Behavioural Processes 68 (2005) 179–184 Short report Left hemispheric advantage for numerical abilities in the bottlenose dolphin Annette Kilian a,b,* , Lorenzo von Fersen b , Onur G ¨ unt¨ urk¨ un a a Department Biopsychology, Inst. Cogn. Neurosci., Faculty of Psychology, Ruhr-University Bochum, 44780 Bochum, Germany b Tiergarten N¨ urnberg, Am Tiergarten 30, 90480 N¨ urnberg, Germany Received 8 September 2004; received in revised form 23 October 2004; accepted 7 November 2004 Abstract In a two-choice discrimination paradigm, a bottlenose dolphin discriminated relational dimensions between visual numerosity stimuli under monocular viewing conditions. After prior binocular acquisition of the task, two monocular test series with different number stimuli were conducted. In accordance with recent studies on visual lateralization in the bottlenose dolphin, our results revealed an overall advantage of the right visual field. Due to the complete decussation of the optic nerve fibers, this suggests a specialization of the left hemisphere for analysing relational features between stimuli as required in tests for numerical abilities. These processes are typically right hemisphere-based in other mammals (including humans) and birds. The present data provide further evidence for a general right visual field advantage in bottlenose dolphins for visual information processing. It is thus assumed that dolphins possess a unique functional architecture of their cerebral asymmetries. © 2004 Elsevier B.V. All rights reserved. Keywords: Bottlenose dolphin; Hemispheric specialization; Monocular vision; Numerical ability 1. Introduction Over the last decades, a large body of experimental studies has accumulated, demonstrating that functional asymmetries are not unique to humans, but represent a characteristic that many vertebrate species have in common (Rogers and Andrew, 2002). This growing * Corresponding author. Tel.: +49 9123 963050; fax: +49 9123 963049. E-mail address: akilian@marine-mammal-cognition.de (A. Kilian). evidence for perceptual and cognitive lateralizations in animals also provides the basis for understanding the biological foundations of hemispheric specializations. So far, only few studies have focused on lateralizations in marine mammals like dolphins. Apart from findings on lateralized motor functions in the bottlenose dol- phin (Norris and Dohl, 1980; Marino and Stowe, 1997), first investigations on cerebral functional asymmetries were carried out by von Fersen et al. (2000) and Kilian et al. (2000) demonstrating a right eye dominance in two different visual tasks for their subjects. Due to the complete decussation of the optic nerves in dolphins 0376-6357/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.beproc.2004.11.003