Dynamics of a brachiating siamang [Hylobates (Symphalangus) syndactylus]

THE slow brachiation which is characteristic of the siamang [Hylobates (Symphalangus) syndactylus] and to a lesser extent the smaller gibbons [Hylobates (Hylobates) spp.] is often described as pendulum-like1,2. This comparison is apt because both the pendulum and the gibbons rotate about a fixed overhead fulcrum and make use of gravitational acceleration to effect movement. Beyond these basic observations, there have been few attempts to elucidate the mechanical aspects of this unique form of locomotion2,3. Analysis of 16 mm cine films of free-ranging and captive siamangs and gibbons reveals details of their locomotor behaviour that can be interpreted as precise adaptations for maximising the forward momentum gained from pendular movement.