Enlarged brains of homeotherms bring behavioural advantages, but incur high energy expenditures for the animal. The ‘Expensive Tissue’ (ET) hypothesis links the evolution of the enlarged brain to increased cognitive abilities (CA) that improved foraging performance, social interactions and allowed for reduction in size of the energetically demanding gut. We tested the directionality of the evolutionary trade-off between brain, gut and CA using experimental evolution model consisting of lines of laboratory mice subjected to artificial selection on basal (BMR) or maximum (VO2max) aerobic metabolism - traits that are implicated in evolution of homeothermy and CA. High BMR mice had bigger guts, but not brains. Yet, they performed better in cognitively demanding tasks and had higher neuronal plasticity than their counterparts. The data indicate that evolutionary increase of CA was initially associated with brain plasticity and fuelled by an enlarged gut, which was not traded off for brain size, as the ET posits.