Social communication in orangutans (Pongo pygmaeus): Use of gestures and facial expressions Katja Liebal1, Simone Pika2 and Michael Tomasello1 1 Max-Planck-Institute for Evolutionary Anthropology, Leipzig, Germany 2 University of Alberta, Edmonton, Canada e-mail: liebal@eva.mpg.de Keywords: Pongo pygmaeus, communication, gestures, facial expressions, flexibility Studies of animal communication systems are essential to understand the evolution of human language. The majority of research focused on vocal communication (Seyfarth, 1987; Snowdon, 1988) what seems to have derived from the analogy to human language (Tomasello et. al, 1997). Other authors have suggested a gestural origin of language (Hewes, 1973; Corballis, 2002; Arbib, 2002). Grigor^Òeva et. al (1987) proposed that the evolution of gestural communication in primates proceeded from a stage involving a majority of tactile gestures to an increasing number of visual gestures. Therefore monkey species should use a wider repertoire of tactile gestures while great apes should use a wider range of visual gestures. The social communication of orangutans is only poorly investigated in contrast to the African great ape species (Goodall, 1986; Tomasello et. al, 1989, 1997; de Waal, 1988; Kano, 1992; Tanner et. al. 1996; Pika et. al, 2003). Some authors suggest that despite their semi-solitary lifestyle orangutans use a wide range of vocal signals and gestures (Kaplan and Rogers, 2000; MacKinnon, 1974) while others argue that they might have lower communication skills (Bennett, 1998). In addition, their natural habitat seems to predict the importance of tactile or vocal signals rather than visual signals (Maestripieri, 1996). The present study is the first systematic investigation of the communicative repertoire of Sumatran orangutans (Pongo pygmaeus) with focus on gestures and facial expressions of two groups of captive orangutans. The main goals were to analyze the signal repertoire with respect to 1) the number and frequency of signals (tactile, visual and auditory gestures, facial expressions), 2) the variability of individual^Òs repertoire as a function of group, age class and gender, and 3) the flexibility of use with respect to ^Ñmeans-end- dissociation^Ò (Bruner, 1981) and adjustment of signals depending on the attentional state of the recipient. 17 individuals (10 adults, 3 subadults, 2 juveniles, 2 infants) were observed using a video camera to record a total of 170 hrs (10 hrs of ^Ñfocal-animal-sampling^Ò per individual). The results show that orangutans use a multifaceted signal repertoire with a majority of visual gestures characterized by flexibility of use and sensitivity to the attentional state of the recipient. 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