Do Mechanical Effectiveness and Recipient Species Influence Intentional Signal Laterality in Captive Chimpanzees (Pan troglodytes)?

Abstract

Studying the relationships between the directions of brain lateralization for handedness and language can shed light on mechanisms underlying hemispheric specialization for manipulation and signalling functions. We investigated the influence of manipulation and communication functions and of recipient species (conspecific- versus human-directed communication) on manual laterality in signalling context, taking several factors into account simultaneously. We assessed laterality in 39 chimpanzees (Pan troglodytes), including 4 manipulators (mechanically effective social actions used to get things done) and 18 gestures (mechanically ineffective social actions implying that the signaller takes the recipient's response into account). We focused on the following factors: interactional context components (e.g., visual fields of both interactants), degree of use of signals (''rare'' for signals performed by only a few subjects in the population or ``common'' for signals performed by many subjects), mechanical effectiveness, subjects' sociodemographic characteristics (e.g., age and hierarchy), and recipient species. We found a significant population-level right-hand bias for one type of human-directed gesture (slap hand). Mechanical effectiveness influenced laterality: right-hand use was more pronounced for conspecific-directed gestures than for conspecific-directed manipulators. The laterality of conspecific-directed gestures overall did not differ from that of human-directed gestures. However, we found an indirect influence of recipient species on laterality as conspecific- and human-directed gestural lateralities were modulated differently by the position of the recipient in the signaller's visual field and by signaller's age. We hypothesize that the communication nature of gestures might have developed from manipulators. Manipulators may have contributed to the emergence and the evolution of the left-lateralized communication system in primates.