Brain size is widely used as a measure of behavioural complexity and sensory-locomotive capacity in avians but has largely relied upon laborious dissections, endoneurocranial tissue displacement, and physical measurement to derive comparative volumes. As an alternative, we present a new precise calculation method based upon coupled magnetic resonance (MR) imaging and computed tomography (CT). Our approach utilizes a novel interactive Fakir probe cross-referenced with an automated CT protocol to efficiently generate total volumes and surface areas of the brain tissue and endoneurocranial space, as well as the discrete cephalic compartments. We also complemented our procedures by using sodium polytungstate (SPT) as a contrast agent. This greatly enhanced CT applications but did not degrade MR quality and is therefore practical for virtual brain tissue reconstructions employing multiple imaging modalities. To demonstrate our technique, we visualized sex-based brain size differentiation in a sample set of Ring-necked pheasants (Phasianus colchicus). This revealed no significant variance in relative volume or surface areas of the primary brain regions. Rather, a trend towards isometric enlargement of the total brain and endoneurocranial space was evidenced in males versus females, thus advocating a non-differential sexually dimorphic pattern of brain size increase amongst these facultatively flying birds.

Department of Biomathematics Institute of Physiology The Czech Academy of Sciences Vídeňská 1083 142 20 Prague Czech Republic

MR Unit Department of Diagnostic and Interventional Radiology Institute for Clinical and Experimental Medicine Vídeňská 1958 9 140 21 Prague Czech Republic Institute of Biophysics and Informatics 1st Medicine Faculty Charles University Prague Czech Republic

Museum of Evolution Uppsala University Norbyvägen 16 SE 752 36 Uppsala Sweden Palaeobiology Programme Department of Earth Sciences Uppsala University Villavägen 16 SE 752 36 Uppsala Sweden

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