UC Davis

Comparative studies provide important insights into the evolution of general features of brain organization as well as features that are unique across species. These unique differences in brain organization are thought to reflect specialized adaptations in body morphology, behavior and lifestyle. While there is a great diversity of mammalian behavioral and morphological specializations, we know little about evolutionary changes in the neocortical structures that support these differences. Using histological collections from the Krubitzer lab, a comparative dataset was created in order to measure the sizes of cortical fields across evolution. In this analysis we focus on 4 cortical fields, the primary somatosensory (S1), visual (V1) and auditory (A1) areas, as well as the motor cortex (M1). These areas were examined in 18 species representing 6 different mammalian lineages: 7 species of Rodentia (rodents), 5 species of Marsupialia (marsupials), 2 species of Chiropotera (bats), 2 species of Eulipotyphla (water shrews and hedgehogs), 1 from Scandentia (tree shrews) and 1 from Afrosoricida (tenrecs). We quantify the lineage-specific changes and general properties of the sizes of primary motor and primary sensory cortical fields using allometric analysis, and discuss several outliers from the trends. We found that larger brains with a larger neocortex devote less space to M1, A1 and S1, whereas V1 becomes exceptionally larger, or alternatively exceptionally smaller in mammals with a small neocortex. We also found several examples of mosaic evolution in S1 and one example in A1. The neocortex is the underlying substrate that supports adaptive sensorimotor specializations, yet this varies across species. We found that not all convergent sensorimotor specializations share the same underlying expansions and contractions of cortical fields, suggesting that there are multiple ways the neocortex can organize to “solve” for the same specialization in different species.