Zegni Triki , Stephanie Fong, Mirjam Amcoff, Sebastien Vàsquez- Nilsson, Niclas Kolm

Executive functions are a set of cognitive control processes required for optimizing goal- directed behaviour. Despite more than two centuries of research on executive functions, both in humans and nonhuman animal species, there is still a knowledge gap of what constitutes the mechanistic basis of evolutionary variation in executive function abilities. Here we show experimentally that changes in a forebrain structure (i.e., telencephalon) size, through mosaic brain evolution, underlie individual variation in executive functions capacities in a fish. For this, we used artificial selection lines of guppies (Poecilia reticulata) with substantial differences in relative telencephalon size. We tested fish from the large and small telencephalon lines in tasks for the three main core executive functions: cognitive flexibility, inhibitory control, and working memory, but also in a basic conditioning test that does not require executive functions. Individuals with larger telencephalons outperformed individuals with smaller telencephalons in all three executive function assays but not in the conditioning assay. Based on our discovery, we propose that the telencephalon is the executive brain in teleost fish. Also, selective enlargement of key brain structures, like the fish telencephalon, through mosaic brain evolution is a potent evolutionary pathway towards evolutionary enhancement of advanced cognitive abilities in vertebrates.