Spatial-numerical association in bottlenose dolphins (Tursiops truncatus)

Spatial-numerical association (SNA) is considered an inherent property of humans who mentally map numbers onto space, positioning smaller values on the left and larger values on the right. Even though cultural factors can influence this spatialization and, in some cases, even reverse it into a right-to-left orientation, evidence of a left-to-right SNA has been reported in human newborns and several non-human animal species, emphasizing its biological origin. In bottlenose dolphins, as well as in other cetacean species, SNA has never been studied before, and it was the focus of this study. Two bottlenose dolphins (Tursiops truncatus), Micha and Golia, were trained to observe a central target for five seconds and touch it after hearing an acoustic signal. During the training phase the target consisted of stimuli depicting 6 items presented in 20 rewarded trials. In the test trials, two identical stimuli displaying either 2 or 18 items were presented simultaneously, and the dolphin had to choose one of them. Golia initially selected the left stimulus for the smaller numerosity, and the right one for the larger numerosity, but this pattern was not consistent across successive trials. Micha consistently showed a right-side bias, choosing the right stimulus in all the test trials. In a second experiment, both dolphins were tested for their ability to discriminate numerosities. Two stimuli were presented simultaneously: one displaying 6 items and the other either 2 or 18. Familiar and novel stimuli were employed to control for continuous physical variables (area and perimeter). In all trials, both individuals invariably showed a right-side bias, which may indicate either a difficulty in numerical discrimination or a misunderstanding of the required task. Future research should refine the experimental design to further investigate numerical discrimination and SNA in bottlenose dolphins.