Lateralization of response to social stimuli in tadpoles

Tadpoles of five anuran species were tested for preferences in the use of the eyes during inspection of their own visual image in a mirror. When tested in a tank with several small mirrors, tadpoles of five different species (Bufo bufo, Bufo viridis, Rana temporaria, Rana esculenta, Bombina variegata) preferentially approached and positioned themselves with the mirror located on their left side, thus looking at the image with the monocular field of their left eye. Similar results were obtained with tadpoles of R. temporaria tested in a task in which they had to choose approaching one or other of two mirrors located on their left and right side. Control experiment showed that the behavioural asymmetry was not due to motor preferences and that it was independent of morphological asymmetries in the positions of the spiracles. The lateral bias typically emerges some minutes after the placement of the animals in the test apparatus. We checked whether such a temporal pattern was associated with lateralisation per se or rather reflected temporal variations in social aggregation. We found that the tendency to move to make social aggregation only appears after about five minutes following placement in a novel environment and this corresponded quite well with the appearance of lateralisation, when tadpoles showed an higher probability of approaching a conspecific appearing on their left rather than on their right hemifield. This is the first demonstration of a functional visual lateralisation among juvenile amphibia before metamorphosis.