From left to right: Humans only?

Adult humans map numbers onto a mental number line oriented from left to right. (Dehaene, 1997, Dehaene et al., 2008). It is not clear if this mapping is of cultural or educational nature, or if it rather is a universal intuition, also present in non-human species. In a task requiring the identification of a certain object on the exclusive basis of its ordinal position in a series of identical objects, few-days-old chicks showed a leftward bias when the spatial orientation of the series was changed from training to test (Rugani et al., 2007). Here, two bird species (i.e. the domestic chick and the Clark’s nutcracker) were tested in a similar task. Birds were trained to identify the 4th or the 6th element in a series of 16 fixed and identical elements, oriented sagittally with respect to the bird in the starting point. When the orientation of the series remained as for the training, birds’ performance was above chance level. When generalizing to the series rotated by 90°, birds’ performance was also for pecking at the correct positions significantly above chance level, but now birds displayed a clear bias for locating such positions in the left hemispace. Birds thus exhibited a left-side bias similar to that displayed by humans. It may be the case that a right hemispheric dominance is at play in this sort of tasks, with the left visual hemifield controlling birds’ behaviour, nevertheless the resemblance with the human’s behaviour is not negligible.