Visual laterality in dolphins: importance of the familiarity of stimuli

We investigated visual laterality expressed spontaneously at the water surface by a group of five common bottlenose dolphins (Tursiops truncatus) in response to various stimuli. The stimuli presented ranged from very familiar objects (known and manipulated previously) to familiar objects (known but never manipulated) to unfamiliar objects (unknown, never seen previously). At the group level, dolphins used their left eye to observe very familiar objects and their right eye to observe unfamiliar objects. However, eyes are used indifferently to observe familiar objects with intermediate valence.Our results suggest different visual cerebral processes based either on the global shape of well-known objects or on local details of unknown objects. Moreover, the manipulation of an object appears necessary for these dolphins to construct a global representation of an object enabling its immediate categorization for subsequent use. Our experimental results pointed out some cognitive capacities of dolphins which might be crucial for their wild life given their fission-fusion social system and migratory behaviour.Laterality, previously considered to be exclusively a human characteristic, has been show in many vertebrates as well as invertebrate species (for example: [1-6]). Thus, the asymmetry of cerebral functions seems to be the rule rather than the exception in the animal kingdom [7]. Analyses of lateralized motor, perceptual or behavioral responses broaden our understanding of cerebral organization and of treatment of information by each cerebral hemisphere.Characteristics of perceived stimuli can be linked to the treatment of the information received by one of the cerebral hemispheres and to the implication of a given hemisphere. This link is modulated by subjects' internal state such as levels of hunger, vigilance or stress [8], as well as their age [9] or social environment [10]. Moreover, stimulus characteristics like emotional value [11,12] and novelty [13,14] are known