The extent to which the evolution of a larger brain is adaptive remains controversial. Trinidadian killifish (Anablepsoides hartii) are found in sites that differ in predation intensity; fish that experience decreased predation and increased intraspecific competition exhibit larger brains. We evaluated the connection between brain size and fitness (survival and growth) when killifish are found in their native habitats and when fish are transplanted from sites with predators to high-competition sites that lack predators. Selection for a larger brain was absent within locally adapted populations. Conversely, there was a strong positive relationship between brain size and growth in transplanted but not resident fish in high-competition environments. We also observed significantly larger brain sizes in the transplanted fish that were recaptured at the end of the experiment versus those that were not. Our results provide experimental support that larger brains increase fitness and are favored in high-competition environments.

The files include the data for the comparative mark-recapture experiments and the transplant experiment described in the manuscript. The data for the comparative mark recapture expeirment tabs associated with the analyses of growth and survval. The file for the transplant experiment inlcudes the data associated with the growth rate analyses. 

Data can be accessed using SPSS.