Experimental transplants demonstrate shifts in predation favor evolution of aggressive behaviors in Trinidadian killifish

Theory asserts larger brains facilitate behaviors that enhance fitness. Research has demonstrated that increased brain size improves cognition and survival. However, the majority of research has focused on cross-species comparisons. Experiments that manipulate selection to investigate the connection between brain size, behavior, and fitness are needed. 

Trinidadian killifish (Anablepsoides hartii) live in communities with (high predation: HP) and without (killifish-only: KO) predators. Predator absence is associated with high population densities, increased intraspecific competition, and evolved larger brain sizes.

We tested for evolutionary shifts in behavior by subjecting second-generation lab-reared fish to a mirror aggression assay. We also quantify selection on brain size and behavior by transplanting wild HP fish to KO sites. 

Lab-reared fish from KO sites—specifically males—exhibited higher levels of aggression than HP fish. In the transplant experiment, HP fish exhibited strong increases in aggression following the introduction to KO sites. Increased brain size was correlated with increased growth in transplanted HP fish, yet there was no association between brain size, aggression, and growth. 

Our results indicate that declines in predation and increased competition favor increases in aggression but further research is needed to determine if and how brain size and behavior are linked through natural selection.

Methods
The data in the accompanying files represent the work of comparative brain size and behavior experiments on common garden lab-reared Trinidadian killifish (Anablepsoides hartii) and a subsequent mark-recapture transplant experiment, both of which are described in full in the manuscript. The transplant data are available in wide and long format to facilitate analyses of growth, recapture statistics, and shifts in behavior.