A single gene called Angiopoietin-1 (Ang-1) drives brain size and intelligence in fish according to a new study. Fish with larger brains and higher intelligence had higher expression of Ang-1, and when expression levels of Ang-1 were experimentally reduced, brains shrunk. These trends were seen in two unrelated species of fish -- guppies (Poecilia reticulata) and zebra fish (Danio rerio) -- indicating expression of Ang-1 is important for brain growth and development in fish generally.