COMPLEX LEAF-GATHERING SKILLS OF MOUNTAIN GORILLAS (GORILLA G-BERINGEI) - VARIABILITY AND STANDARDIZATION

The skills that mountain gorillas use to deal with the stings, tiny ho oks, and spines protecting common plant leaves in their diet were exam ined for variation within and between animals. Many elements of uni- a nd bimanual performance were identified, often involving delicate prec ision and coordination, and varying idiosyncratically, each individual having a different set of preferred elements. Many of these elements are functionally equivalent, and all but one weaned animals showed ful l processing capability; the history of the one exception suggests tha t early experience with the task may be important. Gorillas' idiosyncr asy in manual skill elements is entirely consistent with trial-and-err or learning at this level. By contrast, each individual uses very few techniques (structured sequences of elements) for most processing, and these techniques are the same across the population. Where animals de viate from this generalization, they largely employ the simpler techni que normally used for undefended leaves. Lateralization increases from start to finish, consistent with a logical structure in which each st age has a laterality bias and each stage is sequentially dependent on the last. Variations from their commonest techniques occur in all anim als (on average, about nine variant techniques were recorded from each animal). The repertoire of techniques increases significantly with ag e, whereas the repertoire of elements does not. This points to an init ial reliance on a single logical structuring that is well established by weaning (about 3.5 years), with subsequent development of the abili ty to vary the technique used so as to take advantage of variations in the environment. Standardization of logical organization, despite var iability between different animals in individual elements and behavior al laterality, suggests that the logical ordering of elements and the interrelationships of processing stages is copied by program-level imi tation. (C) 1993 Wiley-Liss, Inc.