Introduction

The study of behavioural lateralisation is a field of research that has recently been receiving extraordinary interest. Paul Broca described in 1861 a morphological asymmetry in human brain hemispheres associated with the use of language. Nowadays there is general agreement that lateralisation is a widespread characteristic and rather common among vertebrates. Behavioural lateralisation is then a very ancient characteristic of our subphylum and likely present in many species as a plesiomorphic character, i.e. an ancient inherited character. Interesting questions are when it arose, which ancestor developed it first and under which context does it occur in these species. Apart from anatomical brain asymmetry alone, motor and/or sensory lateralisation is displayed by a large number of bony fishes, amphibians and squamates. Visual and motor lateralisation are rather frequent in birds and mammals as well. There are several behavioural contexts where lateralisation is displayed: searching for food and prey, auditory vigilance, antipredator vigilance, ‘manipulation’ of food or simply objects, as well as tool use, environmental exploration, as well as in social and reproductive contexts. Another important question is what might be the adaptive advantage of lateralisation. In some species, lateralised individuals can perform two tasks controlled by opposite brain hemispheres at the same time. In others, lateralised individuals could be beneficial to the population as a whole, as social behaviour could be more developed when all members of the group are similarly lateralised. Brain asymmetry also affects the control of emotions, because the left hemisphere controls for focussed attention, and learning, while the right hemisphere is activated in situations such as fear or aggression, and a number of social behaviours.