Advantages of a Lateralised Brain for Reasoning About the Social World in Chicks

Social animals require sophisticated cognitive abilities to recognise other individuals and to deduce hierarchies on the basis of observed interactions among them. The domestic chicken is one such species. Brain lateralisation is also linked to the selective pressures associated with social life. We investigated transitive inference learning in the domestic chick, in order to determine if this may be related to the possession of a lateralised brain, and if this ability is sustained differently by the two hemispheres. Lateralisation was manipulated by exposing eggs to differing levels of light before hatching, which leads to the development of lateralisation of some visual functions. Chicks with strong (Light-incubated) or weak (Dark-incubated) lateralisation were trained to discriminate stimulus pairs, in order to build a hierarchy (A ' B ' C ' D ' E), and were subsequently tested on a pair never seen before together (BD). Light-incubated chicks performed the discrimination BD better than did Dark-incubated ones. Moreover, lateralised chicks using their left eye only (right hemisphere) during test showed a better performance than did right-eye-only (left hemisphere) chicks on the BD task. Results demonstrate that chicks with lateralised brain hemispheres show greater inference and this ability may be largely under right hemisphere control. The findings are discussed with reference to the dominant role of the right brain hemisphere for other behaviours involved in social interactions.