Lateralized prey-catching responses in the cane toad, Bufo marinus: analysis of complex visual stimuli ANDREW ROBINS & LESLEY J. ROGERS Centre for Neuroscience and Animal Behaviour, School of Biological, Biomedical and Molecular Sciences, University of New England (Received 12 August 2003; initial acceptance 1 October 2003; final acceptance 29 December 2003; MS. number: 7822) We tested the responses of Bufo marinus to prey stimuli of varying visual complexity that were moved around the toads in either a clockwise or anticlockwise direction at 1.7 revolutions/min. Predatory responses directed at prey resembling an insect were frequent when the model insect moved clockwise across the visual midline into the right visual hemifield. In contrast, the toads tended to ignore such stimuli when they moved anticlockwise across the midline into the left hemifield. No such lateralization was found when a rectangular strip moved along its longest axis was presented in a similar way. The toads also directed more responses towards the latter stimulus than towards the insect prey. Hence, the results suggest that lateralized predatory responses occur for considered decisions on whether or not to respond to complex insect-like stimuli, but not for decisions on comparatively simple stimuli. We discuss similarities between the lateralized feeding responses of B. marinus and those of avian species, as support for the hypothesis that lateralized brain function in tetrapods may have arisen from a common lateralized ancestor. 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. Vallortigara et al. (1998) found that European common toads, Bufo bufo, European green toads, B. viridis, and South American cane toads, B. marinus, prefer to direct strikes with their tongue at prey in the right visual hemifield. The toads showed this preference when tested using an automated prey stimulus: a wormlike, live Galleria mellonella larva in the case of B. bufo and B. viridis and a live adult cricket (Acheta sp.) in the case of B. marinus. However, although B. marinus showed a pref- erence to tongue-strike at the prey stimulus as it moved through the right hemifield, the magnitude of respon- siveness and the strength of preference were not as strong as in the other two toad species (Vallortigara et al. 1998). Because the result might have been related to the differ- ence in the physical characteristics of the prey stimuli used to test the respective species of toad, in this study we used B. marinus to test the hypothesis that differences in the visual complexity of the prey may influence the degree of lateralization of the predatory responses. The optic fibres in the anuran visual system decussate almost completely, so input received by either eye is processed mainly by neural circuits in the opposite side of the brain, as in birds, reptiles, fish and mammals to laterally placed eyes (Rogers 2002b). In B. marinus, 96% of the retinal fibres cross to the contralateral side of the brain, with 85% of these projecting into the contralateral optic tectum (a mesencephalic structure analogous to the mammalian superior colliculus: Wye Dvorak et al. 1992). A preference to approach and strike at prey in the right visual hemifield might, therefore, reflect preferential control by circuits in the left tectum or higher visual centres in the left forebrain (telencephalon). Such a spe- cialization would be consistent with the general direction of lateralization observed for feeding responses in other vertebrates (Rogers 2002a). However, the right-biased strikes at prey reported by Vallortigara et al. (1998) were within the toad’s binocular visual field, which means that visual inputs would go to both sides of the brain. Toads possess a wide binocular overlap in the horizontal plane (our perimetric analysis of nine B. marinus revealed the mean angle of overlap in the horizontal plane at eye Correspondence and present address: A. Robins, Department of Biological and Physical Sciences, Faculty of Sciences, University of Southern Queensland, QLD 4350, Australia (email: robins@usq. edu.au). L. J. Rogers is at the Centre for Neuroscience and Animal Behaviour, School of Biological, Biomedical and Molecular Sciences, University of New England, NSW 2351, Australia. ARTICLE IN PRESS 567 0003–3472/03/$30.00/0 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. ANIMAL BEHAVIOUR, 2004, 68, 567–575 doi:10.1016/j.anbehav.2003.12.014