Scalable architecture in mammalian brains

Area of Science:

• Neuroscience
• Evolutionary Biology
• Comparative Anatomy

Background:

• Traditional comparisons of mammalian brain parts focus on absolute size, which can be misleading.
• Using body weight as a reference variable obscures true brain size relationships and phylogenetic significance.

Purpose of the Study:

• To introduce a novel measure, the cerebrotype, using the brain itself as a size reference.
• To investigate brain composition across mammalian taxa and test evolutionary hypotheses.

Main Methods:

• Defined the cerebrotype as a species-specific measure of brain composition.
• Analyzed brain volumes across diverse mammalian taxa.
• Examined cerebrotype shifts and neocortical volume fractions in primates.

Main Results:

• The cerebellum consistently occupies a fixed fraction (0.13 +/- 0.02) of total brain volume across mammals, challenging its role as solely serving the neocortex.
• Cerebrotype effectively distinguishes mammalian taxa, enabling quantitative testing of evolutionary relationships.
• Primate cerebrotypes show progressive shifts with increasing neocortical volume fractions from lemurs to hominoids, indicating directed evolutionary pressure.
• Absolute brain size varies greatly within taxa, yet cerebrotype remains relatively uniform, demonstrating brain scalability.

Conclusions:

• Mammalian brains possess a scalable architecture with a stable composition.
• The cerebrotype is a robust measure for understanding brain evolution and phylogenetic relationships.
• Primate brain evolution is characterized by directed selection pressures shaping neocortical expansion.