Abstract
This chapter will review the current knowledge about the communication and social cognitionSocial cognitionof small apes. It will first demonstrate that the new perspective on hylobatid sociality---characterized by a large degree of social flexibility with single-female/multi-male groups representing a common pattern in addition to pair-living, and frequent interactions between groups---has been largely ignored by research into their communicative and socio-cognitive skills. Thus, communication research still considers hylobatids as pair-living, monogamous species adapted to an arboreal habitat. Based on this traditional notion of the hylobatids' social system, their communication is expected to be dominated by their loud, stereotyped species-specific songs, which travel far in the dense vegetation and are most likely used to defend their territory. Because of their small groups, hylobatid communication is thought to be limited to few facial and gestural signals, and their socio-cognitive skills are expected to be rather modest in comparison to great apes that live in socially more complex groups. Research into hylobatid communication largely focused on their songs, with increasing evidence for intra- and interindividual variation in their acoustic structure, while little is known about other vocalizations. Research into their gestures and facial expressions found considerable repertoires and flexible usage across different social contexts, similar to what has been described for great apes. However, unlike in great apes, research into the socio-cognitive skills of hylobatids often produced inconsistent or negative results. Together these findings demonstrate an unexpected variability and flexibility in hylobatid communication, while the socio-cognitive skills of small apes seem to less sophisticated than those of great apes. At the same time, this chapter points to several gaps of knowledge, which need to be addressed by studying gestures and facial expressions across different hylobatid species, in their natural environments, and by comparing their intra- and intergroup communication. Only then it will be possible to grasp the complexity of hylobatid communication and to adequately study their socio-cognitive skills.
