Zegni Triki , Carel van Schaik, Redouan Bshary

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.