The fish challenge to vertebrate cognitive evolution

There is tremendous taxonomic variation in the size, shape and structure of vertebrate brains. While many studies use cross-species comparisons to aim at identifying the ecological factors (social and environmental) that explain what cognitive advantages larger brains confer, a more fundamental divide exists between endotherm and ectotherm vertebrates. Relative to body size, ectotherms have ten times smaller brains than endotherms. Existing ecological hypotheses cannot explain this divide, as some endotherm species with relatively simple social organization 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 hypothesized consequences of brain size differences to propose two non-exclusive solutions. First, the fish brain retains modularity, but it is less efficient in problem-solving than an endothermic brain with a more domain-general organization. Second, brain size variation can be more effectively explained by sensory-motor skills and their integration rather than by cognitive processes. In that case, understanding brain size would require highlighting and emphasizing these aspects when broadly defining animal cognition. Specifically, it would be fitting to amend the classic definition of animal cognition to refer to how animals take in and process sensory information before deciding how to act on it using motor competencies.This article is part of the Theo Murphy meeting issue 'Selection shapes diverse animal minds'.