Lateralized eye use in Octopus vulgaris shows antisymmetrical distribution RUTH A. BYRNE, MICHAEL J. KUBA & DANIELA V. MEISEL Konrad Lorenz Institute for Evolution and Cognition Research (Received 7 October 2003; initial acceptance 10 November 2003; final acceptance 15 March 2004; published online 18 September 2004; MS. number: 7876) Behavioural lateralization has been demonstrated in many species of vertebrates, but there has been scarce evidence for it in invertebrates. We have previously documented lateral asymmetry of eye use in individual octopuses. In the present study we investigated lateralization at the population level. Octopus eyes are on the sides of the head, and these animals prefer monocular to binocular vision. We determined preferential eye use by recording the time that the octopuses watched a stimulus outside the tank while holding on to the front glass of the tank. Thirteen of 25 subjects were highly significantly left-eyed, 10 highly significantly right-eyed, and two showed no preference. Individual octopuses had lateralized eye use but, unlike handedness in humans, eye preference in octopuses at the population level had no systematic bias towards left or right. This behavioural asymmetry follows an antisymmetrical distribution and could therefore be genetically or epigenetically determined. This study extends assessment of lateralization to the population level to include invertebrates in the discussion of the evolution of lateralization. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. A brain is said to be asymmetrical or lateralized if one side is structurally different from the other or performs a different set of functions (Bisazza et al. 1998). According to this definition, one can say that an organism is lateralized if one side of its body is structurally or behaviourally different from the other. Brain lateralization was long thought to be unique to humans, but since the 1970s studies have shown that we share lateralization with many vertebrate species (re- viewed in Bradshaw & Rogers 1993; Davidson & Hughdal 1995; Gu ¨ntu ¨rku ¨n 1997; Bisazza et al. 1998; Rogers & Andrew 2002). Contemporary scientists claim that brain lateralization originated early in vertebrates (Bisazza et al. 1998), and that lateral asymmetries are a homologous trait across all vertebrates (Rogers & Andrew 2002). In their hypotheses for the evolutionary origin of lateralization, they have looked at behavioural lateralization in only one phylum (the chordates) and not across the whole animal kingdom. However, Byrne et al. (2002) showed that individuals of Octopus vulgaris had a lateral preference for either the left or right eye in monocular vision. Accord- ing to this finding, the phenomenon of behavioural lateralization is not limited to vertebrates, and therefore might be a common principle that evolves when bi- laterally symmetrical neuronal systems must cope with complex sensory inputs. There are two sets of definitions concerning individual- or population-level lateralization. One comes from scientists working mainly on behavioural aspects of lateralization (Denenberg 1981; Bisazza et al. 1998; Rogers & Andrew 2002); the other comes from scientists working mainly on structural asymmetries (reviewed in Møller & Swaddle 1998). The position usually taken by scientists working on behavioural aspects of lateralization is that a population is lateralized if more than 50% of the individuals are lateralized in the same direction (Denenberg 1981; Bisazza et al. 1998). Furthermore, they suggest that evidence of lateralization at the population level means that evolu- tionary processes have been at work, indicating that there must have been selection pressure for a particular side to become specialized in the same direction in more than half of the individuals in a population. Laterality at the individual level is evident when most of the individuals in a population are lateralized, but not necessarily in the same direction (Rogers & Andrew 2002). If a brain needs to be lateralized to function efficiently, it may be irrelevant which side is used to conduct a set of functions. The important thing is that, at the level of the individual, Correspondence: R. A. Byrne, Konrad Lorenz Institute for Evolution and Cognition Research, Adolf Lorenz Gasse 2, A-3422 Altenberg, Austria (email: ruth@byrne.at). 1107 0003–3472/03/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2004, 68, 1107–1114 doi:10.1016/j.anbehav.2003.11.027