Lateralisation of predator avoidance responses in three species of toads.

Lateralisation of responses to presentation of a simulated predator was investigated in three species of toads: two European species (the common toad, Bufo bufo, and the green toad, Bufo viridis) and one species introduced to Australia from South America, the cane toad Bufo marinus . First a simulated snake was presented moving rapidly towards the toad in the frontal field of vision and the toad's escape responses, including jumps to the right and to the left, were recorded. No significant bias in left or right side jumping was apparent in this test. Next the simulated snake was presented in the left or right lateral field of vision in random order. Escape and defensive responses were elicited more strongly, in all three species, when the stimulus was on the toad's left side compared to its right side. Reaction times scored in the experiments with B. marinus, alone, did not differ from left to right. There were, however, species differences in the types of escape responses with respect to the laterality: B. viridis and B. marinus showed similar patterns of more sideways jumps with left presentation and more frontal jumps with right presentation. Sideways jumps were not lateralised in B. bufo, but this species showed more frontal jumps when the presentation was on the left side. These findings suggest that the selective involvement of structures located in the right side of the brain (left monocular visual field) in emotional responses (particularly fear responses) could be a phylogenetic ancient trait.