ORIGINAL PAPER Relative Wulst volume is correlated with orbit orientation and binocular visual field in birds Andrew N. Iwaniuk Æ Christopher P. Heesy Æ Margaret I. Hall Æ Douglas R. W. Wylie Received: 15 March 2007 / Revised: 20 November 2007 / Accepted: 24 November 2007 / Published online: 11 December 2007 Ó Springer-Verlag 2007 Abstract In mammals, species with more frontally oriented orbits have broader binocular visual fields and relatively larger visual regions in the brain. Here, we test whether a similar pattern of correlated evolution is present in birds. Using both conventional statistics and modern comparative methods, we tested whether the relative size of the Wulst and optic tectum (TeO) were significantly correlated with orbit orientation, binocular visual field width and eye size in birds using a large, multi-species data set. In addition, we tested whether relative Wulst and TeO volumes were correlated with axial length of the eye. The relative size of the Wulst was significantly correlated with orbit orientation and the width of the binocular field such that species with more frontal orbits and broader binocular fields have relatively large Wulst volumes. Relative TeO volume, however, was not significant correlated with either variable. In addition, both relative Wulst and TeO volume were weakly correlated with relative axial length of the eye, but these were not corroborated by independent con- trasts. Overall, our results indicate that relative Wulst volume reflects orbit orientation and possibly binocular visual field, but not eye size. Keywords Evolution  Wulst  Optic tectum  Binocularity  Eye size Abbreviations GLd Nucleus geniculatus lateralis, pars dorsalis GLv Nucleus geniculatus lateralis, pars ventralis HA Apical hyperpallium HD Densocellular part of the hyperpallium HI Interstitial part of the hyperpallium IHA Intercalated part of the hyperpallium LGN Lateral geniculate nucleus S1 Primary somatosensory cortex TeO Optic tectum V1 Primary visual cortex W Wulst Introduction Orbit orientation varies tremendously among birds from the laterally placed eyes of the woodcock (Scolopax rust- icola; Martin 1994) to the more frontally oriented eyes of owls. The orientation of the orbits has significant impli- cations for the shape and size of the visual field. More laterally oriented orbits result in a broad visual field, but at the cost of a narrower binocular visual field (e.g., Martin 1994). More frontally oriented orbits, however, result in a larger binocular visual field, but at the cost of a large posterior blind field (e.g., Martin 1984). In mammals, species with frontally oriented orbits tend to have broader A. N. Iwaniuk (&)  D. R. W. Wylie Department of Psychology, University of Alberta, Edmonton, AB T6G 2E9, Canada e-mail: brainsize@yahoo.ca C. P. Heesy Department of Anatomy, Midwestern University, Glendale, AZ 85308, USA M. I. Hall Department of Biomedical Sciences, Midwestern University, Glendale, AZ 85308, USA D. R. W. Wylie Centre for Neuroscience, University of Alberta, Edmonton, AB T6G 2E9, Canada 123 J Comp Physiol A (2008) 194:267–282 DOI 10.1007/s00359-007-0304-0