The Fish Challenge to Vertebrate Cognitive Evolution

There is tremendous taxonomic variation in the size, shape, and structure of vertebrate brains. While many studies aim at identifying the ecological factors (social and environmental) that explain brain size variation within taxa, a more fundamental divide exists between endotherm and ectotherm vertebrates. Ectotherms have ten times smaller brains than endotherms. The existing hypotheses cannot explain this divide, as some endotherm species with relatively simpler social organisation and diets still possess larger brains. Furthermore, research demonstrates that at least fishes possess a cognitive "toolkit" equivalent to that of many endotherms. This is the fish challenge to vertebrate cognitive evolution. We review hypothesised causes and consequences of brain size differences to propose two non-exclusive solutions. First, the fish brain achieves modularity at a lower cost, but it is less efficient in problem-solving than an endothermic brain with a more domain- general organisation. Second, brain size variation can be better explained by perception and motor skills rather than by cognitive processes. In that case, understanding brain size would require applying a broad definition of cognition. Specifically, it would be fitting to define animal cognition as how animals take in and process sensory information before deciding how to act on it with motor competencies.