Lack of orienting asymmetries in Barbary macaques: implications for studies of lateralized auditory processing

Shedding light on the origin of specific neuronal mechanisms for processing acoustic stimuli is important for understanding the evolution of primate communication. One of those specializations, lateralized processing of speech in humans, is a well-established finding. Evidence is accumulating that diverse animal taxa also show hemispheric asymmetries in the perception of conspecific sounds. Rhesus macaques, Macaca mulatta, for instance, show a right head-turning bias in response to playback of natural conspecific vocalizations, and a left head-turning bias in response to a heterospecific stimulus and several manipulated conspecific calls. This finding was related to a hemispheric specialization for processing conspecific versus heterospecific vocalizations. We conducted orienting experiments with Barbary macaques, M. sylvanus, living in the enclosure ‘La Forêt des Singes’ in Rocamadour, France. In contrast to rhesus macaques, our subjects showed no orientation preferences in response to conspecific or heterospecific vocalizations. These results add to the puzzling mosaic picture of orienting asymmetries in different mammalian species and, in conjunction with other studies, highlight the importance of substantiating the assumption of a strong coupling of orienting bias and hemispheric asymmetry.