Laughter is an ingroup display which, in humans, occurs in a large variety of forms. Especially its vocal features play a key role in social interaction, since they can trigger laughter on the basis of auditory perception, and so mediate the phenomenon of contagion (Provine and Yong, 1991, Ethology, 89, 115-12). Nevertheless, the acoustic features of laughter have received little attention by investigators in the past, as compared to its non-acoustic features. In our current study on laughter we are concentrating on both the dynamics of its signal parameters and its interactional role. Concurrently, we focus on comparative aspects. The results reported here refer to the sound patterns produced by humans who were tickled and others recorded from chimpanzees during playful encounters which included mutual tickling. In addition, we present data of vocalizations produced by Barbary macaques Macaca sylvanus engaged in playful wrestling (Todt et al., 1992, Primate Report, 32, 19-30). Beside some clear differences, we found a number of similarities among acoustic patterns uttered during such encounters. Differences concerned e.g. the relationship between vocalizations and facial displays: in the Barbary macaque, sound patterns were temporally segregated from the occurrence of a 'play face'. Whereas the latter predominantly preceded body contact of interactors, the vocalizations occurred after contact was established and wrestling became intensive. Congruencies concerned e.g. the rhythm, and specific variations of formant features. Our results contribute to hypotheses on relationships between laughter and playful ingroup behaviours. In addition they allow a novel discussion on laughter homologies as originated e.g. by R. Andrew (1963, Behaviour, 20, 1-109) and J. v. Hoof (1972, in R. Hinde, ed., Non-verbal Communication, Cambridge University Press), or continued by S. Preuschoft (1992, Ethology, 91, 220-236).
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