Environmental Seasonality and Mammalian Brain Size Evolution

Abstract 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. Key Concepts Ecology is one of the main drivers of brain size evolution. Whether selection favours large brains depends on whether the fitness benefits of having a large brain exceed the high costs of brain maintenance. Environmental seasonality is both cognitively challenging and energetically expensive. Large‐brained species use their enhanced cognitive abilities in the form of behavioural flexibility to deal with environmental seasonality by keeping food consumption constant throughout the year (cognitive buffer hypothesis). If species face periods of food scarcity, they will not be able to hold the energy supply for a large brain constant. In this situation, seasonality acts as energetic constraint on brain size (expensive brain hypothesis). Seasonality constrains brain size in all mammalian groups investigated so far and might also play a role in other lineages. Cognitive buffering is mainly found in primates and birds.