or ‘collision’, which is exactly what we feel when we go too high on a swing. This collision is reduced if the gibbon has longer arms – in just the same way as having larger wheels is a good move when driving on a bumpy road.

This work is a small part of John Bertram’s research programme.

Usherwood, J.R., Larson, S.G. and Bertram, J.E.A. (2003). Mechanisms of force and power production in unsteady ricochetal brachiation. Am. J. Phys. Anthropol. 120, 364-372. doi 10.1002/ajpa.10133

Usherwood, J.R. and Bertram, J.E.A. (2003). Understanding brachiation: insight from a collisional perspective. J. Exp. Biol. 206, 1631-1642. doi:10.1242/jeb.00306