© 1981 Markus Kappeler
Extensive descriptions of locomotion exist for the white-handed gibbon (N.l.lar/carpenteri/entelloides; s. CARPENTER [1940,1976], ELLEFSON , BALDWIN & TELEKI ; aspects of the functional anatomy and evolution of gibbon locomotion have been treated by ANDREWS & GROVES , LORENZ  and TUTTLE [1969,1972]).
The statements of these authors are also valid for H.l.moloch, and will therefore not be repeated here. Rather, the conditions determining the occurrence of the various locomotion in the silvery gibbon will be examined.
Locomotion patterns constitute «tool behaviour» («Werkzeug-Verhalten», LORENZ ) and are applied in most functional situations in an ancillary role. While they are rich in variation, certain main types or techniques can be recognized. The type or technique actually used depends upon (1) the functional context, resp. the immediate behavioural goal, and (2) the substrate comprising the chosen route resp. the temporary site of the animal.
In the following, the correlations between locomotion pattern on the one hand, and functional context and substrate on the other hand will be examined. It is convenient to divide this discussion into two sections (A) fast locomotion, and (B) slow.
3.1. Fast locomotion
Characteristic of fast locomotion in the silvery gibbon are sequences of «flying phases» from branch to branch and, possibly, from tree to tree. Contact with the substrate serves to break the fall of the body, and to submit to it the impulse for a new flying phase towards a new arboreal support.
Basically this is brought about by leaping and brachiation. When leaping, the actual take-off - from a transitory bipedal position or reinforced through bipedal running - supplies the energy for a flying phase; when brachiating, the energy of the flying body is transferred into a pendulum-like swing - below the substrate - with the help of one arm at the time, and this swing leads over into the next flying phase.
Undoubtedly, brachiation with flying phases is the swiftest form of locomotion in the gibbon. However, it requires a relatively wide-meshed branching pattern and more or less horizontally growing branches of medium diameter, as well as a direction of motion which is approximately transverse to the main growing direction of the branches.
These substrate conditions are usually fulfilled only at the middle forest level, which explains why the gibbon as a rule moves to this level for fast locomotion. Should the conditions there also impede the continuity of brachiation, phases of bipedal leaping resp. running and leaping are interposed.
This dependence on the substrate for efficient locomotion corresponds to the fact that gibbon groups, in every part of their home range, use a fixed escape route when disturbed and therefore flee in single file. This permits rapid progress with well coordinated technical performance and excludes the risk of erring into a «pathless» situation.
Fast locomotion occurs in various functional contexts, e.g. predator avoidance (chapter 5), female song bouts (chapter 6), territorial disputes (chapter 7), play behaviour of juveniles.
Corresponding to the percentage of time of the total activity time spent by the silvery gibbon on these functions, fast locomotion accounts for a rather small share of the total locomotion.
3.2. Slow locomotion
In by far most cases, movement from one place to another is made by slow locomotion. In this way, usually only very short stretches are travelled at a time; occasionally, however, more or less constant slow locomotion over 50-100 meters occurs.
In slow locomotion, the sequences of flying phases are absent. 1-4 limbs are constantly in contact with the substrate. By temporarily supporting and/or anchoring the body, the limbs prevent it from falling. One hand or one foot alternately relinguishes and establishes contact in the direction of motion.
According to how the limbs are used, the following patterns of slow locomotion may be differenciated:
1. brachiation without flying phases,
2. bipedal walking,
3. brachiation resp. walking with additional temporary use of the legs resp. the arms,
4. quadrupedal climbing.
The scope of slow locomotion, however, occasionally also includes
5. flying phases; they serve to traverse gaps between branches or tree crowns. The impulse is generated by bipedal take-off or a brachiatory, pendulum-like swing; at the next support, the falling body can be caught up by the legs or arms, or both legs and arms together.
Such flying phases are regularly preceded by a check-up of the situation. In this way, not only is the locomotory effort planned; but probably also the motivational component (e.g. the access to resources), which urges the jump, is computed with the expenditure of energy needed for the jump. Moreover, a decision may also be made - within the framework of predator avoidance - between the tendency to move in a conspicuous manner and the tendency to behave cryptically.
The range of slow locomotion patterns gives the gibbon the possibility to use all structures of the tree - from tree trunks of considerable diameter (>1 meter) down to the finest branches (terminal twigs) - and, consequently, all niches of the forest, from the ground up to the top of the canopy. Slow locomotion is therefore not confined to a certain forest level.
In undisturbed situations the members of a gibbon group move in a swarm and rarely follow fixed routes; travelling in single file along a familiar and repeatedly used path only occurs when defiles are present in the home range (e.g. gaps in the substrate which have to be jumped over, or trunks of free-standing trees which have to be climbed).
Slow locomotion occurs mainly in undisturbed situations and serves, summarily, within the framework of survival, to enable the individual to visit resources and functional places associated with self-preservation.