The relationship between growth, brain asymmetry and behavioural lateralization in a cichlid fish.

Cerebral lateralization, the partitioning of cognitive tasks to one cerebral hemisphere, is a widespread phenomenon among vertebrates. Despite this diversity, every species studied to date shows substantial individual variation in the strength of lateralization. The neural basis of this trait is unclear, although asymmetries in cerebral structures have been investigated for over a century. The habenular nuclei, for example, have been shown to present striking neuroanatomical and/or neurochemical asymmetries in species ranging from jawless fish to mammals. In teleost fish, these nuclei are relatively symmetrical in most species. Those teleosts that do have asymmetrical habenular nuclei, show varying patterns of asymmetry in different species. Here we investigate the relationship between individual variation of asymmetry in the habenula of a South American cichlid fish, Geophagus brasiliensis, and behaviour in a commonly used test for visual laterality in fish, the detour task. We show that the strength of asymmetry in the habenula is correlated with strength of behavioural lateralization in the detour task. Both the strength and direction of habenular asymmetry are correlated with individual differences in growth rate. We suggest that this relationship results from processes linking growth rate and sexual differentiation to frequency-dependent variation in life-history strategies. To our knowledge, this is the first study to demonstrate a relationship at the individual level between neural asymmetry and lateralized behaviour in a fish.