Differences and similarities between the natural gestural communication of the great apes and human children

Studies of animal communication systems are essential to understand the evolution of human language. The majority of research focused on vocal communication. Recent studies however provide evidence that gestural signaling plays an important role in the communication of apes and resembles that of prelinguistic and just-linguistic human infants in some important ways. However, ape gestures also differ from the gestures of human infants in some important ways as well, and these differences might provide crucial clues for answering the question of how human language –at least in its cognitive and social-cognitive aspectsevolved from the gestural communication of our ape-like ancestors. The present manuscript summarizes and compares recent studies on the gestural signaling of the great apes (Gorilla gorilla, Pan paniscus, Pan troglodytes, Pongo pygmaeus) to enable a comparison with gestures in children. We focused on the three following aspects: 1) nature of gestures, 2) intentional use of gestures, 3) and learning of gestures. Our results show, that apes have multifaceted gestural repertoires and use their gestures intentionally. Although some group-specific gestures seem to be acquired via a social learning process, the majority of gestures are learned via individual learning. Importantly, all of the intentional produced gestures share two important characteristics that make them crucially different from human deictic and symbolic gestures: 1) they are almost invariably used in dyadic contexts, and 2) they are used exclusively for imperative purposes. Implications for these differences are discussed. One of the enduring questions of the Western intellectual traditions is how spoken language, which is thought to be unique to humans, originated and evolved. One important way to address this question is to compare speech to the systems of vocal communication evolved in other animals, especially in nonhuman primates (hereafter primates) (e.g., Marler, 1977; Seyfarth, 1987; Snowdon, 1988; Zuberbuhler, 2003). The majority of studies investigated vocal communication and revealed that call morphology and call usage seem to have only limited flexibility (Liebermann, 1998; Corballis, 2002). However, recent data provided evidence that vervet monkeys use different alarm calls in association with different predators (leading to different escape responses in receivers) and therefore raised the possibility that some nonhuman species may, like humans, use vocalizations to make reference to outside entities (Cheney & Seyfarth, 1990). But it has turned out since then that alarm calls of this type have arisen numerous times in evolution in species that also must organize different escape responses for different predators, including most prominently prairie dogs and domestic chickens (Owings & Morton, 1998). And importantly, there is currently no evidence that any species of ape has such referent specific alarm calls or any other vocalizations that appear to be referential (Cheney & Wrangham, 1987; however see Crockford & Boesch, 2003). This implies that it is highly unlikely that vervet monkey alarm calls could be the direct precursor of human language unless at some point apes used similar calls and have now lost them. Interestingly, gestural or ideographic communication systems have to some extent been mastered by human-reared great apes (e.g. Gardner, et al, 1989; Savage-Rumbaugh, et al, 1993). Though by no means ‘language’, these projects have shown intentional, referential use of numerous gestures and ideograms (Gardner, et al., 1989; Savage-Rumbaugh, 1986), accurate usage under doubleblind conditions, and understanding of human speech. These findings support the hypothesis that the evolutionary roots of language might have evolved in the visual-gestural modality (e.g., Condillac, 1971; Hewes, 1976; Armstrong et al., 1995; Dunbar, 1996; Arbib, 2002). In addition, recent studies provide evidence that gestural signaling plays an important role in the natural communication of primates and resembles that of pre-linguistic and just-linguistic human infants in some important ways (Plooij, 1978, Tomasello et al., 1985). However, ape gestures differ from the gestures of human infants in some important ways as well, and these differences might provide crucial clues for answering the question of how human language –at least in its cognitive and social-cognitive aspectsevolved from the gestural communication of our ape-like ancestors. The question thus arises: what is the nature of the gestural communication of nonhuman primates, and how do they relate to human gestures and language? The present manuscript is based on observations of the communicative signaling of the four great apes species (Gorilla gorilla, Pan paniscus, Pan troglodytes, Pongo pygmaeus). To enable a qualitative comparison with gestures in children, we focused on the three following aspects: First we investigated the nature of gestures by examining whether they are used dyadic, triadic, imperative (used to get another individual to help in attaining a goal, cf. Bates, 1976) and/or declarative (used to draw another’s attention to an object or entity merely for the sake of sharing attention, cf. Bates, 1976). Second, we investigated if apes use their gestures intentionally, focusing on the key characteristics for intentional communication in children (Piaget, 1952; Bates, 1976; Bruner, 1981), –a) means-ends dissociation and b) special sensitivity to the social context. A) Means-ends dissociation can be characterized by the flexible relation of signaling behavior and goal. An individual uses for instance a single gesture for several goals (touch for nursing and riding) or different gestures for the same goal (slap ground and bodybeat for play). b) Sensitivity to the social context: The sender performs a gesture toward a recipient for the purpose of communication. Evidence for specifically communicative intent includes the signaler’s alternation of gaze between goal and recipient (Bates, 1979; observed in wild chimpanzees, Plooij, 1978), persistence to the goal, or adjustment to audience effects (Tomasello et al., 1997). Our third goal concerned the learning of gestures by focusing on individual and group variability to distinguish between underlying social and individual learning processes. Following Tomasello and colleagues (Tomasello et al., 1994) similarities in the gestural repertoires within a group and group specific gestures would provide evidence for the existence of a social learning process, whereas individual differences that overshadow group differences (i.e., a lack of systematic group differences, idiosyncratic gestures) imply that an individual learning process is involved.