Ecology has been shown to be among the main drivers of brain size evolution. One important ecological aspect is environmental seasonality. Seasonality is related to brain size evolution in two different ways: On one hand, seasonality acts as energetic constraint on brain size because it forces animals to deal with periodic food shortages (expensive brain hypothesis). On the other hand, seasonality may act as a selective pressure to increase brain size, as cognitive and behavioural flexibility helps to overcome periods of food scarcity (cognitive buffer hypothesis). Current evidence suggests that energetic constraints imposed by environmental seasonality play a crucial role in mammalian brain size evolution, cognitive buffering; on the contrary, seems to be less ubiquitous and is mainly found in large‐brained species such as haplorrhine primates and birds.