Gestural communication of the gorilla (Gorilla gorilla): repertoire, intentionality and possible origins

Abstract

Social groups of gorillas were observed in three captive facilities and one African field site. Cases of potential gesture use, totalling 9,540, were filtered by strict criteria for intentionality, giving a corpus of 5,250 instances of intentional gesture use. This indicated a repertoire of 102 gesture types. Most repertoire differences between individuals and sites were explicable as a consequence of environmental affordances and sampling effects: overall gesture frequency was a good predictor of universality of occurrence. Only one gesture was idiosyncratic to a single individual, and was given only to humans. Indications of cultural learning were few, though not absent. Six gestures appeared to be traditions within single social groups, but overall concordance in repertoires was almost as high between as within social groups. No support was found for the ontogenetic ritualization hypothesis as the chief means of acquisition of gestures. Many gestures whose form ruled out such an origin, i.e. gestures derived from species-typical displays, were used as intentionally and almost as flexibly as gestures whose form was consistent with learning by ritualization. When using both classes of gesture, gorillas paid specific attention to the attentional state of their audience. Thus, it would be unwarranted to divide ape gestural repertoires into 'innate, species-typical, inflexible reactions' and 'individually learned, intentional, flexible communication'. We conclude that gorilla gestural communication is based on a species-typical repertoire, like those of most other mammalian species but very much larger. Gorilla gestures are not, however, inflexible signals but are employed for intentional communication to specific individuals.

Fig. 1

Age distribution of gorilla gesture. The percentage of the total gorilla repertoire used by each age class. Error bars represent standard deviation across sites

Fig. 2

Distribution as a function of usage frequency. The total number of observed instances of a gesture in (intentional) use, plotted according to the number of independent sites at which it was recorded

Fig. 3

Gestural flexibility. The frequency of gestures is plotted according to the number of contexts in which they are used. Grey bars represent species-typical gestures, black bars potentially ritualized gestures. Error bars represent standard deviation across sites

Fig. 4

Context specificity of gestures. The frequency of gestures is plotted for each of the situational contexts in which they are used. Grey bars represent species-typical gestures, black bars potentially ritualized ones. Error bars represent standard deviation across sites

Fig. 5

Sensitivity to audience. The percentage of gestures used as a function of the attentional state of the recipient for each sensory modality (visible, audible and tactile). Separate analyses are performed for potentially ritualized gestures (top) and species-typical gestures (bottom). Black bars represent attending, grey bars represent not attending Error bars represent standard deviation across sites