Different ways of evolving tool-using brains in teleosts and amniotes

In mammals and birds, tool-using species are characterized by a high degree of encephalization with a relatively large telencephalon containing a higher proportion of total brain neurons compared to other species. Some teleost species in the wrasse family have convergently evolved tool-using abilities. In this study, we compared the brains of tool-using wrasses with various teleost species from a broad phylogenetic range. We show that in the tool-using wrasses, the telencephalon and the ventral part of the forebrain and midbrain are significantly enlarged compared to other teleost species, but do not contain a larger proportion of cells. Instead, this size difference is due to large fiber tracts connecting the dorsal part of the telencephalon (pallium) to the inferior lobe (IL), a ventral mesencephalic structure absent in amniotes. Such tracts were not present in other teleost species such as trout, zebrafish, or the Astyanax surface fish. The high degree of connectivity between the IL and the pallium in tool-using wrasses suggests that this unique teleostean structure contributes to higher-order cognitive functions. Considering how remarkably different their overall brain organization is, we conclude that, unlike in amniotes, the evolution of non telencephalic structures might have been key in the emergence of higher-order cognitive functions in teleosts.